Susceptibility of Forty-six Lisianthus Cultivars to Fusarium Crown and Stem Rot

Abstract

Fusarium crown and stem rot, caused by Fusarium avenaceum (Fr.:Fr.) Sacc., is a serious disease of lisianthus (Eustoma grandiflorum Raf. (Shinn)). While more than 80 new cultivars of lisianthus have been released for sale in the United States in the last decade, there is a lack of information on their susceptibility to this pathogen. Forty-six cultivars of lisianthus were evaluated for their response to infection by F. avenaceum. Cultivars were grouped according to blue/purple, pink, or white flower colors and evaluated within their color class. Although some plants of all cultivars were susceptible to F. avenaceum, partial resistance was observed as indicated by differences in the length of time to symptom express and in the frequency of diseased plants within each color group. In 21 of the 46 cultivars, 80 to 100% of the plants expressed symptoms within 55 days after inculcation. The lowest frequencies of diseased plants 55 days after inoculation were found in 'Ventura Deep Blue' and 'Hallelujah Purple' (25%), 'Bridal Pink' (23%), and 'Heidi Pure White' (53%) for the blue/purple, pink, and white flower color groups, respectively. Screening cultivars for resistance to F. avenaceum is the first step in breeding resistant cultivars. The methods we developed for these studies should be useful in screening for resistance. These results also ay help growers select cultivars that are less susceptible to F. avenaceum, which should aid in the management of this disease. Fusarium crown and stem rot is one of the most damaging diseases of lisianthus. It has been reported as a disease of lisianthus produced as potted plants and cut flowers (Koike et al., 1996; Ozaki, 1992), but has been especially devastating in cut flower production.

Full text

● October–December 2000 10(4)816
VARIETY TRIALS
Susceptibility of
Forty-six
Lisianthus
Cultivars to
Fusarium Crown
and Stem Rot
Brent K. Harbaugh1 and
R. J. McGovern2
A
DDITIONAL
INDEX
WORDS
.
Eustoma
grandiflorum, Fusarium avenaceum,
plant breeding
SUMMARY. Fusarium crown and stem
rot, caused by Fusarium avenaceum
(Fr.: Fr.) Sacc., is a serious disease of
lisianthus [Eustoma grandiflorum Raf.
(Shinn.)]. While more than 80 new
cultivars of lisianthus have been
released for sale in the United States
in the last decade, there is a lack of
information on their susceptibility to
this pathogen. Forty-six cultivars of
lisianthus were evaluated for their
response to infection by F. avenaceum.
Cultivars were grouped according to
blue/purple, pink, or white flower
colors and evaluated within their
color class. Although some plants of
all cultivars were susceptible to F.
avenaceum, partial resistance was
observed as indicated by differences in
the length of time to symptom
expression and in the frequency of
diseased plants within each color
group. In 21 of the 46 cultivars, 80 to
100% of the plants expressed symp-
toms within 55 days after inoculation.
The lowest frequencies of diseased
plants 55 days after inoculation were
found in ‘Ventura Deep Blue’ and
‘Hallelujah Purple’ (25%), ‘Bridal
Pink’ (23%), and ‘Heidi Pure White’
(53%) for the blue/purple, pink, and
white flower color groups, respec-
tively. Screening cultivars for resis-
tance to F. avenaceum is the first step
in breeding resistant cultivars. The
methods we developed for these
studies should be useful in screening
for resistance. These results also may
help growers select cultivars that are
less susceptible to F. avenaceum,
which should aid in the management
of this disease.
Fusarium crown and stem
rot is one of the most dam-
aging diseases of lisianthus. It
has been reported as a disease of
lisianthus produced as potted plants
and cut flowers (Koike et al., 1996;
Ozaki, 1992), but has been especially
devastating in cut flower production.
For example, in a 1997 survey of crown
and stem rot incidence in three Florida
and four California cut flower produc-
tion sites, plant mortality was as high as
70% (McGovern et al., 1997).
The causal agent of this disease,
Fusarium avenaceum, primarily attacks
the crown and stems of lisianthus, but
may also rot the tap root and large
feeder roots near the soil line
(McGovern and Harbaugh, 1997).
The first above-ground symptom is a
gradual loss of green coloration in
leaves, which is followed by tan leaf
flecks, browning of leaf veins, and a tan
discoloration of entire leaves. Wilting
and a brown stem rot occurs as the
disease progresses, and infected plants
rapidly die. Orange spore masses form
on the bases of rotted stems and are
diagnostically very important.
