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Sorghum Pathology and Biotechnology -A Fungal Disease Perspective: Part II. Anthracnose, Stalk Rot, and Downy Mildew

January 2012

January 2012
6:31-44

Authors:

Christopher R. Little

Kansas State University

Ramasamy Perumal

Kansas State University

Adedayo Adeyanju

Kansas State University

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Foliar diseases and stalk rots are among the most damaging diseases of sorghum in terms of lost production potential, thus commanding considerable research time and expenditure. This review will focus on anthracnose, a fungal disease that causes both foliar symptoms and stalk rots along with the stalk rots caused by Fusarium spp. and Macrophomina phaseolina. Although the downy mildews are caused by oomycetes rather than true fungi, recent outbreaks have revealed resistance to previously effective chemical seed treatments and the evolution of new pathogenic races, once again pointing out the need for continuous vigilance. Sorghum diseases are described with respect to the causal organism or organisms, infection process, global distribution, pathogen variability and effects on grain production. In addition, screening methods for identifying resistant cultivars and the genetic basis for host resistance including molecular tags for resistance genes are described where possible along with prospects for future advances in more stable disease control.

Foliar anthracnose lesions caused by Colletotrichum sublineolum. (A) Infected leaf with elongated necrotic lesions with dark brown acervuli. (B) Acervuli with oval to cylindrical shape. Images courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 2 Foliar anthracnose in the field. (A) Plant infected with C. sublineolum. (B) Field-level infection and foliar senescence associated with severe anthracnose. Images courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 3 Anthracnose-resistant ('SC748-5') (A) and -susceptible ('BTx623') (B) lines. Images courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 4 Severe lodging due to field-scale stalk rot infection. Image courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 5 Examples of artificially inoculated sorghum stalks. (A) Sterile water-inoculated sorghum line '99840'; (B) Fusarium andiyazi-inoculated sorghum hybrid 'Tx2783'; (C) F. thapsinuminoculated sorghum line '25056'; (D) M. phaseolina-inoculated sorghum line 'SC599'. Fig. 6 Example of internal pith discoloration associated with Fusarium stalk rot extending into the peduncle and resulting in head blight. Note reduced caryopsis formation in the split stalk and peduncle at right. Fig. 7 Peronosclerospora sorghi-infected leaf at vegetative stage with sporulating lesions. Fig. 8 Plant with systemic infection by P. sorghi. Fig. 9 Field view of the healthy and P. sorghi-infected plants. Bottom of a mature leaf of the infected plant with abundant sporulation.

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Received: 25 August, 2011. Accepted: 12 November, 2011. Invited Review

Sorghum Pathology and Biotechnology - A Fungal Disease

Perspective: Part II. Anthracnose, Stalk Rot, and Downy Mildew

Tesfaye Tesso1 • Ramasamy Perumal2 • Christopher R. Little3 • Adedayo Adeyanju1 •

Ghada L. Radwan4 • Louis K. Prom5 • Clint W. Magill4*

1 Department of Agronomy, Kansas State University, Manhattan, Kansas 66506, USA

2 Western Agricultural Research Center - Hays, Hays, Kansas 67601, USA

3 Department of Plant Pathology, Kansas State University, Manhattan, Kansas 66506, USA

4 Department of Plant Pathology and Microbiology, Texas A&M University, College Station, Texas 77845, USA

5 Southern Plains Agricultural Research Center, USD A-ARS, College Station, Texas 77845, USA

Corresponding author: * c-magill@ tamu.edu

ABSTRACT

Foliar diseases and stalk rots are among the most damaging diseases of sorghum in terms of lost production potential, thus commanding

considerable research time and expenditure. This review will focus on anthracnose, a fungal disease that causes both foliar symptoms and

stalk rots along with the stalk rots caused by Fusarium spp. and Macrophomina phaseolina. Although the downy mildews are caused by

oomycetes rather than true fungi, recent outbreaks have revealed resistance to previously effective chemical seed treatments and the

evolution of new pathogenic races, once again pointing out the need for continuous vigilance. Sorghum diseases are described with

respect to the causal organism or organisms, infection process, global distribution, pathogen variability and effects on grain production. In

addition, screening methods for identifying resistant cultivars and the genetic basis for host resistance including molecular tags for

resistance genes are described where possible along with prospects for future advances in more stable disease control.

_____________________________________________________________________________________________________________

Keywords: Colletotrichum sublineolum, Colletotrichum graminicola, Fusarium, Macrophomina phaseolina, Peronosclerospora sorghi

CONTENTS

ANTHRACNOSE.................................................................................................................... .................................................................... 31

Introduction ............................................................................................................................................................................................. 31

Biology of Colletotrichum sublineolum................................................................................................................................................... 32

Disease symptoms and losses .................................................................................................................................................................. 32

Pathogenic races ...................................................................................................................................................................................... 32

Disease screening techniques .................................................................................................................................................................. 32

Host resistance......................................................................................................................................................................................... 34

DNA-based diversity and molecular tags ................................................................................................................................................ 34

Future prospects....................................................................................................................................................................................... 35

STALK ROT ................................................................................................................................................................................................ 35

Biology of stalk rot.................................................................................................................................................................................. 36

Disease symptoms and losses .................................................................................................................................................................. 36

Factors associated with stalk rot incidence in grain sorghum.................................................................................................................. 37

Pathogen races ......................................................................................................................................................................................... 38

Disease screening techniques .................................................................................................................................................................. 38

DNA-based diversity tags........................................................................................................................................................................ 39

DOWNY MILDEW ..................................................................................................................................................................................... 39

Biology of Peronosclerospora sorghi...................................................................................................................................................... 39

Disease symptoms and losses .................................................................................................................................................................. 39

Disease screening techniques .................................................................................................................................................................. 40

Disease control and host resistance.......................................................................................................................................................... 40

DNA-based diversity and molecular tags ................................................................................................................................................ 41

Future directions...................................................................................................................................................................................... 41

ACKNOWLEDGEMENTS ......................................................................................................................................................................... 41

REFERENCES............................................................................................................................................................................................. 41

_____________________________________________________________________________________________________________

ANTHRACNOSE

Introduction

Anthracnose, a disease that impacts the health of cereals

and grasses worldwide is caused by a monophyletic group

of taxa in the genus Colletotrichum. During the past decade

there have been important changes in our knowledge of

how the grass-associated Colletotrichum spp. have evolved,

and an increased understanding of the mechanisms by

which these fungi engage in hemibiotrophic interactions

with their host plants (Crouch and Beirn 2009). Anthrac-

nose of sorghum (Sorghum bicolor (L.) Moench) is caused

by the fungus Colletotrichum sublineolum Henn. (syn. C.

graminicola (Ces.) G. W. Wilson [Sutton 1980; Sheriff et al.

1995]. It is widely prevalent and economically significant in

warm and humid regions of Asia, Africa and the Americas

(Pastor-Corrales 1980; Ali and Warren 1992; Leslie 2002;

Mathur et al. 2002; Marley et al. 2004; Figueiredo et al.

2006; Thakur et al. 2007b; Chala et al. 2007). The pathogen

is capable of infecting the stalk, foliage, panicle, and grain,

thereby degrading not only the quantity but also the quality

of both grain and stover. Infection of foliar tissues reduces

photosynthate accumulation while infection of the stalk

leads to stalk rot followed by lodging, a detriment to maxi-

mizing harvestable biomass (Waniska et al. 2001).

Biology of Colletotrichum sublineolum

The causal agent of anthracnose on cereals, including maize

and sorghum, has been long regarded as C. graminicola

(Holliday 1980). However, analyses of rDNA sequences,

DNA fingerprints, mating tests, and appressorium morphol-

ogy have demonstrated that isolates from maize and sorg-

hum belong to distinct species (Sutton 1968; Vaillancourt

and Hanau 1992; Sherriff et al. 1995). Isolates from maize

are now regarded as C. graminicola, whereas those from

sorghum are designated C. sublineolum (Sutton 1980). It

was only after rDNA sequences supported the separation

from C. graminicola that C. sublineolum became the gene-

rally accepted species for sorghum isolates (Singh and

Boora 2008). Conidia are produced terminally and singly on

conidiophores among setae and occur as masses immersed

in the mucilaginous substrate. They are hyaline, non-septate,

uninucleate and sickle or spindle-shaped. Acervuli produced

on the infected host tissue are with or without setae, pink or

dark brown and oval to cylindrical (Fig. 1). Setae in acer-

vuli are long (up to 100 Pm), dark and prominent and are

intermixed with conidia and conidiophores (Thakur 2007).

Wharton and Julian (1996) conducted cytological stu-

dies and showed that C. sublineolum has a two-stage, hemi-

biotrophic infection process on sorghum similar to that of C.

lindemuthianum on bean (O’Connell et al. 1985). The initial

biotrophic phase is associated with intracellular infection

vesicles and primary hyphae, which colonize many host

cells before giving rise to necrotrophic secondary hyphae.

In incompatible interactions, host cells die soon after penet-

ration by infection vesicles, and fungal development is res-

tricted to a single epidermal cell (Wharton and Julian 1996).

Accumulation of red-pigmented cytoplasmic inclusions,

containing 3-deoxyanthocyanidin phytoalexins (Snyder et

al. 1991), occurs in both compatible and incompatible inter-

actions, but this defense response is expressed much earlier

in incompatible interactions (Wharton and Julian 1996; Lo

et al. 1999). Wharton et al. (2001) examined C. subline-

olum infection of susceptible and resistant sorghum cul-

tivars with transmission and scanning electron microscopy

and documented the ultrastructure of the biotrophic inter-

face between intracellular primary hyphae and host cyto-

plasm to determine the effects of infection on host cells and

to examine the ultrastructure of sorghum defense responses.

Disease symptoms and losses

Anthracnose in sorghum was first reported from Togo, West

Africa in 1902 (Sutton 1980). Anthracnose symptoms range

from grain deterioration to peduncle breakage, to stalk rot

and foliar damage. The disease has since been reported in

most sorghum growing regions of the world with yield

losses as high as 50% in susceptible lines when infection is

followed by wet and dry cycles during periods of high tem-

peratures (Harris et al. 1964; Warren 1986; Hulluka and

Esele 1992; Thomas et al. 1995; Waniska et al. 2001;

Gwary et al. 2002; Ngugi et al. 2002). Chala et al. (2010)

surveyed disease severity in different sorghum growing

areas in a total of 487 fields in 49 districts for three years

(2005 to 2007). The maximum observed severity in the

study was 77%.

Although cultural strategies such as rotation may reduce

the impact of the disease, the use of resistant genotypes is

the best means for anthracnose control. However, the vari-

able nature of the pathogen offers challenges in breeding for

durable resistance (Pande et al. 1991; Valèrio et al. 2005).

Three phases of the disease are recognized: foliar anthrac-

nose (Figs. 1, 2), anthracnose stalk rot (Fig. 2), and panicle

and grain anthracnose. Foliar anthracnose, the most com-

mon form and the most destructive phase of the disease,

usually appears 30-40 days after emergence, during growth

stage 4.0 or later (da Costa et al. 2003). Symptoms are cha-

racterized by circular to elliptical spots with few or numer-

ous fungal fruiting bodies (acervuli) within the leaf lesions

(Tarr 1962). Differences in leaf symptoms are common and

may be caused by variations in the pathogen, host reaction,

or the physiological status of the host (Pastor-Corrales and

Frederiksen 1980; Ferreira and Warren 1982). C. subline-

olum may survive as mycelium, conidia and microsclerotia

up to 18 months in crop debris, on or above the soil surface,

in alternate hosts, and as mycelium in infected seeds.

Microsclerotia are produced in sorghum stalks of suscepti-

ble cultivars and survive in crop debris on the soil surface.

