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Diseases of Hydrangea
Yonghao Li, Margaret T. Mmbaga, Boru Zhou, Jacqueline Joshua,
Emily Rotich, and Lipi Parikh
Contents
1 Introduction ................................................................................... 2
2 Fungal Diseases ............................................................................... 2
2.1 Powdery Mildew (Golovinomyces orontii DC) ................................ ........ 2
2.2 Cercospora Leaf Spot (Cercospora hydrangea L.)................................... 4
2.3 Botrytis Blight (Botrytis cinerea Pers.: Fr.) . . .......................................... 6
2.4 Rust (Pucciniastrum hydrangeae (B. & C.) Arth.) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . 8
2.5 Anthracnose (Colletotrichum gloeosporioides (Penz.) Penz. and Sacc.) ............. 10
2.6 Corynespora Leaf Spot (Corynespora cassiicola (Berk. & M.A. Curtis)
C.T. Wei) ................................................................................ 11
2.7 Phoma Leaf Spot (Phoma spp.)..... ................................................... 12
2.8 Myrothecium Leaf Spot and Blight (Myrothecium roridum Tode Ex Fr.)............ 13
2.9 Alternaria Leaf Spot (Alternaria spp.) . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . 14
3 Bacterial Diseases ............................................................................. 15
3.1 Bacterial Leaf Spot (Xanthomonas campestris) ....................................... 15
4 Viral Diseases ................................................................................. 15
4.1 Hydrangea Mosaic Virus (HdMV) . . . . . . . . . . . . . . . . . . . . . . . . ............................. 15
4.2 Hydrangea Ringspot Virus (HRSV) ................................................... 16
4.3 Tomato Ringspot Virus (ToRSV) . . . . . . . . . . . . . . .. . . .................................... 16
4.4 Tobacco Ringspot Virus (TRSV) . . ..................................................... 17
Y. Li (*)
Department of Plant Pathology and Ecology, The Connecticut Agricultural Experiment Station,
New Haven, CT, USA
e-mail: Yonghao.Li@ct.gov
M.T. Mmbaga •J. Joshua •E. Rotich •L. Parikh
Department of Agricultural and Environmental Sciences, Tennessee State University, Nashville,
TN, USA
e-mail: Mmmbaga@tnstate.edu;jomega84@gmail.com;erotich@my.tnstate.edu;lp.
parikh@gmail.com
B. Zhou
Department of Forest Protection, Northeast Forestry University, Harbin, China,
e-mail: boruzhou@yahoo.com
#Springer International Publishing AG (outside the USA) 2016
R.J. McGovern, W.H. Elmer (eds.), Handbook of Florists' Crops Diseases, Handbook of
Plant Disease Management, DOI 10.1007/978-3-319-32374-9_36-1
1
4.5 Tomato Spotted Wilt Virus (TSWV) . . . ................................................ 17
4.6 Cherry Leaf Roll Virus (CLRV) ........................................................ 17
4.7 Alfalfa Mosaic Virus (AMV) ........................................................... 17
References . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18
Abstract
Hydrangea, a native to Asia and America, is a popular ornamental plant due to its
interesting large, showy flowers with different colors and inflorescence forms.
Many hydrangea species are often used to design landscapes and gardens. In
greenhouses, nurseries, and landscapes, hydrangea production and values are
affected by fungal, bacterial, and viral diseases. In this chapter, the biology of
pathogens, development of diseases, and strategies for their control of common
diseases of hydrangea were described.
Keywords
Hydrangea •Powdery mildew •Leaf spot •Rust •Anthracnose •Bacterial •Virus
1 Introduction
Hydrangeas are among the most loved flowering shrubs and vines in landscapes.
Many species in the genus Hydrangea vary in shrub size, texture, flower shape, and
color, and their aesthetic values include popular floral arrangements and crafts
(Mmbaga et al. 2008). The most common hydrangea is the bigleaf/French/garden
hydrangea (H. macrophylla (Thunb.) Ser.) that thrives in hardiness US zones 6–9
(Dirr 2004). Although more than 700 cultivars have been described in
H. macrophylla, only about 25% of these are available in the US trade (Dirr 2004;
Van Gelderen and Van Gelderen 2004). Several other common species in this genus
are climbing hydrangea (H. anomala), oakleaf hydrangea (H. quercifolia), smooth
hydrangea (H. arborescens), and Pee Gee hydrangea (H. paniculata) (Dirr 2004).