Chemical and cultural control
measures have been improved over the
last few years as a result of research on
the etiology and biology of F.
avenaceum (McGovern and Harbaugh,
1997; McGovern and Harbaugh,
1998). However, two approaches to
disease management that have received
inadequate attention are 1) the evalu-
ation and use of resistance in existing
lisianthus cultivars, and 2) the breed-
ing of resistant cultivars. There has
been an explosion of new cultivars
released within the last decade with
over 85 cultivars available in the United
States in 1999. However, we are not
aware of breeding efforts specifically
aimed at developing resistance to F.
avenaceum. The objective of this re-
search was to evaluate 46 cultivars of
lisianthus to determine if resistance
exists that could either be used in
current control strategies or as a first
step toward the breeding of resistant
cultivars.
Material and methods
GENERAL.
Sixteen blue-purple, 15
pink, and 15 white-flowering cultivars
of lisianthus were selected for this study
(Tables 1–3). Cultivars were selected
from five different breeding programs:
American Takii, Inc., Salinas, Calif.;
PanAmerican Seed Company, Elburn,
Ill; Fukukaen Seed Company, Japan;
Sakata Seed America, Inc., Morgan
Hill, Calif.; and University of Florida,
Bradenton, Fla. Each color group was
evaluated separately due to space limi-
tations in the growth chamber and, as
a result, there were differences in cer-
tain production practices that will be
noted for each group.
Plants used in these studies were
produced as follows. Seeds were ger-
minated at 72 to 75 oF (22 to 24 oC)
with a photosynthetic photon flux
(PPF) of 30 µmol·m–2·s–1 for 12 h from
cool-white fluorescent lamps. The soil
medium was, by volume, a 3 Canadian
peat : 2 vermiculite: 1 perlite mix with
an initial pH of 6.7 ± 0.2. Germination
trays were moved to a glass green-
house 2 weeks after sowing. Tempera-
tures were maintained between 60 oF
(15.5 oC) and 95 oF (35 oC). Seedlings
were fertilized twice per week with a
15N–7P–14K water soluble fertilizer
(15–16–17 Peat-Lite Special; Scotts
Co., Marysville, Ohio) solution con-
taining N at 250 ppm (mg·L-1).
Seedlings were transplanted about
35 to 40 d after sowing into 128-cell
plug trays using the same soil medium.
We used eight plants of each cultivar
per tray, and each tray represented a
plot. Seedlings at this stage were fertil-
ized twice per week with a 15N–2P–
12.4K water soluble fertilizer (15–5–
15 Ca–Mg Excel; Scotts Co.,
Marysville, Ohio) solution containing
N at 500 ppm.
Before inoculation with F.
avenaceum and for the rest of each
study, seedlings were moved to a
growth chamber maintained at 66 ± 2
oF (18.9 ± 1 oC) with a PPF of 180
µmol·m–2·s–1 provided by cool-white
fluorescent lamps for 16 h·d–1. Plug
trays were placed in individual 1-inch
(2.5-cm) deep trays and plants pro-
vided water and fertilizer solution via
subirrigation. Seedlings were at the 8
University of Florida, Gulf Coast Research and Educa-
tion Center, 5007 60th Street East, Bradenton FL
34203.
Florida Agricultural Experiment Station journal series
R-07542. We thank the Fred C. Gloeckner Founda-
tion Inc. and the American Floral Endowment for
partial funding of this project. We also thank American
Takii, Inc., PanAmerican Seed Company, Park Seed
Company, and Sakata Seed America, Inc., for supply-
ing seed of cultivars used in this research. The cost of
publishing this paper was defrayed in part by payment
of page charges. Under postal regulation this paper
must therefore be hereby marked advertisement solely
to indicate this fact.
1Department of Environmental Horticulture. To whom
reprint requests should be addressed; e-mail
brenth@nersp.nerdc.ufl.edu.
2Department of Plant Pathology.

817 ● October–December 2000 10(4)
to 12 leaf stage and , if they had bolted,
had one or two internodes on the
bolted stem at the time of inoculation.