Pathogenic races

C. sublineolum is considered to be a very heterogeneous

species, primarily based on the large number of pathotypes

that have been described based on differential virulence to

host lines (da Costa et al. 2003). An abundance of patho-

genic races of C. sublineolum exist in nature. More than 40

races/pathotype have been reported from different geogra-

phical areas of the world, using different sets of host dif-

ferentials (Thakur 2007). Pande et al. (1991) noted 9 patho-

types from as many isolates obtained in India. The existence

of five pathotypes of C. sublineolum among 16 isolates

from the major sorghum growing zones of Nigeria was

detected based on reactions on 8 host differentials (Marley

et al. 2001). Mathur et al. (2002) used a set of 15 sorghum

differentials grown in 16 locations in Africa, Asia and the

United States; the interactions showed that different patho-

types prevailed at each location. In Brazil, Casela et al.

(1992) separated isolates of C. sublineolum first into 8

pathotype groups based on resistant or susceptible reactions

on 9 common differentials and then added other differen-

tials to further delineate the isolates into 32 pathotypes.

Later, Valério et al. (2005) reported 22 pathotypes among

37 isolates using an additional differential, 'SC748-5'. In the

United States, by using 8 common differentials, Ali and

Warren (1987) recorded 3 pathotypes from 9 C. subline-

olum isolates, Cardwell et al. (1989) reported 8 pathotypes

from 12 isolates, while Moore et al. (

2008) established 13

new pathotypes from 87 isolates collected from Arkansas.

Recently, 20 out of 232 isolates (collected from three

geographically distinct regions of Texas, and from Arkansas,

Georgia, and Puerto Rico between 2002 and 2004) were

selected based on amplified fragment length polymorphism

(AFLP) genetic diversity and tested for pathogenicity on 14

sorghum lines previously used in Brazil and the United

States and 4 from Sudan. Seventeen of the 20 isolates were

identified as new pathotypes (Prom et al., unpublished data).

Since not all studies have used the same set of host differen-

tials, it is not possible to directly compare the extant patho-

gen populations.

Disease screening techniques

Pande et al. (1991) inoculated both leaf surfaces of the plant

with a conidial suspension of C. sublineolum using a hand

sprayer. One hour after inoculation humidifiers were run

continuously for 18 hr to create 100% humidity. To further

promote disease development, humidifiers were run conti-

nuously for 6 days for 8 hr/day. To enhance infection, the

author conducted the experiments five times. Mehta (Mehta

2002; Mehta et al. 2005) followed a similar inoculation

Sorghum anthracnose, stalk rots and downy mildew. Tesso et al.

BC D

Fig. 1 Foliar anthracnose lesions caused by Colletotrichum sublineolum. (A) Infected leaf with elongated necrotic lesions with dark brown acervuli. (B)

Acervuli with oval to cylindrical shape. Images courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 2 Foliar anthracnose in the field. (A) Plant infected

with C. sublineolum. (B) Field-level infection and foliar senescence associated with severe anthracnose. Images courtesy of John Jaster, Pioneer Hybrid

Seeds. Fig. 3 Anthracnose-resistant ('SC748-5') (A) and -susceptible ('BTx623') (B) lines. Images courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 4

Severe lodging due to field-scale stalk rot infection. Image courtesy of John Jaster, Pioneer Hybrid Seeds. Fig. 5 Examples of artificially inoculated

sorghum stalks. (A) Sterile water-inoculated sorghum line '99840'; (B) Fusarium andiyazi-inoculated sorghum hybrid 'Tx2783'; (C) F. thapsinum-

inoculated sorghum line '25056'; (D) M. phaseolina-inoculated sorghum line 'SC599'. Fig. 6 Example of internal pith discoloration associated with

Fusarium stalk rot extending into the peduncle and resulting in head blight. Note reduced caryopsis formation in the split stalk and peduncle at right. Fig.

7 Peronosclerospora sorghi-infected leaf at vegetative stage with sporulating lesions. Fig. 8 Plant with systemic infection by P. s o r g hi . Fig. 9 Field view

of the healthy and P. so rg hi -infected plants. Bottom of a mature leaf of the infected plant with abundant sporulation.

method by spraying approximately 3-5 mL of a conidial

suspension (106 conidia ml-1) onto the leaves and the whorl

of each plant, using backpack sprayers or a tractor-mount

sprayer at night or in the early morning so as to have low

light intensity. However, Mehta (2002) reported that the

number of lesions on susceptible plants within susceptible

or segregating F2:3 lines was very low compared to the in-

fection observed on susceptible checks, which made ac-

curate scoring difficult. An inoculation system (Erpelding

and Prom 2006) where colonized sorghum grains are

dropped into the whorls of young plants has proven

effective in lowering the problem of escapes. In a 2007 field

evaluation at College Station, Texas, 95% of the accessions

from 100 advanced germplasm lines and 55% from the exo-

tic lines were found to be susceptible using this inoculation

method. Thakur (2007) assigned disease ratings on a 1 to 9

scale, where 1 = highly resistant (HR), no lesions or a

hypersensitive reaction with mild yellow flecks and ratings

2 to 9 are based on leaf area covered with lesions: 2 (1-5%)

and 3 (6-10%) - resistant (R); 4 (11-20%) and 5 (21-30%) -

moderately resistant (MR); 6 (31-40%) and 7 (41-50%) -

susceptible (S); 8 (51-75%) and 9 (>75%) - highly suscepti-

ble (HS). The rating scale is useful for differentiating

between lines with minor differences in resistance. How-

ever, for large scale field screening Prom et al. (2009) intro-

duced a modified 1 to 5 scale, where 1 = no symptoms or

chlorotic flecks on leaves; 2 = hypersensitive reaction (red-

dening or red spots) on inoculated leaves, but no acervulus

formation and no spread to other leaves; 3 = lesions with

small acervuli in the center of leaves up to one third of plant

height from the bottom; 4 = necrotic lesions with acervuli

on all leaves except the flag leaf; and 5 = necrotic lesions

with abundant acervuli covering the entire plant. Disease

assessments are conducted 30 days post-inoculation and

thereafter on a weekly basis for four consecutive weeks

until flowering, allowing disease progression to be analyzed.

Host resistance

Plants have a wide array of physical (surface features, struc-

tural barriers) and chemical strategies (phytoalexins,

hydroxyproline-rich glycoproteins, wall papillae) to defend

themselves from invasion by pathogens. Young sorghum

leaves accumulate phytoalexins in the form of a complex of

phenols having fungitoxic activity in response to invasion

by both pathogenic and non-pathogenic fungi (Singh and

Boora 2008). The fact that many cultivars are susceptible to

anthracnose suggests differences in levels or rate of res-

ponse in these defense mechanisms have a genetic basis.

Breeding for stable host plant resistance has been difficult

even in regions with endemic anthracnose because of the

hypervariable nature of C. sublineolum along with strong

environmental effects on symptom development and disease

spread. Consequently, even though several sources of gene-

tic resistance are known, an understanding of the basis for

anthracnose resistance is still lacking. Studies have exa-

mined anthracnose resistance in sorghum germplasm from

the USDA-TAES sorghum conversion program (Cardwell

et al. 1989). Coleman and Stokes (1954) reported that resis-

tance to anthracnose in the sorghum line 'Sart' is encoded by

two closely linked dominant genes, each conferring resis-

tance to different phases of the disease. Jones (1979) found

single gene dominant resistance for leaf anthracnose in one

cross but two dominant genes segregated for resistance in a

second cross. Tenkouano et al. (1993) reported that

resistance to anthracnose in 'SC326-6' was controlled by a

single genetic locus with multiple allelic forms while in

progeny of a cross made by Boora et al. (1998) resistance in

'SC326-6' segregated as a simple recessive as was also the

case for 'G73' crossed to a highly susceptible cultivar

(Singh and Boora 2008). Erpelding and Prom (2004) eval-

uated 270 Mali accessions to study the mode of inheritance

during the dry and wet seasons in 2003 at Puerto Rico and

41 accessions exhibited both dominant and recessive gene

action. Mehta et al. (2005) identified four converted lines

that displayed unique, but simply inherited sources of an-

thracnose resistance. Resistance from 'SC748-5' (Fig. 3A)

was the most stable across environments. Resistance to

foliar anthracnose in sorghum accession 'Redlan' was found

to segregate as a simple dominant trait whereas infection of

the leaf midrib was found to be controlled by a single un-

linked recessive gene (Erpelding 2007).

Breeding for host plant resistance provides an econo-

mical approach for controlling diseases and stabilizing crop

production, but pathogen populations are variable and evol-

ving; therefore, the identification of new sources of resis-

tance is essential. Plant germplasm collections have been

established by the United States National Plant Germplasm

System in Griffin, Georgia to preserve genetic variation for

utilization in crop improvement programs. Exotic germ-

plasm materials are continuously being evaluated at the

United States of America Department of Agriculture, Agri-

cultural Research Service, National Plant Germplasm Sys-

tem (USDA, ARS, NPGS) Tropical Agriculture Research

Station in Isabela, Puerto Rico. Twenty-two Mozambique

accessions (Erpelding and Prom 2006), four Chinese ac-

cessions ('PI430471', 'PI563905', 'PI563924' and 'PI563960';

Prom et al. 2007), 11 Zimbabwe accessions (Erpelding

2008a), 119 Mali accessions (Erpelding 2008b) and six

Uganda accessions ('PI534117', 'PI534144', 'PI576337',

'PI297199', 'PI533833', and 'PI297210'; Prom et al. 2011)

were identified as potential resistance sources over multiple

growing seasons, at least to the anthracnose pathotypes at

the research site in Puerto Rico. Marley and Ajayi (2002)

identified lines 'R 6078', 'IS 14384' and 'CCGM 1/19-1-1' as

resistant to anthracnose when evaluated under natural infec-

tion at Samaru and Bagauda (Nigeria) in 1996 and 1997.

Thakur et al. (2007b) tested 15 sorghum lines collected

from the Sorghum Anthracnose Virulence Nursery (ISAVN)

at 14 anthracnose hotspots in India, Thailand, Ethiopia,

Kenya, Zambia, Nigeria and Mali for 4 to 7 years (1992-

1998) and identified 'IS 6928', 'IS 18758', and 'IS 12467' as

the most resistant across the environments (locations and

years). Pereira et al. (2011) identified four parental lines

'CMSXS657', 'ATF14', 'ATF08' and 'CMSXS210' as resis-

tant sources against 20 virulent isolates collected from dif-

ferent sorghum producing areas in Brazil. Results of these

evaluations suggest that exotic germplasm is an important

source of anthracnose resistance and that ecogeographic

information could aid in the selection of germplasm and

increase the likelihood of identifying additional sources of

resistance.

DNA-based diversity and molecular tags

Due to environmental influences on the stability of morpho-

logical traits, differentiation between Colletotrichum iso-

lates based on conidial morphology or features such as

colony color, size, and shape or host origin are not suffici-

ent for assessing genetic diversity. DNA markers including

random amplified polymorphic DNA (RAPD) (Guthrie et al.

1992) and restriction fragment length polymorphisms

(RFLPs) have also been used to examine diversity in the

pathogen. Vaillancourt and Hanau (1992) reported signifi-

cant differences between the restriction fragment patterns of

maize and sorghum isolates, while Rosewich et al. (1998),

using seven low-copy DNA probes, detected nine RFLP

haplotypes among 411 C. sublineolum isolates collected

from one site in Georgia. However, DNA comparisons have

revealed less diversity than might have been anticipated.

For example, in one study, seven DNA hybridization probes

detected multiple RFLP-based haplotypes, but the most

common patterns were found in samples collected from

Georgia, Honduras, Zambia and Texas (Gale 2002). Inter-

genic spacer regions of nuclear ribosomal DNA (Latha et al.