2 Fungal Diseases
2.1 Powdery Mildew (Golovinomyces orontii DC)
Geographical occurrence and impact. Powdery mildew of hydrangea has been
reported in the Unites States and Korea (Park et al. 2012). The disease is a common
problem in bigleaf hydrangea (H. macrophylla) in greenhouses, nurseries, and
landscapes when environment conditions are warm and humid. The disease also
affects H. serrata as well as smooth (H. arborescens), bigleaf (H. macrophylla), and
panicle (H. paniculata) hydrangeas (Hagan and Mullen 2001).
Symptoms/signs. Early signs of powdery mildew on hydrangea consist of
circular light gray patches composed of mycelia and spores on both sides of the
2 Y. Li et al.
leaf surface. These pustules of fungal tissue eventually turn into yellow or purple
blotches on the leaf (Fig. 1). As the disease progresses, disfiguring white mycelium
develops on leaf surfaces along with extensive chlorosis or yellowing of the leaves
(Fig. 2). Premature defoliation may occur on severely infected plants. Growth of the
plant may be impacted with the reduction in active leaf area and shoot elongation
directly reducing the production of photosynthesis (Hagan et al. 2004; Sinclair and
Lyon 2005). Powdery mildew is rarely fatal to plants in landscape settings. Its major
damage is the reduction in aesthetic quality and plant vigor. In addition economic
values of infected plants are lowered and may even render plants unmarketable.
Biology and epidemiology. The bigleaf hydrangea powdery mildew is caused by
G. orontii (formerly Erysiphe polygoni DC.) and E. poeltii (Pscheidt 2014). Powdery
mildew caused by Oidium hortensiae on H. macrophylla was first reported in Korea
in 2012 (Park et al. 2012). The pathogen is an obligate parasitic fungus that occupies
the leaf surface of the plant (Li et al. 2009a). The disease is favored by dry
conditions, high humidity, and warm day/cool night temperatures. The pathogen
obtains its nutrition from epidermal cells that are parasitized by root-like structures
called haustoria (Li et al. 2008). The fungus overwinters as hyphae or spores in buds
of previously infected plants in the landscape and living plants in greenhouses.
Management
•Cultural practices –Propagation should be done using cuttings from healthy
stock plants. Efforts to reduce the relative humidity in the greenhouse and
increasing plant spacing to improve air circulation will suppress powdery mildew.
Placing plants in sunny locations and avoiding overhead sprinkler irrigation will
also help to reduce the incidence of disease. Growers should remove infected
plants as well as remove debris from previous crops as soon as possible to reduce
the amount of inoculum and infection source.
•Chemical –Fungicides labeled for use against powdery mildew on hydrangea are
available. Applications should be initiated when initial symptoms of the disease
are observed or conditions are favorable for disease development. Fungicide
applications should be repeated at 7–14-day intervals or by following label
recommendations to provide effective control. Some labeled fungicides reported
to control powdery mildew on hydrangea include azoxystrobin, fenarimol, par-
affinic oil, and thiophanate-methyl (Hagan et al. 2004; Hagan and Mullen 2001).
Fungicide rotations should be practiced to minimize the development of fungicide
resistance. Rotations should include fungicides from different fungicide groups
(FRAC groups) that operate using different modes of action.
•Biorationals –Applications of biorational products are also effective in control-
ling powdery mildew (Mmbaga and Sauvé 2004). However products have to be
applied preventatively when initial symptoms of powdery mildew are observed.
Thorough coverage of the tops and undersides of leaves is recommended. Sodium
bicarbonate (baking soda) marketed as Armicarb™(potassium bicarbonate
(MilStop)) and fatty acids from neem seed oil marketed as Triact 70
1
are labeled
for powdery mildew control (Hagan et al. 2005; Horst et al. 1992; Mmbaga and
Oliver 2007).
Diseases of Hydrangea 3
•Biocontrols –Bacillus subtilis strain QST 713 (Hagan et al. 2004) is labeled as a
biological control agent against powdery mildew in hydrangea.
•Resistance –The best way to manage powdery mildew is by growing disease-
resistant selections. Previous reports indicate that cultivars belonging to
H. macrophylla ssp. serrata are more resistant than H. macrophylla ssp. macro-
phylla cultivars (Dirr 2004). Out of 90 H. macrophylla cultivars evaluated for
resistance to powdery mildew over a 3-year period, three cultivars, “Amagi
Amacha,”“Shirofuji,”and “Veitchii,”were among the most powdery mildew-
resistant cultivars each year (Windham et al. 2011). Other cultivars, namely,
“Diadem,”“Komachi,”and “Omacha,”were highly resistant in 2006 and 2008,
but only moderately resistant in 2007 (Windham et al. 2011). Bigleaf hydrangea
“Veitchii”also showed higher resistance to powdery mildew in vitro tests
(Li et al. 2009b).