Inoculum was prepared by blend-
ing carnation leaf agar plates of 5 to 7-
d-old cultures of a highly virulent iso-
late (96-62B) of F. avenaceum in tap
water at a ratio of one plate per 100 mL
(3.4 fl oz) of water. Ten milliliters of
the F. avenaceum inoculum was ap-
plied to the soil around each plant
which saturated the soil medium in
each cell.
The frequency of plants express-
ing characteristic well developed symp-
toms of F. avenaceum (browning of
leaf veins, stem lesion and or crown
rot) was recorded at 25, 40, and 55 d
after inoculation. Although seedlings
often have a slight chlorosis before
fully developed symptoms are ex-
pressed, this chlorosis is similar to that
caused by Pythium Pringsh. or nutri-
tional imbalances and thus not defini-
tive for signaling infection by F.
avenaceum. Symptomatic plants were
randomly sampled throughout all tests
to confirm infection by F. avenaceum
by reisolation of the pathogen on
Komada’s medium (Komada, 1975).
The experimental design was a
randomized complete block with five
blocks, and eight plants per block rep-
Table 2. Susceptibility of pink-flowering lisianthus cultivars to Fusarium avenaceum at 25, 40, or 55 d after inoculation.
Seeds were sown on 7 Apr. 1998 and plants inoculated 13 May 1998. Each value (percentage) represents the mean of five
replications with eight plants per replication as the experimental unit.
Seed Plants showing symptoms (%)
Cultivar sourcezDay 25 Day 40 Day 55
Maurine Pink U.Fla. 58 ay90 a 98 a
Florida Pink U.Fla. 38 a–c 60 bc 90 ab
Lisa Pink PanAm. 45 ab 65 ab 85 ab
Mermaid Pink Sakata 25 bc 58 b–d 85 ab
Echo Pink Sakata 3 e 33 c–e 83 ab
Royal Pink Takii 18 cd 33 c–e 80 bc
Tiara Pink Takii 38 a–c 58 b–d 78 bc
Tyrol Rose Pink Sakata 5 e 25 d–f 73 b–d
Flamenco Rose Pink Sakata 8 de 23 ef 58 c–e
Hallelujah Pink Fukukaen 18 c–e 33 c–e 53 d–f
Ventura Rose PanAm. 3 e 13 ef 50 d–f
Heidi Rose Pink Sakata 3 e 20 ef 40 ef
Laguna Pink PanAm. 5 de 13 ef 40 ef
Mariachi Pink Sakata 5 de 8 f 40 ef
Bridal Pink Takii 5 de 13 ef 23 f
zTakii = American Takii, Inc.; PanAm. = PanAmerican Seed Company; Fukukaen = Fukukaen Seed Company; Sakata = Sakata Seed America, Inc.; U.Fla. = University of Florida.
yMean separation within columns by Duncan’s multiple range test, P ≤ 0.05. Arcsine square root transformation was performed before data analysis; nontransformed means
are presented.
Table 1. Susceptibility of blue/purple-flowering lisianthus cultivars to Fusarium avenaceum at 25, 40, or 55 d after
inoculation. Seeds were sown on 4 Dec. 1997 and plants inoculated 25 Feb. 1998. Each value (percentage) represents the
mean of five replications with eight plants per replication as the experimental unit.
Seed Plants showing symptoms (%)
Cultivar sourcezDay 25 Day 40 Day 55
Mermaid Blue Sakata 90 ay100 a 100 a
Tiara Purple Takii 55 b 70 b 93 ab
Maurine Blue U.Fla. 53 bc 73 b 85 ab
Florida Blue U.Fla. 43 b–d 65 bc 80 bc
Flamenco Purple Sakata 29 b–e 39 c–e 59 cd
Laguna Deep Blue PanAm. 48 bc 53 b–d 58 cd
Echo Blue Sakata 18 de 29 de 56 d
Lisa Blue PanAm. 28 b–e 43 c–e 50 de
Heidi Deep Blue Sakata 18 de 28 de 49 de
Tyrol Blue Sakata 30 b–e 40 c–e 45 d–f
Mariachi Blue Sakata 25 c–e 35 c–e 40 d–f
Yodel Blue Sakata 13 e 20 e 38 d–f
Royal Purple Takii 18 de 30 c–e 33 d–f
Bridal Violet Takii 20 de 23 e 30 ef
Ventura Deep Blue PanAm. 10 e 18 e 25 ef
Hallelujah Purple Fukukaen 18 de 23 e 25 f
zTakii = American Takii, Inc.; PanAm. = PanAmerican Seed Company; Fukukaen = Fukukaen Seed Company; Sakata = Sakata Seed America, Inc.; U.Fla. = University of Florida.
yMean separation within columns by Duncan’s multiple range test, P ≤ 0.05. Arcsine square root transformation was performed before data analysis; nontransformed means
are presented.