2003) or isozymes (Horvath and Vargas 2004) used to

estimate genetic variation among isolates revealed that host

origin plays a more important role than geographic origin in

the genetic diversity of anthracnose isolates. (It must be

noted, however, that these and similar studies did not clas-

Sorghum anthracnose, stalk rots and downy mildew. Tesso et al.

sify C. sublineolum separate from C. graminicola). Valèrio

et al. (2005) used RAPD and RFLP-PCR markers to study

the molecular diversity of 37 Colletotrichum isolates col-

lected from four distinct regions of Brazil and recorded

polymorphic differences among isolates belonging to the

same race as defined on 10 sorghum differentials. However,

no association between virulence phenotypes and molecular

profiles was observed. Figueiredo et al. (2006) used dif-

ferent molecular markers (SDS-PAGE, RAPD, ARDRA

(amplified rDNA restriction analysis) and rDNA sequen-

cing) for identifying C. sublineolum pathotypes and con-

cluded RAPD and rDNA sequencing revealed a high degree

of polymorphism among the five pathotypes in Brazil.

Chala et al. (2011) assessed diversity through amplified

fragment length polymorphism (AFLP) analysis using 102

isolates of C. sublineolum collected from different sorg-

hum-producing regions of Ethiopia. The results of this

study confirmed the presence of a highly diverse pathogen,

which is in agreement with the existence of diverse host

genotypes and widely ranging environmental conditions in

sorghum-producing regions in that country.

On the host side, Wang et al. (2006) used microsatellite

markers for ninety-six accessions randomly selected from

the core collection database of the Germplasm Research

Information Network (GRIN) to evaluate genetic diversity

in relation to rust and anthracnose disease response. The in-

formation from genetic classification was used for choosing

parents to make crosses in sorghum breeding programs and

classifying sorghum accessions in germplasm management.

Molecular markers linked to gene(s) of interest are one

possible strategy to permit selection for anthracnose resis-

tance without concern for pathogen pressure. A vast array of

genome resources for sorghum has been developed in the

past 10 to 15 years. DNA-based molecular markers showing

genetic linkage to disease resistance loci in sorghum have

been reported for anthracnose by different workers using

different marker techniques. Boora et al. (1998) identified

RAPD markers linked to a recessive gene conditioning

anthracnose resistance, butt those markers have not been

mapped to a specific sorghum chromosomal location.

Panday et al. (2002) used bulk segregant analysis and iden-

tified two RAPD-based DNA markers (OPI 16 and OPD

12) linked to anthracnose disease resistance in sorghum

accession 'SC326-6', found to segregate as a simple reces-

sive trait when crossed with the susceptible cultivar

'BTx623' (Fig. 3B). Singh et al. (2006) were able to show

that an anthracnose resistance gene in sorghum line 'G73'

maps to the long arm of chromosome 8 on the basis of

linked RAPD (OPJ 01 with 3.26 cM distance from the

gene) and SCAR (SCJ 01-1 and SCJ 01-2) markers. Singh

and Boora (2008), using bulked segregant analysis, found

parental bands from OPA 12, OPJ 01, OPF 07 and OPL 04,

OPI 12 and OPD 12 (RAPD markers), Xtxp 61 and Xtxp

212 (simple sequence repeat or SSR) markers and SCA 12

and SCJ 01 (SCAR markers) from two mapping popula-

tions ['HC136' (susceptible to anthracnose) u 'G73' (an-

thracnose resistant) and 'SC326-6' (anthracnose resistant) u

'BTx623' (anthracnose susceptible)] provide closely linked

markers to the anthracnose resistance gene. Perumal et al.

(2009) identified AFLP marker Xtxa6227, previously

mapped to the end of sorghum linkage group LG-05, which

mapped within 1.8 cM of the anthracnose resistance locus

Cg1, a dominant gene for resistance originally identified in

cultivar 'SC748-5'. BAC clones spanning this chromosome

led to the discovery that Xtxp549, a polymorphic (SSR)

marker, mapped within 3.6 cM of the anthracnose resistance

locus. The efficacy of Xtxa6227 and Xtxp549 were examined

for marker-assisted selection and 13 breeding lines derived

from crosses with sorghum line 'SC748-5' were genotyped.

In 12 of the 13 lines the Xtxa6227 and Xtxp549 polymor-

phism associated with the Cg1 locus was still present, sug-

gesting that Xtxp549 and Xtxa6227 could be useful for

marker-assisted selection and for pyramiding Cg1 with

other genes conferring resistance to C. sublineolum in sorg-

hum for more stable disease resistance. These markers

could also facilitate marker-assisted selection in breeding

for anthracnose resistance gene and map-based cloning of

resistance gene(s).

Future prospects

Variation in the natural populations of C. sublineolum and

the powerful selective advantage of a natural or mutant

strain that is able to reproduce on a previously resistant host

means that screening and identification of resistant cultivars

must persist. The use of molecular tools to combine dif-

ferent sources of genetic resistance may prolong the useful

life of a cultivar, but there can be no guarantee of perma-

nence. Genetic transformation provides another avenue for

increasing tolerance to plant diseases such as anthracnose.

Introducing genes encoding proteins such as chitinases and

chitosanases that hydrolyze fungal cell walls is a potential

strategy. For example, Kosambo-Ayoo et al. (2011) used

particle bombardment genetic transformation in sorghum by

introducing genes encoding proteins such as chitinases

(harchit) and chitosanases (harcho) that hydrolyze fungal

cell walls. Chitinases endolytically hydrolyze the beta-1,4-

linkages of chitin whereas, chitosanases hydrolyze the beta-

1,4-linkages between N-acetyl-D-glucosamine and D-gluco-

samine residues in a partially acetylated chitin polymer. The

two antifungal genes introduced into sorghum genome

could be introgressed into other sorghum lines for fungal

diseases resistance.

STALK ROT

Introduction

Stalk rots are among the most prevalent diseases of sorg-

hum in most places where the crop is cultivated (Zummo

1984). They are also common in other crop species inclu-

ding maize, millet, soybean, and sunflower (Pappelis and

BeMiller 1984; Rane et al. 1997). This disease complex

occurs in wide geographic regions both in tropical and tem-

perate environments (Tarr 1962). It is a widespread disease

in the west and central Africa region extending from Chad

to Senegal (Frowd 1980; Zummo 1980). The disease is also

prevalent in North and East African countries including

Egypt, Sudan, Uganda, Kenya, Somalia (Gray et al. 1991;

Hulluka and Esele 1992) and the semi-arid Rift Valley

region of Ethiopia (Gebrekidan and Kebede 1979). It is one

of the major diseases of sorghum in India, particularly

during the dry rabi season (Khune et al. 1984; Seetharama

et al. 1987), and Australia mainly in New South Wales

(Trimboli and Burgess 1982). In the United States, stalk rot

is a common problem in the southern states and in the cen-

tral Great Plains extending from Texas to Kansas (Edmunds

1964; Edmunds and Zummo 1975). There are also reports

of stalk rot incidence as far north as Nebraska (Reed et al.

1983; Duncan 1983) and the Southeastern United States

(Duncan 1983). It is also an important disease of sorghum

in Brazil and most of Latin American countries (Foster and

Frederiksen 1979; Frederiksen 1984).

The disease usually develops at later growth stages

during grain filling period and is characterized by degrada-

tion of the pith tissue at or near the base of the stalk causing

death of stalk pith cells (Edmunds 1964). This premature

death of stalk cells may result in reduced transportation of

nutrients and water, thus disrupting photosynthetic activity

and may also cause breakage of the stalk at the zone of

infection leading to lodging (Mughogho and Pande 1984;

Hundekar and Anahahusor 1994; Maranville and Clegg

1984). These phenomena may slow down or inhibit the

grain-filling process and thus result in shriveled seeds

(Zummo 1980). Growing interest in the use of the crop as

an alternative cellulosic feedstock for bio-fuel production

requires cultivation of taller and high biomass cultivars; the

apparent relationship between stalk rot incidence and crop

lodging is a major source of concern to exploitation of the

crop as bio-fuel feedstock.

Biology of stalk rot

Several genera and species of fungi are known to cause

stalk rot diseases in sorghum. Only one species of bacteria,

Erwina chrysanthemi pv. zeae (Sabet) Victoria, has been

reported to cause stalk rot in sorghum (Saxena et al. 1991).

It is a very aggressive pathogen that can cause complete

death of the plant within two to three days after the ap-

pearance of initial symptoms, a brownish lesion in the in-

fected area.

Most of the common stalk rotting fungal organisms

belong to the genera Macrophomina, Fusarium, Alternaria,

Colletotrichum, Nigrospora, Trichoderma, Periconia and

Cephalosporium (Reed et al. 1983; Frederiksen 1984;

Hassan et al. 1996). The mechanism by which the patho-

gens infect and establish in the host tissue may vary

between species. However, most of them survive unfavora-

ble condition as conidia or sclerotia in infected plant debris

at or near the soil surface. Infection usually starts in the

roots and gradually advances to aboveground stalks. Two or

more pathogen species usually attack the same plant for-

ming a disease complex making it difficult to distinguish

the primary invader (Reed et al. 1983; Trimboli and Bur-

gess 1983; Mughogho and Pande 1984). Reed et al. (1983)

identified more than eight Fusarium spp. from the same

infected sorghum plant tissue. Very few of these organisms,

on only a few genotypes, cause significant rot on sorghum

prior to flowering. One of these, Periconia circinata (M.

Mangin) Sacc. has been reported to cause early rot ("Milo

disease") on susceptible genotypes in the southwestern Uni-

ted States, but this disease has been overcome by incor-

porating a simply inherited gene that is now available in

most available elite germplasm (Dodd 1980). Colletotri-

chum graminicola has also been reported to cause early rot

in susceptible sorghum genotypes, although the species that

attacks sorghum has been renamed C. sublineolum (see

Anthracnose section). Some strains of F. moniliforme sensu

lato (also now separated into a number of species) cause

infection in certain hybrids as early as 30 days after plan-

ting (Khune et al. 1984). However, the majority of stalk

rotting organisms colonize the stalk and cause the disease

during the post-flowering period (Reed et al. 1983; Jardine

and Leslie 1992).

Despite the complexities caused by the number of

pathogens involved, stalk rots of sorghum are broadly cate-

gorized into two classes based on the major causal orga-

nisms responsible for the diseases. These are charcoal rot

caused by Macrophomina phaseolina (Tassi) Goid., and

Fusarium stalk rot, also called ‘soft rot’, caused by Fusa-

rium spp. Although several other pathogens are involved in

causing stalk rot disease these are reported as the most

widespread types in ecologically diverse areas in the tropics,

subtropics as well as temperate regions (Tarr 1962).

Charcoal rot, known to be the most widespread and

destructive stalk rot disease of sorghum (Mughogho and

Pande 1984) is caused by M. phaseolina. Charcoal rot is

also common in India, especially during the dry rabi season

(Khune et al. 1984; Seetharama et al. 1987), Australia

(Trimboli and Burgess 1982) and the United States, par-

ticularly in the southern states of Texas, Georgia and Ari-

zona (Edmunds 1964; Edmunds and Zummo 1975; Duncan

1983). M. phaseolina is a plurivorus pathogen attacking

over 75 different plant families and about 400 plant species

(Mughogho and Pande 1984). It is a root inhabiting fungus

with little or no saprophytic growth in either soil or host

cells of infected plants (Edmunds 1964). In the absence of

the host, the pathogen survives predominantly as small

black microsclerotia in debris from diseased plants or in soil

after decay of the plant material (Smith 1969). The process

and mechanisms by which M. phaseolina penetrates and

colonizes roots are not clearly known, but it is reported that

the growing hyphae can infect the roots only when the

plants are subjected to both moisture and temperature

stresses (Odvody and Dunkle 1979). Once the roots are in-

vaded, the pathogen quickly moves to above ground basal

stalk portions, attacking the lower internodes and eventually

resulting in poor grain filling or premature death of the

plant. The distinctive charcoal rot symptom can be revealed

by splitting the infected stalks. Doing so reveals charac-

teristic grey to black pigmentation of the entire infected

tissue, with bundles of sclerotia covering the invaded area.