2.2 Cercospora Leaf Spot (Cercospora hydrangea L.)
Geographic occurrence and impact. This disease generally affects bigleaf and
smooth types of hydrangea as well as oakleaf hydrangea in both landscape and
Fig. 1 The early symptom of
powdery mildew of hydrangea
with white mildew and brown
patches (Photo by Yonghao
Li)
Fig. 2 Symptoms of
hydrangea powdery mildew
with infected leaves covered
by heavy white mildew (Photo
by Yonghao Li)
4 Y. Li et al.
nursery settings (Vann 2010; Mmbaga et al. 2012,2015). The disease is particularly
important in warmer regions of southeastern United States including Arkansas,
Alabama, Georgia, and Tennessee (Vann 2010; Hagan and Mullen 2001). This
leafspot disease hardly ever kills the plant, but it can cause significant premature
defoliation and reduce the plant vigor and flower bud set (Vann 2010; Hagan and
Mullen 2001).
Symptoms/signs. Leaf spot symptoms differ based on the type of hydrangea
infected. Older leaves at the bottom of the plant usually exhibit the first visible
symptoms and then spread upward toward the top of the plant (Vann 2010; Hagan
and Mullen 2001). Early spots on the leaves are purple and small with a circular
shape (Fig. 3). This is followed by enlargement of the spot that turns angular or
irregular in shape and acquires a tan or gray center enclosed by a purple or brown
border commonly referred to as frogeye leaf spot pattern in bigleaf hydrangea (Vann
2010; Hagan and Mullen 2001). Alternatively, leaf spots on oakleaf hydrangea are
angular shaped and dark brown to purple and may become yellow green and
defoliate. Leaf spotting often commences in the midsummer and becomes visible
by early fall.
Biology and epidemiology. The primary source of inoculum is the conidia of
C. hydrangeae that overwinter in the leaf debris.The spores are disseminated to
healthy lower foliage via splashing rain or overhead irrigation as well as wind
(Hagan and Mullen 2001; Vann 2010). Recurring summer rain showers can signif-
icantly accelerate the rate of disease spread, level of spotting, and defoliation. On the
other hand, suppression of the disease development and spread can be prolonged by
drought periods and prolonged exposure to full sun (Hagan and Mullen 2001;
Li et al. 2008; Mmbaga et al. 2012).
Management
•Cultural –Application of sufficient nitrogen will help ensure good growth of the
plants (Hagan and Mullen 2001). Drip irrigation or using soaker hoses and wide
plant spacing that allow air movement and reduce leaf wetness have been reported
to reduce Cercospora leaf spot incidences in landscape plantings. Proper site
selection is imperative in order to avoid known problematic areas. Choice of site
by a grower may not always be feasible, but the use of well-drained areas and
partial shade with direct exposure to morning sunlight promotes better growth of
the hydrangea and reduces foliage disease incidence (Li et al. 2008). A good
sanitation strategy requires the removal and destruction diseased leaf debris that
act as the source of infection for new plants. This will help to minimize infection
level and disease severity.
•Chemical –Using protective fungicides is advocated for high-value landscape
plants that annually show noticeable damages. The fungicide application should
commence when first leaf spot symptoms are observed and continue as needed.
Products containing chlorothalonil, myclobutanil, or thiophanate-methyl are
highly effective if applied at the onset of the disease. The use of protective
chlorothalonil or mancozeb programs was found to significantly reduce
Cercospora leaf spot disease severity in bigleaf hydrangea (Morrison 1980).
Diseases of Hydrangea 5
Since protection by these products is not guaranteed for new growth that is not
covered by the fungicide, multiple fungicide applications at 10–14-day intervals
are necessary to ensure continuous leaf protection and good disease control.
•Resistant varieties –Selection of resistant and disease-free cultivars for new
planting is important to prevent the introduction of the disease to new areas.
Mmbaga et al. (2012) reported ten cultivars of H. macrophylla,“Ami Pasquier,”
“Ayesha,”“Blue Bird,”“Forever Pink,”“Fuji Waterfall”(“Fujinotaki”),
“Miyama-yae-Murasaki,”“Seafoam,”“Taube,”“Tricolor,”and “Veitchii,”for
expressing resistance or moderate resistance to multiple pathogens including
Cercospora leaf spot in full sun, full shade, and partial shade environments
(Li et al. 2008; Mmbaga et al. 2012).
2.3 Botrytis Blight (Botrytis cinerea Pers.: Fr.)
Geographic occurrence and impact. Botrytis blight, also called gray mold, is a
serious disease problem in the production and postharvest of floral crops. B. cinerea
has a wide host range including many herbaceous annual and perennial plants (Plant
Disease Diagnostic Clinic 2015). The fungus is considered an opportunistic patho-
gen that infects weakened or injured leaves and flowers especially in cloudy, humid,
and rainy weather conditions. All species of hydrangea are susceptible to this
disease, but bigleaf hydrangea is most susceptible (Hagan and Mullen 2001).