● October–December 2000 10(4)818
VARIETY TRIALS
resented the experimental unit. Statis-
tical analyses were performed on data
using analysis of variance, and means
separated where appropriate using
Duncan’s multiple range test, P ≤ 0.05
(PROC ANOVA, SAS Inst., Cary,
N.C.). An arcsine square root transfor-
mation for percentage data was per-
formed before data analysis (Little and
Hills, 1972).
BLUE CULTIVARS.
The blue culti-
vars were planted 4 Dec. 1997, trans-
planted into plug trays on 13 Jan.
1998, and moved to the growth cham-
ber and inoculated on 25 Feb. 1998.
Seedlings were treated with mefenoxam
(Subdue Max, Novartis, Greensboro,
N.C.) on 20 Mar. 1998 to suppress
Pythium.
PINK CULTIVARS.
The pink culti-
vars were sown 7 Apr. 1998, trans-
planted 13 May 1998, and moved to
the growth chamber 28 May. Plants
were inoculated on 19 June 1998.
Mefenoxam was applied on 17 June to
suppress Pythium and diflubenzuron
(Adept, Uniroyal Chemical,
Middleburg, Conn.) On 22 June 1998
to control fungus gnats [Bradysia
Winnertz (Diptera: Sciaridae)].
WHITE CULTIVARS.
The white culti-
vars were sown 7 July 1998 and trans-
planted 17 Aug. 1998. Because the
greenhouse temperatures were high
enough to cause rosetting of some of
the cultivars (Harbaugh, 1995;
Harbaugh et al., 1992), seedlings were
moved to the growth chamber and
Table 3. Susceptibility of white-flowering cultivars to Fusarium avenaceum at 25, 40, or 55 d after inoculation. Seeds
were sown on 7 July 1998 and plants inoculated 17 Aug. 1998. Each value (percentage) represents the mean of five
replications with eight plants per replication as the experimental unit.
Seed Plants showing symptoms (%)
Cultivar sourcezDay 25 Day 40 Day 55
Florida White U.Fla. 4 by8 bc 100 a
Malibu White PanAm. 0 b 3 bc 100 a
Maurine White U.Fla. 0 b 13 bc 100 a
Tiara White Sakata 0 b 3 bc 100 a
Yodel White Sakata 5 b 18 ab 100 a
Royal White Takii 28 a 28 a 98 a
Ballet White Takii 0 b 10 bc 93 a
Tyrol White Sakata 0 b 0 c 90 a
Flamenco White Sakata 0 b 3 c 88 a
Maurine White/Blue U.Fla. 3 b 33 a 88 a
Mermaid White Sakata 0 b 3 bc 85 a
Mariachi Pure White Sakata 0 b 5 c 83 a
Ventura White PanAm. 5 b 8 bc 60 b
Lisa White PanAm. 0 b 0 c 55 b
Heidi Pure White Sakata 0 b 3 bc 53 b
zTakii = American Takii, Inc.; PanAm. = PanAmerican Seed Company; Fukukaen = Fukukaen Seed Company; Sakata = Sakata Seed America, Inc.; U.Fla. = University of Florida.
yMean separation within columns by Duncan’s multiple range test, P ≤ 0.05. Arcsine square root transformation was performed before data analysis; nontransformed means
are presented.
grown at 75 ± 2 oF (23.9 ± 1 oC) day
and 65 ± 2 oF (18.3 ± 1 oC) night. On
22 Sept. 1998, the temperature was
set to 50 ± 2 oF (10 ± 1 oC) to limit
lisianthus seedling growth until inocu-
lation. Plants were inoculated 14 Oct.
1998 and the temperature finally set to
66 ± 2 oF (18.9 ± 1 oC). Seedlings were
treated 11 Sept., 13 Oct., and 19 Nov.
1998 with mefenoxam to prevent
Pythium development.
Results and discussion
BLUE CULTIVARS.