Fusarium stalk rot develops under moisture stress after

flowering so is associated with a reduction in photo-

assimilate translocated to the roots an event either triggered

by stress induced metabolic changes in the plant or as a

result of competition for photo-assimilate by the developing

grain, or both (Dodd 1980). Though the pathogen is not

virulent enough to cause infection on young vigorous plants,

it is capable of spreading rapidly and can destroy the whole

sorghum fields in 2-3 days if the plants are subjected to

predisposing factors or when they are in senescence due to

maturation (Zummo 1980). Edmunds (1964) pointed out

that plants must be in early milk to dough stages of seed

development for the pathogen to attack.

Although Fusarium species are generally regarded

weak pathogens or secondary invaders (Clark and Miller

1980), they may become aggressive and spread quickly

when environmental conditions are favorable for disease

development. At least eight different Fusarium spp. have

been reported to cause stalk rot in sorghum (Leslie et al.

1990). Isolation and characterization of the pathogens iden-

tified F. moniliforme sensu lato as the most important causa-

tive agent of Fusarium stalk rot, while other species occur

at lower frequency (Reed et al. 1983). Several other virulent

strains previously recognized as different mating popula-

tions of F. moniliforme sensu lato (Gibberella fujikuroi spe-

cies complex) are recently recognized as separate species

including F. andiyazi Marasas, Rheeder, Lamprecht, Zeller

& Leslie, F. brevicatenulatum Nirenberg, O'Donnell, Kros-

chel & Andrianaivo, F. nygamai Klaasen & Nelson, F. pro-

liferatum (Matsushima) Nirenberg, F. pseudoanthophilum

Nirenberg, O'Donnell & Mubatanhema, F. thapsinum

Klittich, Leslie, Nelson & Marasas, and F. verticilloides

(Saccardo) Nirenberg, all of which are commonly found in

sorghum though some prefer other species as major host

(Leslie 1991).

Fusarium stalk rot also occurs in maize and millet. In

the United States, the disease is generally found in the same

area where charcoal rot occurs. It is particularly important

on the high plains from Texas to Kansas (Edmunds and

Zummo 1975). F. moniliforme sensu lato is the primary

pathogen associated with sorghum stalk rot in Kansas (Jar-

dine and Leslie 1992).

Similar to M. phaseolina, infection by Fusarium spp.

usually occurs when the host is weakened by predisposing

factors (Dodd 1980). The pathogens persist in soil, on crop

residue and on weed hosts as mycelium or conidiomata,

such as sporodochia. The infection starts on the cortical tis-

sues of the roots and eventually invades the vascular tissues

as the pathogen progresses towards the stalk (Zummo 1980).

The rate of invasion by Fusarium is less rapid compared to

charcoal rot (Zummo 1980). However, in certain susceptible

hybrids, Fusarium can spread very fast, beginning as early

as 30 days after planting (Khune et al. 1984). Unlike M.

phaseolina, Fusarium spp. do not produce microsclerotia.

But upon sudden occurrence of dry conditions, the fungus

appears to form a sclerotia-like structure for its survival

(Khune et al. 1984). Symptoms of the disease are prevalent

in tissues that are injured or damaged by insects. The in-

fected plant parts contain large areas of reddish pith and the

upper internodes have discolored vascular bundles. Pre-

mature plant death, poor grain development and crop lod-

ging are some of the characteristic symptoms of Fusarium

stalk rot (Fig. 6).

Disease symptoms and losses

Initial symptoms of M. phaseolina develop on roots

appearing as water-soaked lesions that turn brown or black

with age. The fungus continues to invade the host starting

Sorghum anthracnose, stalk rots and downy mildew. Tesso et al.

from the crown and causes watersoaking and discoloration

in the pith. Infected tissue eventually disintegrates leaving

only the vascular strands intact. Numerous, small, black

microsclerotia form on the vascular strands and are easily

visible when stalks are split open. The most characteristic

outward symptom of the disease is stalk lodging, which

often occurs in the driest portions of the field (Fig. 4). Other

yield reducing physiological events associated with stalk

rotting are poor grain filling and premature ripening. In

addition to lodging, bleaching of outer stalk tissue may be

evident (Jardine 2006).

One of the earliest signs of Fusarium root and stalk rot

diseases is the premature death of scattered individual

plants or adjacent plants in small patches within a given

field. Such plants often exhibit leaves that appear to be frost

damaged or suffering from desiccation stress. The most

characteristic symptom of Fusarium stalk rot, however, is

the shredding of the internal tissue of the lower internodes.

This shredded region may be pink or reddish-brown depen-

ding on the plant pigment type. As with charcoal rot, the

decay of interior stalk tissue results in stalk lodging (Jardine

2006).

Though little or no quantitative crop loss assessment

results have been reported in the recent past, the widely

recognized effect of the disease on standability and grain

weight indicate that yield losses as a result of the disease

may be economically significant. In mechanized agriculture,

yield loss due to stalk rot is directly proportional to percen-

tage lodging since lodged plants cannot be harvested (Mug-

hogho and Pande 1984). However, the magnitude of yield

loss may vary from region to region depending on the seve-

rity of the disease and the type of cultivars grown. In Kan-

sas, although the average yield loss is estimated at 4%, the

loss is believed to reach 50% in areas of high disease inci-

dence (Jardine and Leslie 1992).

Charcoal rot has been reported to cause significant yield

reduction in major sorghum growing areas of Africa (Geb-

rekidan and Kebede 1979; Frowed 1980; Hulluka and Esele

1992). Garud and Changule (1984) reported an average

yield loss of 48 to 67% due to charcoal rot infestation in

India. In India and Sudan, charcoal rot infection has been

reported to result in up to 100% lodging in susceptible

hybrids (Mughogho and Pande 1984). In India, yield losses

of 56 to 65% and 19 to 30% have been reported in fields

infected with M. phaseolina and F. moniliforme sensu lato,

respectively (Hundekar and Anahusor 1994).

Factors associated with stalk rot incidence in

grain sorghum

External abiotic factors and overall crop management are

recognized as elements linked with development and spread

of stalk rot diseases (Jordan et al. 1984; Flett 1996). Stalk

rotting pathogens are often considered weak parasites, since

they are capable of invading host and causing a disease only

when environmental conditions are favorable for disease

development. Conditions that are adverse to growth of

plants such as water deficit, high temperature, and un-

balanced mineral nutrition are reported to be the major pre-

disposing factors to stalk rot disease (Dodd 1980; Seetha-

rama et al. 1987). These stresses, especially when encoun-

tered during later stages of growth, create favorable con-

ditions for disease spread (Odvody and Dunkle 1979). In

the case of charcoal rot, moisture deficit accompanied by

hot, dry weather during the grain filling period has been the

major factor that aggravates the disease (Rao et al. 1980).

Water stress after flowering has been reported to lead to in-

creased charcoal rot incidence by up to 90% (Edmunds

1964). Odvody and Dunkle (1979) showed that M. phase-

olina only penetrated host cells after application of stress.

Seetharama et al. (1987) demonstrated the impact of water

stress on charcoal rot incidence using a line source sprinkler

irrigation technique that produced a gradient of water defi-

cit by decreasing the water supply with increasing distance

from the sprinkler line. These authors recorded high inci-

dence of charcoal rot and low grain yield from plants grown

farther from the water source (i.e., exposed to more severe

stress). Edmunds (1964) reported total death of sorghum

plants five days after inoculation at soil temperatures of

35°C and less than 25% available soil moisture.

Fusarium stalk rot, unlike charcoal rot, has been parti-

cularly important during wet conditions following hot, dry

weather (Zummo, 1980), but exposure to stress is essential

for the spread of Fusarium spp. Trimboli and Burgess

(1983) reproduced basal stalk rot in greenhouse studies by

growing sorghum plants on F. moniliforme sensu lato-infes-

ted soil at optimal soil moisture until flowering, then sub-

jected to a gradual development of severe moisture stress

between flowering and mid-dough stage, followed by

rewetting. Stalk rot did not develop in plants grown to

maturity at optimal soil moisture, although many of these

plants were infected by F. moniliforme sensu lato. Stalk rot

developed on the majority of stressed plants, including

those grown on soils initially non-infested but contaminated

by F. moniliforme sensu lato after planting.

Studies have also indicated that fertilizer application has

a direct relationship with charcoal rot and Fusarium stalk

rot incidence (Edmunds and Zummo 1975). Mote and Ram-

she (1980) have shown the impact of high levels of nitrogen

fertilizer on increasing charcoal rot incidence, regardless

host genotype differences. They observed an increase in dis-

ease incidence from 8 to 41% when the nitrogen level was

increased from 0 to 50 kg ha-1. Similarly, Patil et al. (1982)

observed a marked increase in charcoal rot infestation from

34% in unfertilized plots to 58% in plots that received 90 kg

ha-1 nitrogen. Avadehany and Mallanagouda (1979) found a

similar result in earlier work.

Certain plant biochemical compounds (sugars, phenols

and proteins) are associated with the incidence of stalk rot

in both sorghum and maize (Craig and Hooker 1961). Stalk

rot resistant maize and sorghum genotypes have been repor-

ted to contain higher total sugar and phenols in the inter-

nodes than the susceptible genotypes. This was at first

attributed to preferential use of these compounds by the

fungus. However, reports by Clark and Miller (1980) con-

firmed that stalk sugar content is positively correlated with

stalk rot resistance. An earlier study by Odvody and Dunkle

(1979) where male-fertile sorghum plants subjected to post-

flowering drought stress exhibited more rapid spread of M.

phaseolina than male-sterile plants agrees with this obser-

vation. Removal of the panicle from grain sorghum plants is

also reported to reduce the rate of senescence of stalk pith

tissue and the spread of C. graminicola (Frederiksen 1984).

Both male sterility and panicle removal reduce sink size and

consequently enable the plant to maintain higher sugar

content in the stalk. The increased incidence of stalk rot in

plants where leaves are removed (Rajewiski and Francis

1991) also indicates the possible role of stalk sugar on stalk

rot development. Hence, these findings generally support

the hypothesis that stalk rot is more severe on high yielding

cultivars that mobilize the reserve carbohydrates in the stem

for grain development than low yielding types that maintain

high stem carbohydrate at the expense of low grain yield

(Seetharama et al. 1991). Interest in the use of sorghum sto-

ver as a source of bioenergy led Funnell-Harris et al. (2010)

to test disease response to plants with reduced lignin as a

result of ‘brown midrib’ mutations bmr6 and bmr12. The re-

sults showed that the low-lignin plants differed in the array

of colonizing Fusarium spp. and that lesion size was sig-

nificantly reduced following toothpick inoculations com-

pared to the near-isogenic normal plants.

Green leaf area retention, also called “staygreen” is an-

other physiological character related to stalk rot resistance

(Duncan 1983). Described as a reduced progressive sense-

cence resulting in increased functional leaf area during

grain filling and an extension of photosynthetic capability

after grain maturity (Oosterum et al. 1996), staygreen helps

plants reduce the need for translocation of stored assimi-

lates and enables them to maintain high concentrations of

soluble sugar in the stem. Senescence is associated with the

loss of RNA, DNA, total nitrogen, chlorophyll, protein and

dry weight in plants (Potter 1971; Hoffman 1972). This

condition may dictate plant response to both abiotic and

biotic stresses and thus resistance to stalk rot diseases. Non-

senescent genotypes such as 'SC599' and 'B-35' have been

shown to express excellent levels of resistance to Fusarium,

Colletotrichum and Macrophomina spp. (Duncan 1983;

Tesso et al. 2005). The trait has also been reported as

having high and positive correlation with lodging resistance

in sorghum Woodfin et al. (1988).