Symptoms/signs. When flowers are infected, the initial symptoms are water-
soaked spots on petals, which develop reddish-brown irregular blotches that may, in
time, cover the whole flower later. Affected flowers quickly turn brown and wither.
The fuzzy gray growth (spores and conidiophores of the fungal pathogen), a
characteristic symptom of Botrytis blight, is usually noticed on diseased flowers
when environment conditions are wet or highly humid (Hagan and Mullen 2001).
Leaves can be infected and show irregular brown necrotic lesions on leaves when
diseased petals or other infected debris are fallen on them (Fig. 4).
Biology and epidemiology.B. cinerea can survive in plant debris as a saprophyte
or as a pathogen on a broad range of host plants (Daughtrey et al. 2000). The fungus
also overwinters as black resting structures (sclerotia) on dead plant tissues or in soil
Fig. 3 Small circular brown
necrotic lesions with
ash-colored centers on a
hydrangea leaf infected by
Cercospora sp. (Photo by
Margarete Mmbaga)
6 Y. Li et al.
(Hagan and Mullen 2001; Plant Disease Diagnostic Clinic 2015). The pathogen can
be transmitted by wind, water splash, insects, and human activities from affected
plants. Spore germination needs high relative humidity or water films on plant
surfaces. A 5- to 8-h wet period is required for infection of B. cinerea (Jewett and
Jarvis 2001). Germinated spores directly penetrate healthy blossoms and leaves and
also enter through natural openings and wounds. The favorable conditions for the
disease development are unbalanced nutrition, low light intensity, cool temperature,
and high humidity, especially in the condition of several consecutive days of cloudy,
humid, rainy weather (Sinclair and Lyon 2005). The optimal temperature for the
onset of Botrytis blight in the greenhouse is approximately 15 C. All species of
hydrangea are susceptible to this disease, but damage is noted most often on the
bigleaf hydrangea (Hagan and Mullen 2001).
Management
•Cultural practices –Venting and concurrently heating greenhouses at dusk can
expunge humidity. Avoiding overhead irrigation will help to keep leaf canopies
dry and reduce relative humidity which can suppress the disease (Daughtrey et al.
2000). Increasing plant spacing and pruning weakened leaves from the lower part
of plants to improve air circulation and encourage rapid evaporation of moist
from plants and soil surfaces, avoiding wounds of leaves or shoots which can
promote further spread of the disease. In addition, watering plants in the morning
to allow rapid evaporation of excess moisture from plant surfaces is
recommended.
•Sanitation –Proper sanitation is crucial in controlling this disease. But, sanitation
should not be conducted when plants are wet with dew or rain because it will
spread fungal spores into the air. Removing infected leaves and damaged flower
heads from plants and destroying them to reduce the inoculum in greenhouses and
landscapes. In the greenhouse, clearing of benches and beds should be done
before bringing in fresh plant materials of next crops. This can be followed by
Fig. 4 Irregular necrotic
lesions on hydrangea leaves
affected by Botrytis blight
(Photo by Margarete
Mmbaga)
Diseases of Hydrangea 7
treatment of wooden surfaces in propagation and production areas with 2%
disinfectant (Hagan and Mullen 2001).
•Fungicide –Protection of plants against infection can be easily managed using
fungicides. The first fungicide application should be made in the spring when
continuously cool and wet weather is predicted or where Botrytis blight has been
a problem in the previous year. Choice of fungicides is based on the site and type
of plant(s) to be treated. Some effective fungicides recommended include
iprodione, mancozeb, and thiophanate-methyl (Hagan and Mullen 2001). To
prevent fungicide resistance development, always alternate fungicide applications
between chemicals with different modes of actions. Resistance to thiophanate-
methyl and iprodione has been reported in Botrytis populations.
•Biological –Some biofungicides, such as Mycostop (Streptomyces griseoviridis
Strain K61), actinovate (Streptomyces lydicus), are effective to control Botrytis
blight.
•Resistant varieties –The use of resistant cultivars is the most effective; however,
no major resistance gene to Botrytis has been identified in hydrangeas.
2.4 Rust (Pucciniastrum hydrangeae (B. & C.) Arth.)
Geographic occurrence and impact. In nurseries, gardens, and landscapes, hydran-
gea rust is most commonly found on smooth hydrangea, a native shrub in the eastern
North America. The pathogen can also affect hemlock, but it is more commonly
found on hydrangeas. The disease is reported from New York to Illinois, Arkansas,
Tennessee, and Georgia (Sinclair and Lyon 2005; Li et al. 2010).