The frequency of
diseased plants (i.e., percentage of
plants with characteristic symptoms
caused by infection with F. avenaceum)
ranged from a low of 10% for ‘Ventura
Deep Blue’ to a high of 55% for ‘Tiara
Purple’ 25 d after inoculation (Table
1). By day 40, ‘Ventura Deep Blue’
continued to have the lowest percent-
age disease (18%), while 100% of ‘Mer-
maid Blue’ plants were diseased. The
frequency of diseased plants at day 55
ranged from 25% for ‘Ventura Deep
Blue’ and ‘Hallelujah Purple’ to 85%
for ‘Maurine Blue’, 93% for ‘Tiara
Purple’, and 100% for ‘Mermaid Blue’.
PINK CULTIVARS.
The frequency of
diseased plants ranged from a low of
3% for ‘Echo Pink’, ‘Heidi Rose Pink’,
and ‘Ventura Rose’ to a high of 58%
for ‘Maurine Pink’ 25 d after inocula-
tion (Table 2). By day 40, cultivars
with the lowest percentage disease were
‘Mariachi Pink’, ‘Bridal Pink’, ‘La-
guna Pink’, and ‘Ventura Rose’ (8 to
13%), while the greatest number of
diseased plants occurred with ‘Maurine
Pink’ (90%). The percentage of dis-
eased plants at day 55 ranged from
23% for ‘Bridal Pink’ to 98% for
‘Maurine Pink’.
WHITE CULTIVARS.
Only a few cul-
tivars expressed disease symptoms by
25 d after inoculation with the highest
frequency at 28% for ‘Royal White’
(Table 3). By day 40, only ‘Lisa White’
and ‘Tyrol White’ did not show symp-
toms, while ‘Maurine White-on-Blue’
reached 33% diseased plants. How-
ever, by day 55, the lowest percentage
of diseased plants was 53% for ‘Heidi
Pure White’ and five cultivars had 100%
diseased plants (‘Florida White’,
‘Malibu White’, ‘Maurine White’, ‘Ti-
ara White’, and ‘Yodel White’).
The white cultivars appeared to
respond differently than the blue/
purple or pink-flowering cultivars
(evaluated in the other tests) because
the disease development was slower.
However, direct comparisons can not
be made since the plant production
conditions were not the same for all
three tests. For example, white-flow-
ering cultivars were held at 50 ± 2 oF
(10 ± 1 oC) for 22 d while cultivars
within the other color groups did not
receive this treatment. It is possible the
white-flowering cultivar seedlings were
less succulent than the blue/purple
and pink cultivars at the time of inocu-
lation. Many of the white cultivars
were chlorotic 25 and 40 d after inocu-

819 ● October–December 2000 10(4)
lation and we suspected plants were
infected, but these symptoms are not
conclusive evidence of F. avenaceum
infection (McGovern and Harbaugh,
1997). However, more definitive
symptoms of infection by F. avenaceum
(browning of leaf veins, stem lesion
and or crown rot) occurred between
day 40 and 55 and then plants rapidly
started to die. Thus, if the production
practices for the white-flowering culti-
vars were responsible for the delay in
symptom expression, their effects ap-
peared to have diminished toward the
end of the test. Additional experiments
will be designed to compare the best
blue/purple and pink-flowering culti-
vars with the best white-flowering cul-
tivars in order to determine if the
white-flowering cultivars have a ge-
netic basis for delayed disease develop-
ment.
In summary, all cultivars tested
were considered susceptible to F.
avenaceum since a percentage of all
cultivars became infected. However,
distinct differences occurred between
cultivars within a flower color group in
the length of time for symptom devel-
opment and in the frequency of dis-
eased plants, indicating different de-
grees of susceptibility. The genetic basis
for this delayed symptom development
or partial resistance needs to be stud-
ied in order to take advantage of these
findings for breeding more resistant
cultivars. Incorporation of resistance
to F. avenaceum into breeding efforts
could significantly improve control
measures and potentially reduce fun-
gicide use.
Reisolation of the pathogen on all
symptomatic plants randomly sampled
throughout these tests confirmed in-
fection by F. avenaceum. We believe
the method we developed for screen-
ing lisianthus cultivars for their re-
sponse to infection by F. avenaceum
would be valuable in a breeding pro-
gram to develop resistant cultivars.
Growers currently experiencing severe
losses from F. avenaceum may benefit
from this research by using cultivars
found to be less susceptible to F.
avenaceum as an aid to management of
this disease.
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