Pathogen races

The two major stalk rotting organisms M. phaseolina and

Fusarium spp. and other causal agents are known to harbor

wide ranges of pathogenic races. While the recent re-clas-

sification of the F. monliforme species complex identified

several independent species that were originally recognized

as different isolates of the same species, similar effort in

Macrophomina did not lead to a conclusive re-grouping of

the pathogens. Although a wide variety subdivision with

markedly variable virulence is available among Macropho-

mina isolates, most isolates attack multiple hosts. RAPD

AFLP marker genotyping of different M. phaseolina iso-

lates have shown significant genetic variability (Rajkumar

and Kuruvinashetti 2007). Further, using AFLPs and ribo-

somal gene internal transcribed spacer (ITS) sequences,

Saleh et al. (2010) found that a group of 143 M. phaseolina

isolates from both natural and agroecosystems in Kansas

could be divided into four clusters based upon host origin,

where one cluster contained isolates from sorghum, which

were distinguishable from the clusters containing isolates

from maize and soybean, wild plants (including Ambrosia

sp., Asclepias sp., Cornus drummondii, Helianthus sp.,

Lespdeza capitata, Panicum virgatum, and Physalis sp.),

and goldenrod (Solidago canadensis L.). Similarly, charac-

terization of large number of isolates within Fusarium spe-

cies causing stalk rot diseases in both sorghum and maize

have shown marked variability (Leslie et al. 1996; Chulze

et al. 2000). It is possible that some of these variations may

represent inherent genetic differences in pathogenicity

among isolates.

Disease screening techniques

Quantitative description of diseases requires techniques for

accurate scoring of disease symptoms and methods for arti-

ficial inoculation. Over years, various techniques have been

developed and used to quantify stalk rot diseases in sorg-

hum. Direct measurement of pathogen effects such as per-

cent plants infected (disease incidence) and length of lesion

in the stalk and numbers of diseased (disease severity)

nodes have been used as disease scoring parameters (Desh-

mane et al. 1979; Bramel-Cox and Claflin 1989).

Earlier studies classified stalk rot disease reactions into

five categories based on the percentage of plants infected:

highly resistant (plants are completely free of disease),

resistant - less than 10% infection, moderately resistant - 10

to 30% infection, susceptible - 30 to 50% infection and

highly susceptible - greater than 50% infection (Deshmane

et al. 1979; Patil et al. 1980). Similarly, Bramel-Cox and

Claflin (1989) rated genotypic reaction based on stalk

disintegration using a numeric score of 1 to 6 with 1 being

resistant and 6 highly susceptible. Scores of 1 to 4 are based

on discoloration within the first internode; scores from 4 to

5 indicate discoloration of the first internode and a certain

proportion of the second internode. Scores of 5 to 6 depend

on the number of internodes greater than two discolored,

with a score of 6 being premature death of the main stalk.

One or a combination of these techniques may be applied

for scoring stalk rot disease under natural or artificial infec-

tion condition.

In addition to disease scoring, it is also important to

develop inoculation techniques that allow uniform and con-

sistent exposure of sorghum genotypes to pathogens in

germplasm evaluation. Since natural infections normally

involve more than one pathogen species and lack of uni-

formity in inoculum dose, it is difficult to rely on natural

infection to rate genotypic reactions. For this reason efforts

have been made to develop effective inoculation procedures

for germplasm screening. The techniques were primarily

applied for the most common pathogen species M. phaseo-

lina and Fusarium spp., but with slight modifications they

can be adopted for other pathogens as well.

One of the methods used for M. phaseolina is soil

inoculation. This method involves the use of dried micro-

sclerotia to inoculate sterilized soils. The procedure for

inoculum preparation and inoculation is described by Abawi

and Pastor-Corrales (1989). A similar method was used for

F. moniliforme sensu lato as well (Trimboli and Burgess

1983. Because of the amount of soil that can be sterilized,

this technique is limited to small-scale screening in green-

houses. The other widely practiced inoculation method is

the use of infected toothpicks. This method works well for

both Macrophomina and Fusarium spp. Sterilized tooth-

picks are incubated with pathogens in potato dextrose agar

until they are fully colonized by the growing mycelia. The

infected toothpicks are then inserted in the stalks of target

plants. The method is used for large-scale germplasm

screening in the field as well as for small-scale greenhouse

studies (Fig. 5). This method has been widely utilized for

screening genotypes for charcoal rot and Fusarium stalk rot

resistance both in maize and sorghum (Bramel-Cox and

Claflin 1989; Afolabi et al. 2008; Tesso and Ejeta 2011).

A more recent procedure is the liquid inoculation

method. This method is particularly effective for conidia-

producing Fusarium spp. Here pure cultures of the patho-

gen are incubated in potato dextrose broth and the conidia

are separated from the mycelia by straining the suspension

through cheesecloth. The inoculum concentration is deter-

mined by counting the number of conidia using a hemacyto-

meter and the inoculum concentration adjusted to the

required dose. The pathogens are then injected into the stalk

using a modified syringe and needle that is calibrated to

deliver a uniform inoculum suspension. Detailed procedure

for inoculum preparation and inoculation is described in

Tesso et al. (2009). This method has also been modified to

suit M. phaseolina. Though Macrophomina does not pro-

duce conidia, the mycelial mass can be broken into small

fragments that can be plated on nutrient agar to determine

colony forming unit numbers.

Each method has advantages and disadvantages. Soil

inoculation under controlled conditions seems appropriate

in that it mimics natural infection that occurs through the

root. However, soil inoculation does not allow large-scale

screening of germplasm due to the limits of greenhouse

space and the method cannot be applied in the field with

same level of environmental control. Toothpick inoculation

allows screening of relatively large number of genotypes

both in the field and greenhouse. But controlling inoculum

delivery for each plant is difficult resulting in variable

results. The liquid inoculation procedure seems to overcome

some of these difficulties in that fairly uniform amounts of

inoculums can be delivered to each plant.

Host resistance

Earlier studies have shown lodging resistance as closely

related to stalk characteristics in both grain sorghum and

corn (Craig and Hooker 1961; Thompson 1963; Esechie et

al. 1977; Dodd 1980). Some of these characters include

basal internodes and peduncles with larger diameters, shor-

ter peduncle length, shorter plant height, higher weight of 5

cm basal and peduncle stalk sections, and a thicker stalk

rind (Thompson 1963; Esechie et al. 1977). Sorghum plants

with these characteristics have been reported to have strong

stalks and are classified as resistant to lodging. Crushing

strength and pith density have also been found to signifi-

cantly (negatively) correlate with lodging resistance in corn

(Craig and Hooker 1961; Thompson 1963). Resistant sorg-

Sorghum anthracnose, stalk rots and downy mildew. Tesso et al.

hum types also appeared to be more perennial in habit and

thus were resistant to senescence, particularly in tall acces-

sions. Recent studies indicate that lodging in the semi dwarf

modern cultivars and hybrids is mainly associated with the

incidence of stalk rot diseases (Seetharama et al. 1987).

Genotypes susceptible to the disease, regardless of their

morphological attributes have shown to have higher degree

of lodging than those that are resistant to the disease. Most

genotypes that have been identified as resistant to stalk rot

including 'SC33', 'SC35', and 'SC599' have been reported to

be staygreen types (Bramel-Cox et al. 1988; Woodfin et al.

1988). As a result many researchers suggest that indirect

selection for staygreen characteristics can be used to iden-

tify genotypes with superior stalk rot resistance. Several

attempts have been made to determine the mode of in-

heritance of this character (Tuinstra et al. 1997; Tenkuano et

al. 1993; Walulu et al. 1994; Oosterom et al. 1996).

As for all other biotic and abiotic stresses, host plant

resistance is the most effective way for reducing losses

incurred by stalk rot diseases. However, a disease caused by

a complex of pathogens and confounded by environmental

stresses has complicated the development of resistant cul-

tivars. These complications and the lack of effective inocu-

lation procedures that mimic naturals condition have pre-

sented challenges to genotype screening under field con-

ditions. Nevertheless, progress has been made in identifying

genotypes resistant to the predominant causal agent of stalk

rot, M. phaseolina and some genotypes against Fusarium

species under common predisposing environmental factors

(Dodd 1980; Reed et al. 1983).

Using one or a combination of the screening tools des-

cribed above, several genotype-screening experiments have

been conducted. The results reveal the existence of genetic

variability for stalk rot resistance and the potential for deve-

loping resistant varieties or hybrids (Tesso et al. 2005).

Because of the strong relationship between drought stress

and onset of stalk rot infection, indirect selection for

drought tolerance has contributed to improvement of stalk

rot resistance. Most of the public releases that are tolerant to

post-flowering drought stress have been shown to contri-

bute resistance (or tolerance per se) to stalk rot diseases

caused both by Macrophomina and Fusarium spp.

Efforts to understand the mode of inheritance of resis-

tance traits have generated different results. Most of the stu-

dies indicate that stalk rot resistance is controlled by domi-

nant and additive gene action for both Fusarium stalk rot

and charcoal rot (Thakur et al. 1996). Bramel-Cox et al.

(1988) reported significant general combining ability

(GCA) effects for both M. phaseolina and F. moniliforme in

some selected lines. The works by Tesso et al. (2004, 2005)

also showed additive genes with dominant inheritance as

being more important in determining resistance to both

Fusarium and charcoal rot. Most resistance sources, inclu-

ding 'SC599', 'SC134', 'SC1039' and 'SC35', have signifi-

cant GCA for resistance under both greenhouse and field

conditions, while resistance in a few other sources such as

'SC564' appears to be recessive (Tesso et al. 2005). Most of

the sources resistant to Fusarium appear to have consistent

expression across pathogen species, but resistance to Fusa-

rium and Macrophomina seem to be under different genetic

control even though both diseases are aggravated by similar

predisposing conditions. Many of the known sources of

resistance to Fusarium are not resistant to charcoal rot. But

there are exceptions: some genotypes, such as 'SC599', ex-

press a high level of resistance to both pathogen groups, but

genes associated with resistance to the different pathogen

species may differ.

DNA-based diversity tags

Perhaps due to the difficulty associated with evaluating

large genotype cohorts, little has been done to characterize

and tag genomic regions associated with stalk rot resistance

in sorghum. As a result, few, if any marker tools are availa-

ble to use in breeding programs to improve this trait. The

only recent attempt is the one made by a group in India

where a set of F9 recombinant inbred lines initiated from a

cross between 'IS22380' (susceptible) and 'E-36-1' (resis-

tant) were used to map charcoal rot resistance QTLs (Reddy

et al. 2008). An attempt to genetically characterize potential

stalk rot resistance sources using unlinked SSR markers not

only revealed marked variation among the genotypes, but

also resulted in a fascinating genetic structure where the

different sources expressed distinct patterns of association.

Most of the resistance sources from the durra race of east

Africa were grouped together showing a clear geographic

pattern of variability (Tesso et al. 2005). This and the fact

that most of the post-flowering drought tolerant staygreen

sources are among the gene pool from the east African

region indicates that this area harbors germplasm sources

that may be of interest for stalk rot resistance and post-

flowering drought tolerance.