Symptoms/signs. On hydrangea leaves, symptoms initiate as small yellow or
orange spots, which then develops to necrotic angular lesions because fungal growth
is normally restricted by major veins (Fig. 5). An orange dust (urediniospores) forms
on the lower surface of infected leaves when the fungal fruiting bodies (uredinia) are
mature (Fig. 6). As the disease develops, symptoms are noticeable on the whole plant
(Fig. 7).
Biology and epidemiology. To complete its life cycle, the fungus, P. hydrangea,
needs two genetically distinctive hosts, hemlock and hydrangea, which includes
Tsuga canadensis (eastern hemlock), T. caroliniana (Carolina hemlock),
H. arborescens (smooth hydrangea), and H. paniculata (panicle hydrangea).
Urediniospores formed on hydrangea leaves repeat several infection cycles on
hydrangeas during a season. Flat, reddish-brown telia develop within the epidermal
cells of both sides of leaf surfaces of hydrangea in late summer and fall. The
teliospores germinate and produce basidiospores that infect hemlock needles in the
spring (Sinclair and Lyon 2005). In late spring to early summer, cylindric aecia with
light cream color are formed on the lower surface of hemlock needles. When aecia
are mature, orange-yellow aeciospores are released and infect hydrangea leaves in
the spring.
8 Y. Li et al.
Management
•Cultural practices –Reduction of relative humidity and the duration of plant
tissue wetness are critical for the disease control.
•Fungicide –Chlorothalonil 720 SC and Daconil Weatherstik fungicides are
labeled for hydrangea rust. The first fungicide application should be made in
the spring to protect leaves. Fungicide doses and replications need to follow the
fungicide labels.
•Resistance –Significant differences in resistance to hydrangea rust were reported
between cultivars of smooth hydrangeas (Li et al. 2010). Among seven tested
varieties, “Frosty”was highly resistant, “Green Dragon”was highly susceptible,
and the other five varieties, “Ryan Gainey,”“Pink Pincushion,”“White Dome,”
“Mayes Starbust,”and “Annabelle,”were intermediate (Li et al. 2010).
Fig. 5 Yellow spots on a
smooth hydrangea leaf
infected with hydrangea rust
(Photo by Yonghao Li)
Fig. 6 Yellow to orange
pustules on the lower surface
of a hydrangea leaf infected
with hydrangea rust (Photo by
Yonghao Li)
Diseases of Hydrangea 9
2.5 Anthracnose (Colletotrichum gloeosporioides (Penz.) Penz.
and Sacc.)
Geographic occurrence and impact. Anthracnose of hydrangea is caused by
C. gloeosporioides and found periodically in landscape and field plantings of bigleaf
hydrangea. This disease affects both leaves and blossoms of hydrangea plants.
Symptoms/signs. Initially, circular or slightly irregular brown spots are formed
on hydrangea leaves with the center of the spots measuring up to 2.5 cm or more in
diameter and turning light brown to tan in color (Hagan and Mullen 2001). This is
followed by alternating dark and slightly lighter rings of dead tissue forming a bull’s
eye or target-spot appearance (Fig. 8). Large, dark brown, irregular blotches may
extend across the flower petals and leaves when environmental conditions are
favorable for the disease development.
Biology and epidemiology.C. gloeosporioides has a very broad host range in
woody shrubs and trees. The pathogen overwinters in diseased leaf debris of
hydrangea and other plant debris. The disease is favored by hot, wet weather
conditions. Masses of spores ooze from fruiting bodies (acervuli) embedded in leaf
debris. In the presence of continuous leaf wetness, spores spread to other leaves and
flower petals by splashing water (Hagan and Mullen 2001). The pathogen is able to
quickly penetrate and colonize the host tissues at warm temperatures of 24–32 C.
The rate of disease infection and symptom appearance are promoted by prolonged
periods of heavy fog, frequent showers, and dew. Hydrangeas that are extensively
fertilized may be more sensitive to infection by C. gloeosporioides (Hagan and
Mullen 2001).
Management
•Cultural –The use of disease-/symptom-free plant cuttings can significantly
minimize the chances of spreading the disease. Removing and destroying dis-
eased leaf debris or blighted blooms eliminate and reduce the inoculum source.
•Chemicals –The use of protective fungicide treatments is a viable option to
protect susceptible hydrangea from anthracnose. The application should be done
Fig. 7 A hydrangea plant
with rust-infected leaves
(Photo by Yonghao Li)
10 Y. Li et al.
at 10–14-day intervals during the summer for good efficacy. Some of the
recommended fungicides include chlorothalonil and thiophanate-methyl, and
they should be administered as per manufacturer’s instructions (Hagan and
Mullen 2001).