DOWNY MILDEW

Introduction

The disease sorghum downy mildew (SDM) is caused by

the oomycete Peronosclerospora sorghi. As for all host-

pathogen interactions, environment has an important role in

the occurrence and spread of disease. In the case of downy

mildews, dew formation is critical for the production of

sporulating lesions on susceptible host varieties (Fig. 7). It

is the resulting white fluffy appearance of the lesions on the

underside of leaves that accounts for the "downy mildew"

name. Other than for Australia (Ryley et al. 2002) where

measures have been taken to prevent the introduction of P.

sorghi, SDM occurs wherever sorghum is grown, but the

disease is especially damaging in more tropical climates. P.

sorghi can also cause SDM on maize, although oospores are

typically not produced on this alternate host. Similarly,

there are related Peronosclerospora species that can attack

sorghum. Examples include P. m a yd i s (Ryley et al. 2002), P.

sacchari (Bonde and Peterson 1983), P. heteropogoni (Nair

et al. 2004) and P. philippinensis (Bonde and Peterson

1983). Because the latter species is especially virulent on a

wide range of important host crops, including maize, it has

been subject to strict quarantine and was added to the U.S.

list of select agents as a potential bioterrorism threat.

Biology of Peronosclerospora sorghi

Because downy mildews have a growth pattern typical of

fungi, they have always been classified as fungal diseases.

Even now that molecular data show they are more closely

related to alga and have been assigned to a separate clas-

sification, for practical reasons, downy mildews are still

typically grouped with fungal diseases. P. sorghi produces

conidial spores asexually and produces oospores from the

union of oogonia ("female" gametangia) and antheridia

("male" gametangia) born on the same vegetative hyphae in

infected leaf tissues. The conidia, produced abundantly in

lesions on the underside of the leaf, are very fragile but

serve to spread the infection when environmental conditions

are appropriate. Oospores by contrast are relatively long-

lived and are able to overseason in leaf debris to initiate

infections through roots in the next season. Failure to find

different mating types while still finding both antheridia and

female structures led Pawar (1986) to conclude that P.

sorghi is homothallic, which is unusual among obligate bio-

trophs. The fact that P. sorghi can only be grown on living

susceptible hosts greatly increases the work required to

identify pathotypes that differ in the ability to cause disease

on host differentials.

Disease symptoms and losses

Plants infected through the roots via oospores develop a

systemic infection (Fig. 8). Infected plants that survive

through the seedling stage are stunted, chlorotic and typic-

ally fully or partially sterile (Fig. 9). As the plant matures,

lesions characterized by fluffy white mycelia and conidia

develop on the underside of leaves. By maturity, the leaves

of severely affected plants become shredded, releasing fresh

inoculum into the soil. Local lesion infections generally

arise as secondary infection from conidia, which are formed

only at night and require a wet leaf surface to infect. Cool

nights that lead to dew formation greatly increase the level

of infection. Symptoms in this case seldom turn systemic,

but are limited to the "downy mildew" seen at the site of

infection; they do not produce oospores or the associated

degree of leaf shredding (Frederiksen and Odvody 2000).

Although high planting rates can minimize yield loss

even when up to 30% of the plants are systemically infected,

losses to SDM can be severe. High losses are typically seen

when susceptible sorghum varieties are planted in oospore-

contaminated soils and under conditions favorable to dis-

ease development. The highest loss noted in the literature is

78% in cv. 'DMS 652' in India (Thakur and Mathur 2002).

Yield losses of up to 11.7% have been reported in Africa

(Bock et al. 1998). Within Brazil, SDM was initially restric-

ted to the Southern region but has now also spread to the

Southeast and Central-West regions, causing significant

losses specifically in areas of seed production (Barbosa et al.

2006). In the United States, epidemic outbreaks were first

seen in Kansas and Texas in the 1960s with subsequent

outbreaks of economic significance reported in Kansas

(1978-9) and Nebraska (1987) (Jensen et al. 1989). For the

most part, SDM has been controlled through the use of re-

sistant sorghum varieties and seed treatment with metalaxyl

(methyl N-(2,6-dimethylphenyl)-N-(methoxyacetyl)-DL-

alaninate). However in 2001 and 2002, the disease reap-

peared at high levels in sorghum fields in the upper gulf

coast of Texas that had been planted with treated seed. It

was subsequently determined that not only was a new strain

of metalaxyl-resistant P. sorghi present, but that a new

pathotype had arisen, likely as a result of growers who had

not used resistant varieties, which allowed rampant repro-

duction via the spread of conidia from local lesions (Isakeit

et al. 2003; Isakeit and Jaster 2005).

Pathogen races

The discovery of a new race, subsequently labeled "patho-

type 6" (P6), led to screening programs to identify new

resistance sources. Prom et al. (2010) found that 6 of 20

accessions in the U.S. sorghum germplasm that originated

from Chad were resistant when grown in the area where the

new race is prevalent and that 6 of 40 accessions originating

from China were also resistant to P6 (Prom et al. 2007). A

larger greenhouse screening was undertaken with a set of

245 accessions assembled from the nearly 40,000 lines in

the ICRISAT germplasm collection to provide a ‘minicore’

collection that includes much of the genetic diversity of

sorghum (Upadhyaya et al. 2009). The tests also included

breeding lines from the Kansas State and Texas A&M

sorghum breeding programs as well as commercial hybrids.

Overall, only 52 of the minicore accessions were resistant

to the new pathotype, but nearly half of the elite breeding

lines and commercial hybrids from both states showed re-

sistance (Radwan et al. 2011).

Disease screening techniques

Several methods have been developed to screen for resis-

tance, including inoculation of soil with oospore-laden leaf

fragments and the use of ‘downy mildew nurseries’ (Thakur

et al. 2007a). However, due to the exacting conditions

under which sporulation and infection will occur, green-

house based tests allow for the most controlled experiments

and the lowest frequency of escapes. Even if a dew chamber

is not available, relatively simple adaptations can be made

to allow inoculation and screening of flats of germinated

seedlings. The "sandwich procedure" (Thakur et al. 2007a)

as illustrated in Radwan et al. (2011) proved to be very

effective in the screening tests for P6 described previously.

Disease control and host resistance

Sorghum downy mildew can be effectively controlled by a

combination of methods, which include cultural, chemical,

and host resistance. Cultural practices such as crop rotation,

deep plowing, early planting, and the use of trap crops have

been shown to significantly reduce downy mildew inci-

dence (Frederiksen 1980; Schuh et al. 1987; Pande et al.

1997). Other than in areas where the previously mentioned

fungicide-resistant strain has developed, metalaxyl or

fosetyl-Al applied as a seed treatment can provide effective

chemical control of SDM (Odvody and Frederiksen 1984).

The recent severe epiphytotic of SDM observed in Texas

and the emergence of a new race that is pathogenic to pre-

viously resistant cultivars has increased efforts to identify

alternate sources of host plant resistance for managing the

disease.

Navi et al. (2003) evaluated 1671 S. bicolor accessions

(945 elite landraces and 726 breeding lines) for multiple

disease reaction under field conditions and natural inocula-

tion during the 2001 rainy season at ICRISAT. Of the 945

landraces from 46 countries, 82 accessions from India,

Lesotho, Botswana, Zimbabwe, Russia, Swaziland, Came-

roon, Sudan, Ethiopia, Malawi and Nigeria were free from

all diseases including downy mildew, ergot, head smut and

anthracnose.

In Brazil, sources of resistance for downy mildew

among 42 sorghum genotypes were identified through field

evaluation under natural infection. Cultivar 'SC283' served

as a susceptible check. The tests were conducted in two

separate nurseries in Sete Lagoas and Minas Gerais expec-

ted to have different pathtypes of P. sorghi. While 15 cul-

tivars were classified as highly resistant in both nurseries,

only 'SC170-6-17' and '9910296' showed 0% systemic in-

fection (Barbosa et al. 2005).

Forty Chinese sorghum accessions maintained by the

USDA-ARS, Plant Genetic Resources Conservation Unit,

Griffin, Georgia were evaluated by Prom et al. (2007) for

multiple disease resistance. The level of sorghum downy

mildew (SDM) infection with systemic infection and local

lesion development for infected plants was low to very high.

Accessions 'PI511832', 'PI563519', 'PI563521', 'PI563850',

'PI610677' and 'P610724' were the most resistant to sorg-

hum downy mildew, whereas 'PI610692' and 'PI610720'

were the most susceptible to SDM.

In another study by Prom et al. (2010), 78 accessions

from Chad, Central Africa and 20 photoperiod insensitive

accessions from Uganda were evaluated for downy mildew

resistance in Ocotlan, Mexico in 2004 and 2005. Several

sources of downy mildew resistance were identified. Three

accessions 'PI282860', 'PI282864', and 'PI563505' from

Chad were shown to possess high levels of downy mildew

resistance in Mexico and Texas, whereas 'PI282843',

'PI282877', 'PI549196', and 'PI563438' also from Chad

exhibited high levels of resistance to the disease in Texas.

Accessions 'PI297210', 'PI576386' and 'PI576395' from

Uganda also showed downy mildew resistance in Mexico

and Texas. These sorghum accessions from Chad and

Uganda can be utilized in breeding for downy mildew resis-

tance in Mexico and Texas.

Sharma et al. (2010) in India and Radwan et al. (2011)

in the United States screened the sorghum "minicore" germ-

plasm collection, a subset of the sorghum germplasm col-

lection maintained at ICRISAT. The minicore collection

includes 245 representatives assembled to include much of

the diversity of the overall collection of over 37,000 acces-

sions (Upadhyaya et al. 2009). In these tests, the accessions

were screened in the greenhouse following a sandwich

inoculation technique, with '296B' and 'QL3' respectively,

serving as susceptible and resistant checks. Downy mildew

resistance (mean incidence  10%) was observed only in six

accessions in India ('IS 28747', 'IS 31714', 'IS 23992', 'IS

27697', 'IS 28449', and 'IS 30400'), whereas 52 resistant

Sorghum anthracnose, stalk rots and downy mildew. Tesso et al.

accessions (28 were photoinsensitive) were identified by

Radwan et al. (2011) in the United States against the P6

pathotype.

DNA-based diversity and molecular tags

DNA based analysis techniques have been applied both to

attempt to tag genes for resistance in the host and to identify

species and races of the downy mildews that can attack

sorghum. Yao et al. (1991b) were able to use DNA extrac-

ted from P. sorghi spores to show that the pathogen could

be seedborne in maize, but this is not considered a problem

if the seed is dried below 20% moisture. They subsequently

showed that repeated sequence clones readily detect RFLP

fingerprint differences among P. sorghi, P. m a y d i s and P.

sacchari, which were not differentiated from P. philippinen-

sis. A Thailand isolate previously presumed to be a deri-

vative of P. sorghi was different and thus was renamed P.

zeae (Yao 1991). He also found differences in the base

sequences of the internal transcribed spacer (ITS2) of the

ribosomal DNA of the different species (Yao et al. 1992)

and developed PCR primers that specifically amplify DNA

from all the Peronosclerospora spp. and from P. sorghi only

(Yao et al. 1991a).

In a study by Mathiyazhagan et al. (2008), DNA was

extracted from P. sorghi isolates from sorghum and maize

collected from different locations of Tamil Nadu, India, and

their genetic variability was investigated using restriction

fragment length polymorphism (RFLP) analysis of the

PCR-amplified internal transcribed spacer (ITS) region of

ribosomal DNA. PCR amplification of the ITS regions of

the P. sorghi isolates from sorghum and maize gave prod-

ucts of approximately 550 bp in length with slight varia-

tions among isolates. RFLP analysis of the ITS regions of

nuclear rDNA of P. sorghi with HhaI, EcoRI and MspI

revealed differences among P. sorghi isolates sampled from

sorghum and those from maize. Subsequently, Ladhalak-

shmi et al. (2009) developed SCAR primers based on a

sequence identified via PCR amplification with a RAPD

primer that amplified an 800 bp fragment only from isolates

from maize. Because inoculum from one host can generally

be used to infect the other, such host specific unique se-

quences suggest that P. m a y d i s or P. z e a e may have been

present, although the possibility exists that host-specific

formae speciales have evolved within P. sorghi. In an

attempt to discover the origin of pathotype 6 and to develop

DNA-based markers to distinguish among pathotypes, Peru-

mal et al. (2008) used AFLP and SSR to characterize a

number of isolates of pathotypes 1, 3 and 6. Although the

results strongly suggest that pathoype 6 arose from patho-

type 3, unfortunately none of the individual markers showed

total concordance with pathotype (Perumal et al. 2006,

2008).

Kamala et al. (2006) used 10 simple sequence repeat

(SSR) marker loci and 20 phenotypic traits to assess diver-

sity among 36 downy-mildew-resistant sorghum accessions.

The lines were chosen at random from 130 (out of ~16,000

screened) that showed a high level of resistance (0 to 5%

mean disease incidence of the total plants systemically in-

fected) to the ICRISAT isolate of P. sorghi. High gene

diversity and allelic richness were observed in sorghum

races durra caudatum and guinea caudatum and in acces-

sions from east Africa. The pattern of SSR-based clustering

of accessions was more in accordance with their geographic

proximity than with race designations based on phenotypic

traits. Eleven pairs of resistant accessions had a SSR gene-

tic distance of more than 0.70 suggesting they will likely

provide different sources of resistance and be useful as

parents in sorghum downy mildew resistance-breeding

programs.

With respect to tagging host genes for resistance, the

first success came from the use of RFLPs and RAPD pri-

mers. Because the two parents used in creating an RFLP

map for sorghum differed in susceptibility to SDM

('BTx623' is resistant to P1 while 'IS3620C' is susceptible),

Gowda et al. (1995) were able to show that clone txs552

revealed an RFLP linked to resistance. Also using the same

clones developed to create an RFLP map for sorghum, Oh

et al. (1996) identified two RFLPs that segregated with a

resistance gene in F2 progeny of a cross between 'SC325'

(resistant to P1, P2 and P3) in a cross to 'RTx7078' (suscep-

tible to all pathotypes). Although the markers were 5 to 7

map units from the gene for resistance they were useful in

determining that the two sources of resistance were not the

same gene. In a third cross segregating for resistance

('SC414', resistant and 'RTx7078', susceptible) a screen of

674 primers revealed two that amplified bands that seg-

regated relatively closely with resistance. The same dif-

ferences did not show up in amplification products of the

mapping parents, suggesting that this represents a third

independent source of SDM resistance, at least to P1. In

each of these cases resistance has been associated with a

single gene. However, there are also cases in maize where

SDM resistance has been shown to segregate as a quanti-

tative (polygenic) trait (Agrama et al. 1999).

Future directions

Further research in the area of race determination in P.

sorghi perhaps taking advantage of the advances in low-cost

DNA sequencing should help to identify differences

between races that could be used to greatly simplify patho-

type identification and resolve the question of the origins of

new pathotypes. In addition, DNA markers should help to

identify those sources of resistance that differ among sorg-

hum cultivars, and enable subsequent gene stacking that is

predicted to enhance the durability of resistance.

ACKNOWLEDGEMENTS

This publication is Contribution No. 12-052-J from the Kansas

Agricultural Experiment Station, Manhattan. Support from the

Global Crop Diversity Trust, the United Sorghum Checkoff Prog-

ram, the Center for Sorghum Improvement (KSU), and Texas

Agrilife Research is gratefully acknowledged.

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Full-text available

Oct 2023

Maize belongs to the family Poaceae, a staple crop grown worldwide (Naz et al., 2019), which originated from Central America and Mexico, also known as Indian corn or corn (Ranum et al., 2014). It is an annual crop with 2–3 m in height; the stem consists of nodes and internodes, with 8–21 leaves per plant extending from each internode. The leaves may be lance-like (lanceolate) or linear, having visible primary veins (midrib), and their length ranges from 30 to 100 cm (Gunjan et al., 2021). The female and male inflorescence appear singly on the plant; the female inflorescence is known as the ‘ear,’ while the male inflorescence is ‘tassel’ (Bortiri and Hake, 2007). There may be 1–3 spikes (modified ear) per plant. The kernels (maize grains) are 30–1000 per spike and are enclosed in husks and may be yellow, black, white, purple, or red (Gunjan et al., 2021). Maize and cornmeal are staples in many countries worldwide (Gwirtz and Garcia‐Casal, 2014). Its ears may be cooked and consumed as a vegetable, or various products like flour and cereals prepared from the kernels. It is also a chief source of starch, which is further processed into high fructose corn syrup and oils. It is also used as livestock feed (Orhun, 2013).

Genetic response of hybrid maize line to downy mildew caused by Peronosclerospora spp. infection

Article

Full-text available

Sep 2023

Maize is the second staple food commodity in Indonesia. Apart from being the main source of carbohydrates and protein, the production of maize continues to increase along with the escalation of population growth and animal feed requirements in the last few years. The potential to increase the national production of maize is still feasible because of the yield gap between the potential yields of new superior varieties and the level of yields obtained by farmers. The yield gap caused by biotic stress in maize is mainly caused by pathogens such as downy mildew due to Peronosclerospora spp. Downy mildew distribution is sporadic that can infect a wide area. In Indonesia, it spreads widely and significantly reduces yields in the areas of maize production centres in East Java, Central Java, South Sulawesi, North Sulawesi, Gorontalo, Lampung, and Sumatera. These obstacles can be overcome by integrated pest and disease control technology. One strategy is to discover downy mildew-resistant varieties that can be combined with other control treatments. The phenomenon of resistance to downy mildew infection of several hybrid maize strains began to be detected in the vegetative growth phase, with symptoms beginning at 14 days after planting (DAP), increasing with plant age, and reaching its peak after 28-35 DAP and then the symptoms will gradually disappear until no infection after 42 HST. This study analyses the resistant maize varieties from 2020 to 2022. The data showed the susceptible comparison variety (Anoman) was infected with 88.94% to 100%. In 2020, BMD73 showed a resistant reaction to Philippinensis species, other strains were classified as susceptible in P. maydis except BMD 76. All strains showed highly susceptible and susceptible reactions. In 2021, in P. philippinensis all strains FCP10-FCP16 showed a resistant reaction, but line FCP10-FCP16 had a resistance response that was classified as moderately resistant to P. maydis . The incidence of genetic response in 2022, except BMD86, strains BMD81-BMD85 were classified as resistant, as well as in the P. maydis endemic area all strains showed the same reaction.

Screening of M5 Generation Mutant Lines for Charcoal Rot Resistance in Sorghum [Sorghum bicolor (L.) Moench]

Article

Apr 2023

This study was carried out in augmented design during rabi 2021 (November‒May) at Agriculture research station, Hagari, Karnataka, India to identify the charcoal rot resistant mutant line. Total 200 mutants and 7 checks were used to study the charcoal rot resistance in the present experiment. Charcoal rot is a major disease in the dry sorghum-growing regions of Asia, Africa, Americas and Australia. Charcoal rot disease is caused by Macrophomina phaseolina (Tassi) Goid. It appears in severe form on the improved varieties in hot dry weather with soil moisture stress. The process of mutation is recognized as one of the driving forces of evolution. Induced mutation breeding is a relatively quick method of creating variability in quantitatively inherited traits between plants. The parameters used in charcoal rot studies were lodging per cent, mean number of nodes crossed, mean length of spread and Charcoal rot index (CRI). The screening results revealed that 66 mutant lines shown moderate resistant reaction compared to the resistant check DSV-4 (0.5) and E-36-1 (0.27), among them eight mutants had exact only one node crossed by the pathogen. These mutant lines exhibited comparatively lowest number of mean nodes crossed. 84 mutant lines shown moderate resistant response to charcoal rot index trait. These resistant lines can be used for further confirmation and also for future resistant breeding programme.

Identification of disease resistant bmr sorghum recombinant inbred lines derived from diverse donor and recurrent parents

Article

May 2024

Brown midrib mutants discovered in maize, sorghum and pearl millet through mutagenesis constitute an important genetic resource. Studies indicated that the discovered bmr mutants not only have lower lignin content than their wild-type counterparts, but they are also associated with an improved fodder digestibility and improved process yield in biofuel production. There is need to combine this novel trait with disease resistance to enhance the commercial utilisation of sorghum cultivars by end users. We have assessed reaction of a random set of F2:4 generations that represented progenies from 46 unique cross combinations involving 12 recurrent parents and 5 donor parents against anthracnose, leaf blight, and charcoal rot diseases. The genotypes were artificially inoculated with the pure cultures of the disease-causing organisms in separate experiments. There were signif icant genotypic differences for the disease severity and area under disease progress curve (AUDPC) in leaf blight and anthracnose, and number of nodes crossed and lesion length in charcoal rot disease. Progeny expressing bmr12 and bmr6bmr12 were generally superior in resistance to foliar diseases and charcoal rot than bmr6. The reaction of most of the bmr lines against the diseases was similar to their corresponding wild-type parents. Principle component and hierarchical cluster analysis identified sets of genotypes with bmr6, bmr12, and bmr6bmr12 loci that combine desirable levels of resistance to two or all the three diseases. These sets of bmr derived lines could be the potential sources to develop disease resistant bmr hybrids and pure line varieties, and improved breeding populations in sorghum.

Genome wide association studies for red leaf spot (Colletotrichum sublineolum) in sorghum

Preprint

Full-text available

Aug 2023

Evaluation of red leaf spot disease ( Colletotrichum sublineolum) resistance in Sorghum bicolor (L.) Monech crop is recognized as a necessary objective towards anthracnose resistance in warm and humid climates. A fixed diversity set of 121 genotypes were assessed for red leaf spot disease response for two years during 2020-21(E1) and 2021-22 (E2). On the basis of differential response to red leaf spot, a panel of 86 genotypes were selected for genotyping by sequencing (GBS). The best linear unbiased predictors (BLUPs) of disease score and area under disease progress curve (AUDPC) along with identified SNPs in GBS were used for genome wide association studies (GWAS). A large number of significant marker-trait associations were identified for test traits. A total of 30 SNPs were identified for final disease score of plant at harvest and 38 SNPs were identified for AUDPC during E1 and E2. Chromosome S02 (six) and S06 (six) harboured maximum number of SNPs followed by S04 (five) for disease score. Linkage disequilibrium decay (LD) in population was estimated to be ~ 50kb. Annotations helped to predict candidate genes in the genomic regions associated SNPs for test traits. Important candidate genes involved were RLP, WRKY, F-box protein, CDP, zinc finger and putative disease resistance protein. Four lines IS 7017, IS 8015, IS 8256 and SCL 4 were free from infection, indicating resistance to red leaf spot and can also serve as donors in cultivar improvement breeding for red leaf spot.

Charcoal rot: a major root and stalk disease of sorghum

Chapter

Jan 2023

Recent advancements in the breeding of sorghum crop: current status and future strategies for marker-assisted breeding

Article

Full-text available

May 2023

Sorghum is emerging as a model crop for functional genetics and genomics of tropical grasses with abundant uses, including food, feed, and fuel, among others. It is currently the fifth most significant primary cereal crop. Crops are subjected to various biotic and abiotic stresses, which negatively impact on agricultural production. Developing high-yielding, disease-resistant, and climate-resilient cultivars can be achieved through marker-assisted breeding. Such selection has considerably reduced the time to market new crop varieties adapted to challenging conditions. In the recent years, extensive knowledge was gained about genetic markers. We are providing an overview of current advances in sorghum breeding initiatives, with a special focus on early breeders who may not be familiar with DNA markers. Advancements in molecular plant breeding, genetics, genomics selection, and genome editing have contributed to a thorough understanding of DNA markers, provided various proofs of the genetic variety accessible in crop plants, and have substantially enhanced plant breeding technologies. Marker-assisted selection has accelerated and precised the plant breeding process, empowering plant breeders all around the world.