2.6 Corynespora Leaf Spot (Corynespora cassiicola (Berk. & M.A.
Curtis) C.T. Wei)
Geographic occurrence and impact. This disease infects leaves, stems, and flower
petals and causes irregularly shaped brown necrotic lesions. The disease may spread
rapidly during hot and wet weather and has the potential to kill small plants and
severely reduce the aesthetic value of the plants.
Symptoms/signs. Symptoms of this disease are characterized as various sized
brown spots on hydrangea leaves (Fig. 9) that are similar to those of Cercospora leaf
spot. Like Cercospora leaf spot, Corynespora leaf spot is favored by the similar
environments and is often isolated as part of a disease complex (Hagan et al. 2004;
Zaher et al. 2005; Smith and Schlub 2007).
Fig. 8 Circular to irregular-
shaped brown spots with
concentric rings and ash-gray
center on hydrangea leaves
infected by Colletotrichum
gloeosporioides (Photo by
Margarete Mmbaga)
Fig. 9 Various sized brown
spots on hydrangea leaves
infected with Corynespora
cassiicola (Photo by
Margarete Mmbaga)
Diseases of Hydrangea 11
Biology and epidemiology. Although C. cassiicola has been reported on over
300 plant species, a high degree of host specificity was reported in the fungal
populations (Pereira et al. 2003). The pathogen survives in infested plant debris
and on stems of previously infected plants.
Management
•Cultural –Removal and destruction of diseased leaf debris that act as the source
of infection for new plants are important. Avoid overhead irrigation and plant
crowding.
•Chemicals –Both Cercospora and Corynespora leaf spot diseases tend to have
annual outbreaks when the pathogens have become established in an area (Hagan
and Mullen 2001; Hagan et al. 2004; Zaher et al. 2005; Smith and Schlub 2007;
Mmbaga et al. 2012). Thus, protective fungicide applications are advocated for
high-value landscape plants that annually show noticeable damages. The fungi-
cide application should commence when first leaf spot symptoms are observed
and continue as needed.
•Resistant varieties –Selection of resistant and disease-free cultivars for new
planting is important to prevent the introduction of the disease to new areas.
Cultivars “Ami Pasquier,”“Ayesha,”“Blue Bird,”“Forever Pink,”“Fuji Water-
fall”(“Fujinotaki”), “Miyama-yae-Murasaki,”“Seafoam,”“Taube,”“Tricolor,”
and “Veitchii”were rated resistant or moderate resistant to multiple pathogens
including Corynespora leaf spot in full sun (Mmbaga et al. 2012).
2.7 Phoma Leaf Spot (Phoma spp.)
Geographic occurrence and impact. This disease can occasionally cause leaf spots
in hydrangea. Phoma exigua has been reported to cause leafspot disease in hydran-
gea in different parts of the United States and central Italy (Garibaldi et al. 2006;
Mmbaga et al. 2009).
Symptoms/signs. Phoma leaf spot is characterized by small necrotic spots
surrounded by chlorotic haloes on the upper side of infected leaves (Garibaldi
et al. 2006). The color of the leaf spots may differ slightly based on cultivars as
observed by Mmbaga et al. (2010) where H. macrophylla cultivar “Lady in Red”
developed small reddish-brown lesions and cultivar “Seafoam”had large lighter
brown lesions during pathogenicity tests with P. exigua (Fig. 10).
Biology and epidemiology. Phoma leaf spot is favored by relatively low tem-
peratures (24 3C) for symptom development (Mmbaga et al. 2010). Severely
infected leaves become chlorotic and abscised leading to premature defoliation.
Infected plants rarely die, but the presence of lesions on mature plants decreases
aesthetic quality and subsequent market values.
Management
•Cultural practices –Sanitation and removal of diseased leaf debris help to
eliminate source of infection.
12 Y. Li et al.
2.8 Myrothecium Leaf Spot and Blight (Myrothecium roridum
Tode Ex Fr.)
Geographic occurrence and impact. The disease was first reported in Tennessee in
2010 (Mmbaga et al. 2010; Mmbaga 2010).
Symptoms/signs.InH. macrophylla, brown necrotic lesions with concentric
rings and ash-colored centers (Fig. 11) characterized this disease and are similar to
symptoms from M. roridum in other ornamental plants such as salvia, gardenia,
begonia, and New Guinea impatiens (Mmbaga et al. 2010; Mangandi et al. 2007).
The pathogen causes blighting of the stems, mostly beginning at the soil line, tan
concentric leaf spots, and yellowing and wilting of foliage.