Accumulation of sorghum phytoalexins induced by Colletotrichum graminicola at the infection site

Article

Full-text available

Dec 1991

Microspectrophotometry was performed on intact, pigmented vesicle-like inclusions within living sorghum cells that were accumulating phytoalexins as a response to attempted fungal infection. The results indicate that the deoxyanthocyanidin phytoalexins are present in inclusions. Moreover, the phytoalexin concentration within a single inclusion, based on luteolinidin, was calculated to be 0·15 m. The amounts of luteolinidin and apigeninidin in cells involved in the phytoalexin response at individual infection sites were also determined. The data showed that luteolinidin accumulated to levels of 0·48–1·20 ng/cell whereas apigeninidin accumulated to levels of 0·24–0·91 ng/cell. The results of both analyses confirmed that at the infection site the deoxyanthocyanidins accumulate to levels in substantial excess of those required for inhibition of the fungus Colletotrichum graminicola.

Sorghum Anthracnose and Sustainable Management Strategies in West and Central Africa

Article

Full-text available

Jan 2004

Anthracnose caused by Colletotrichum graminicola remains the most important foliar disease of sorghum in West and Central Africa. This paper describes the advances made in sorghum anthracnose research that has led to a better understanding of pathogenic and genotypic diversity, epidemiology and important disease management strategies. We further highlight how understanding pathogen diversity interplays with the major sustainable anthracnose management strategies such as the use of host plant resistance and crop residue management within the region.

Incidence and Severity of Sorghum Anthracnose in Ethiopia

Article

Full-text available

Dec 2010

A two year survey was conducted to determine incidence and severity of sorghum anthracnose in different sorghum growing regions in Ethiopia. A total of 487 fields in 49 districts were surveyed in each of the 2005 and 2007 production season. Incidence of sorghum anthracnose was assessed as the percentage of plants with visible symptoms in a field and anthracnose severity was evaluated as the percentage of leaf area with symptoms. Also, the relationship of the incidence and severity of the disease to the altitude of the fields and weather conditions were determined. Results from the 2 years survey revealed that sorghum anthracnose is present in most (84%) of the survey districts. However, both incidence and severity of the disease varied significantly (p<0.0001) among the survey areas. Anthracnose incidence ranged from 0 to 77% and severity of the disease varied between 0 and 59% on average for the two years. The two year average anthracnose severity classes ranged from trace (<5%) to severe (up to 59%) and the disease was generally more severe in the Southwest and South regions. However, some districts in the East and North Ethiopia also had fields with severe anthracnose infection. It was also found out that the prevailing weather conditions especially rainfall has a significant impact on both anthracnose incidence and severity.

Genetic Structure and Temporal Dynamics of a Colletotrichum graminicola Population in a Sorghum Disease Nursery

Article

Full-text available

Nov 1998
PHYTOPATHOLOGY

ABSTRACT Restriction fragment length polymorphisms (RFLPs) were used to study the population genetics of Colletotrichum graminicola (= C. sublineolum), the causal agent of sorghum anthracnose. Screening of 80 anonymous probes from a genomic library detected polymorphisms in 81% of 299 probe-enzyme combinations among nine international isolates. Seven single- or low-copy probes were used to study a collection of 411 isolates sampled during 1991 to 1993 from a sorghum disease nursery in Georgia. Nei's gene diversity was moderately high, with = 0.215 on average, while genotypic diversity was extremely low with an average genotypic diversity value of G = 1.513. Only nine multilocus haplotypes were identified, with one haplotype being present at a frequency of approximately 80% each year. Two other haplotypes were found at significant frequencies (4 to 10%). Allele and haplotype frequencies did not differ over the 3 years, indicating that this population was stable. Our findings suggest that genetic drift and gene flow were not major contributors to genetic structure, while asexual reproduction had a significant effect.

Effect of pattern and severity of moisture deficit stress on stalk rot incidence in Sorghum I. Use of line source irrigation technique, and the effect of time of inoculation

Article

Feb 1987
FIELD CROP RES

Water deficit occurring after flowering in sorghum [Sorghum bicolor (L.) Moench] predisposes the crop to stalk rot, mainly due to infection by the charcoal rot causal fungus Macrophomina phaseolina (Tassi) Goid., resulting in lodging. The line source (LS) sprinkler irrigation technique which produces a gradient of water deficit stress due to decreasing amount of water supply with increasing distance from the sprinkler line was used to study the relationships between applied water levels, time of inoculation and the degree of charcoal rot incidence.Grain yields were reduced in proportion to the deficit in water supply along the gradient, whereas the incidence of charcoal rot increased. Inoculation with the fungus increased the incidence of disease when natural incidence was low. Sorghum was more susceptible to disease during the later stages of grain filling than during the period immediately after flowering. The rows of plants farthest from the LS, which received the least irrigation, showed disease incidence earlier than those nearest, which were better watered. This was apparent for each of the three different, but highly correlated, parameters of disease spread: percentage of soft stalks; number of nodes crossed; and the length of fungal spread (cm). It is concluded that the LS is an effective method to study the quantitative relationship between severity of water stress and the degree of charcoal rot incidence during rainfree dry seasons.

Effect of pattern and severity of moisture-deficit stress on stalk-rot incidence in sorghum. II. Effect of source/sink relationships

Article

Jun 1991
FIELD CROP RES

Stalk-rot of grain sorghum [Sorghum bicolor (L.) Moench] is most commonly associated with weakly parasitic root and stalk-rot fungi when the host plants are subjected to environmental stresses. The incidence of rots is generally more in high-grain-yield environments associated with high plant density or fertilizer application. The effects of time of occurrence and degree of moisture deficit stress (moisture stress) on grain and biomass yields, and the natural incidence of stalk-rots were studied.Stress during grain-filling had a greater effect on incidence of rots than stress at earlier stages. Grain-yield and disease incidence were differently affected by timing and severity of moisture stress. Stalk-rot incidence was most commonly and strongly associated with moisture stress at the terminal stage of growth. However, stress during panicle development induced changes in the sink size (grain number) or root-growth pattern, which in turn influenced both the timing and extent of stalk-rot incidence. The amount of biomass produced during the later part of the grain-filling period was positively correlated with lower disease susceptibility. The distribution index (i.e., the ratio of grain-yield to biomass produced after flowering) could be generally used to predict disease susceptibility. There were no simple correlations between biomass, grain-yield or yield components, and stalk-rot.Implications of these findings for sorghum production and for stalk-rot resistance screening are discussed.

Genetic differentiation and gene flow in Colletotrichum sublineolum in Ethiopia, the centre of origin and diversity of sorghum, as revealed by AFLP analysis

Article

Nov 2010
PLANT PATHOL

Isolates of Colletotrichum sublineolum were collected from different sorghum-producing regions of Ethiopia and divided into five groups based on their geographic origin. The growth rate of 50 isolates showed considerable variation: 1·7–5·8 mm day−1, mean 3·3 mm day−1. However, the isolates displayed little variation in colony colour and colony margin, except for isolates from the north, which were different from the others. Amplified fragment length polymorphism analysis of 102 isolates revealed much greater variations among the different groups. Dice similarity coefficients ranged from 0·32 to 0·96 (mean 0·78). Cluster analysis and principal coordinate analysis revealed a differentiation of the isolates according to their geographic origin, and both methods clearly indicated a genetic separation between the southern, the eastern and the other isolates. Analysis of molecular variance (amova) indicated a high level of genetic variation both among (42%) and within (58%) the C. sublineolum sampling sites in Ethiopia. The amova also indicated a high level of genetic differentiation (FST = 0·42) and limited gene flow (Nm = 0·343). The results of this study confirmed the presence of a highly diverse pathogen, which is in agreement with the existence of diverse host genotypes and widely ranging environmental conditions in sorghum-producing regions of the country. Such diversity should be taken into account in future breeding programmes to achieve an effective and sustainable disease management strategy.

Assessment of anthracnose (Colletotrichum graminicola) resistance in sorghum (Sorghum bicolor) germplasm under field conditions in Nigeria.

Article

Mar 2002
J AGR SCI

Twenty-one elite sorghum lines were assessed for resistance to foliar, peduncle, rachis, grain and overall panicle anthracnose under natural infection at Samaru and Bagauda in 1996 and 1997. Three genotypes (R 6078, IS 14384 and CCGM 1/19-1-1) were completely resistant to all forms of the disease while Nagawhite was completely resistant to foliar, peduncle and rachis but moderately resistant to panicle anthracnose. The data indicate vertical disease progression from leaves onto peduncle and the rachis. More genotypes were susceptible to grain than rachis anthracnose thus indicating that grain infection could be direct or through vertical progression and direct pathways.

Variation among foliar isolates of Colletotrichum sublineolum of sorghum in Nigeria

Article

Feb 2001
FIELD CROP RES

Foliar anthracnose, caused by Colletotrichum sublineolum, is a major disease of sorghum in Nigeria. Pathogenic diversity was studied among foliar isolates of C, sublineolum collected from sorghum cultivars in farmers fields in the Sahel, Sudan, northern Guinea and southern Guinea zones of Nigeria during the 1997 crop season. For the study a total of 50 isolates were identified based on typical symptom types and cultural characteristics. On the basis of growth in culture and morphological characteristics, the isolates were classified into nine morphological groups (MGs). Pathogenicity tests were done on a set of known sorghum differential lines by inoculating pot-grown seedlings in a greenhouse. On the basis of disease reaction and disease severity scores on the differential lines, the 50 isolates were classified into seven pathogenic groups (PGs). Sixteen representative isolates of the MGs and the PGs were further evaluated for virulence and aggressiveness on the differential lines and were classified into five distinct groups using the centroid method of cluster analysis. The existence of five races of C. sublineolum in major sorghum growing zones of Nigeria is suggested.

Article

Nov 2009
MOL ECOL

Agricultural and wild ecosystems may interact through shared pathogens such as Macrophomina phaseolina, a generalist clonal fungus with more than 284 plant hosts that is likely to become more important under climate change scenarios of increased heat and drought stress. To evaluate the degree of subdivision in populations of M. phaseolina in Kansas agriculture and wildlands, we compared 143 isolates from maize fields adjacent to tallgrass prairie, nearby sorghum fields, widely dispersed soybean fields and isolates from eight plant species in tallgrass prairie. Isolate growth phenotypes were evaluated on a medium containing chlorate. Genetic characteristics were analysed based on amplified fragment length polymorphisms and the sequence of the rDNA-internal transcribed spacer (ITS) region. The average genetic similarity was 58% among isolates in the tallgrass prairie, 71% in the maize fields, 75% in the sorghum fields and 80% in the dispersed soybean fields. The isolates were divided into four clusters: one containing most of the isolates from maize and soybean, two others containing isolates from wild plants and sorghum, and a fourth containing a single isolate recovered from Solidago canadensis in the tallgrass prairie. Most of the sorghum isolates had the dense phenotype on media containing chlorate, while those from other hosts had either feathery or restricted phenotypes. These results suggest that the tallgrass prairie supports a more diverse population of M. phaseolina per area than do any of the crop species. Subpopulations show incomplete specialization by host. These results also suggest that inoculum produced in agriculture may influence tallgrass prairie communities, and conversely that different pathogen subpopulations in tallgrass prairie can interact there to generate 'hybrids' with novel genetic profiles and pathogenic capabilities.

Sorghum Pathology and Biotechnology -A Fungal Disease Perspective: Part II. Anthracnose, Stalk Rot, and Downy Mildew

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