Biology and epidemiology.M. roridum resides mostly in the soil and has a broad
host range consisting of several vegetable, agronomic, and ornamental crops
(Ponappa 1970). Infected plant materials and infested soil disseminate spores of
M.roridum, and fungal spores may remain viable for 2–3 months in field soil at
20 C (Murakami et al. 2000).
Management
•Cultural practices –Some of the management strategies comprise of soil disin-
festation using fumigants or steam and soil solarization to kill the pathogen in the
soil. Preventative measures focus on sanitation practices to reduce disease
incidence.
•Host resistance –When available, disease resistance is the best method for
controlling hydrangea leaf spot diseases. Out of 88 cultivars of H. macrophylla
exposed to natural airborne leaf spot pathogens in McMinnville, Tennessee, USA,
ten cultivars, “Ami Pasquier,”“Ayesha,”“Blue Bird,”“Forever Pink,”“Fuji
Waterfall”(“Fujinotaki”), “Miyama-yae-Murasaki,”“Seafoam,”“Taube,”“Tri-
color,”and “Veitchii,”were rated resistant or moderately resistant in full shade,
full sun, and partial shade environments (Mmbaga et al. 2012). These cultivars
may be considered to have multiple resistance to the pathogens including
Cercospora spp., C. cassicola,C. gloeosporioides,M. roridum, and P. exigua
(Mmbaga et al. 2012,2015).
Fig. 10 Brown spots on a
hydrangea leaf infected by
Phoma exigua (Photo by
Margarete Mmbaga)
Diseases of Hydrangea 13
2.9 Alternaria Leaf Spot (Alternaria spp.)
Geographic occurrence and impact. Alternaria leaf spot of hydrangea can be
caused by several Alternaria species that has been reported in Italy and the United
States (Daughtrey 1995; Garibaldi et al. 2006,2008). Infected plants rarely die, but
the presence of lesions reduces the aesthetic quality and subsequently the commer-
cial value.
Symptoms/signs. Initial symptoms of Alternaria leaf spot are small brown spots
with yellow halos that appear on the upper surface of the infected leaves. And then,
lesions become dark brown or black and often coalesced into large necrotic areas.
Symptoms are commonly observed on the leaf margins and near the petioles.
Severely affected plants are defoliated (Garibaldi et al. 2006,2008).
Biology and epidemiology. Two species, A. alternate and A. compacta, have
been identified on bigleaf hydrangea and climbing hydrangea, respectively, in Italy
(Garibaldi et al. 2006,2008). The pathogens of Alternaria leaf spot can survive as
mycelium or spores in plant debris. In moist and wet conditions, spores of the
pathogen are spread by wind, rain, or irrigation, which causes disease epidemics in
landscapes and nurseries. Consequently, periods of moist conditions from rain and
overhead irrigation are favorable for the disease development.
Management
•Cultural practices –Spacing plants to minimize dense canopy formation and
improve air circulation. Avoiding overhead irrigation, watering plants in the
morning to allow water on leaf surfaces to be evaporated quickly. Appropriately
heating and ventilating greenhouses to reduce humidity.
•Sanitation –Removing infected leaves from infected plants and destroying them
or putting them in trash.
•Fungicides –Fungicide application is a part of integrated disease management
programs of the disease. Since most fungicides are preventative, the first appli-
cation should be started when the disease is detected in nurseries or landscapes.
The interval of applications needs to follow label recommendations.
Fig. 11 Concentric ring
spots on a bigleaf hydrangea
leaf infected by Myrothecium
roridum (Photo by Margarete
Mmbaga)
14 Y. Li et al.
3 Bacterial Diseases
3.1 Bacterial Leaf Spot (Xanthomonas campestris)
Geographic occurrence and impact. Bacterial leaf spot caused by a pathovar of
X. campestris is a common problem on oakleaf hydrangea, smooth hydrangea, and
bigleaf hydrangea. The disease has been reported in the United States (Uddin et al.
1996).
Symptoms/signs. On oakleaf hydrangea, the early symptoms of the disease
initiate as water-soaked spots on the lower surface of leaves and then develop to
angular purple lesions because lesions are usually restricted by leaf veins (Fig. 12).
Normally, the disease starts on the lower leaves and spreads upward on the plant. On
bigleaf hydrangea and smooth hydrangea cultivars, symptoms are small purple
angular spots (Fig. 13). At the late stage of lesion development, the necrosis
develops in the center of lesions which dries, turns tan to brown, and can frequently
fall out giving a shot-hole appearance.
Biology and epidemiology. Humid and moderately wet conditions are favorable
for the disease epidemic in greenhouses, nurseries, and landscapes. Infection begins
on the lower leaves and then spreads to upper leaves and adjacent plants as the
pathogen is transported by splashing water. Small water-soaked spots develop on
leaves within 3–5 days after inoculation, with typical purple spots developing within
7 days (Uddin et al. 1996).
Management
•Cultural practices –Avoid overhead irrigation to prevent the spread of the
inoculum through splashing water. Space plants adequately to improve air circu-
lation. Water plants in the morning and avoid handling plant when they are wet.
•Sanitation –Promptly remove and discard severely infected leaves during the
growing season. At the end of a season, remove and destroy fallen leaves.
•Fungicide and biological control –Kaolin clay (Surround) and neem oil (Neem
Gold) were reported to reduce disease severities of bacterial leaf spot in the field
experiments (Mmbaga and Oliver 2007). Copper-containing fungicides may
provide some control when applied in late spring, but will not be effective in
plants that are grown under conditions favoring the disease.
4 Viral Diseases
4.1 Hydrangea Mosaic Virus (HdMV)
The typical symptom of HdMV on bigleaf hydrangea is chlorotic mosaic on leaves.
The virus can be transmitted mechanically by grafting, by leaf contact, or by tools,
but not by aphids or through seed. Control of HdMV includes using virus-free
transplants and making cuttings from healthy plants (Thomas 1983).
Diseases of Hydrangea 15
4.2 Hydrangea Ringspot Virus (HRSV)
The early symptoms of HRSV on hydrangea leaves are brown spots or rings, and
then plants show leaf distortion, leaf roll, and stunted growth. The virus can be
transmitted mechanically through saps, but aphids or seed does not transmit HRSV
(Daughtrey et al. 1995; Williams-Woodward and Daughtrey 2001). Cultural control
of HRSV includes buying clean stock and sanitizing pruning tools.
4.3 Tomato Ringspot Virus (ToRSV)
ToRSV has a wide host range including woody and herbaceous plants. On hydran-
gea, the virus causes leaf distortion and chlorosis and stunted plant growth. The virus
can be transmitted by nematodes, but not by pruning tools. Control strategies for
ToRSV are weed management and the use of nematode-free soil mix for container-
grown plants (Williams-Woodward and Daughtrey 2001; Brierley 1954; Daughtrey
et al. 1995).
Fig. 12 Angular brown
lesions of bacterial leaf spot
on an oakleaf hydrangea leaf
(Photo by Yonghao Li)
Fig. 13 Angular brown
lesions of bacterial leaf spot
on a bigleaf hydrangea leaf
(Photo by Yonghao Li)
16 Y. Li et al.
4.4 Tobacco Ringspot Virus (TRSV)
TRSV is widespread in woody and herbaceous plants in North America and also
occurs in Europe, Asia, and Australia. Various symptoms can be found on infected
plants, which include leaf distortion, leaf chlorosis, and stunted plant growth.
Nematodes in the genus Xiphinema are major vectors of the virus (Williams-Wood-
ward and Daughtrey 2001). Strategies for TRSV control include planting clean
stocks in containers in soilless media and controlling weeds that may be reservoirs
the virus and nematode vectors.
4.5 Tomato Spotted Wilt Virus (TSWV)
TSWV infects over 1,000 plant species. The virus is transmitted by thrips. Preven-
tion is the first step of the disease control, which can be achieved by purchasing
virus-free plant materials. In greenhouses, control of TSWV can be achieved by
destroying infected plants and thrip management (Williams-Woodward and
Daughtrey 2001).
4.6 Cherry Leaf Roll Virus (CLRV)
CLRV is widespread in Eurasia and North America and also occurs in Australia and
New Zealand. Symptoms of CLRVon hydrangea are leaf deformation, chlorosis, and
necrotic spots. This virus can be transmitted through seed and pollens and also by
grafting (Veerakone et al. 2012). Since CLRV is a relatively new disease on
hydrangea and no effective control methods have been reported, using clean and
virus-free plant materials is effective to prevent the introduction of the virus to
nurseries and landscapes.
4.7 Alfalfa Mosaic Virus (AMV)
AMV has a wider host range including 150 plant species and is widely distributed in
the world (Jaspars and Bos 1980). This virus was first reported on Hydrangea spp. in
British Columbia (Chiko et al. 1986). In the United States, occurrence of AMV in
hydrangea was first reported in 2013 (Lockhart et al. 2013). A typical symptom of
AMV on hydrangea is yellow leaf blotching. AMV can be transmitted by various
aphid species. Control of AMV is achieved by weed management and aphid control
(Daughtrey et al. 1995).
Diseases of Hydrangea 17
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Diseases of Hydrangea 19
