Fungal Diseases 3



Plectosporium tabacinum (synonym = Microdochium tabacinum)
United States, Europe and Asia
Plectosporium tabacinum has a host range that includes cucurbits, peanut, snap
bean, soybean and sunflower. It remains unclear whether these alternate hosts play a
role in the epidemiology of Plectosporium blight of cucurbits, as it appears that some
host specialization by isolates occurs. In the United States, pumpkin and squash are
the most commonly affected cucurbit crops. Plectosporium tabacinum can infect all
parts of the plant and often causes significant crop losses. Lesions on stems, leaf
veins, petioles and peduncles are often sunken, spindle- or diamond-shaped and tan
to white in appearance. Initially, stem lesions are small but can quickly enlarge and
coalesce over the entire stem, turning it white. Leaf infections are confined to leaf
veins and do not spread to interveinal leaf tissue. Infected petioles and peduncles can
quickly turn dry and brittle, resulting in death of attached leaves or flowers. When a
plant is severely infected, complete defoliation and plant death may occur. Infected
fruit develop small, circular, tan to white raised lesions. Fruit lesions are usually
constricted, but can expand to form a corky, necrotic panel. Fruit lesions often serve
as entryways for secondary soft-rot organisms that cause various fruit rots.
The disease cycle of Plectosporium tabacinum is not well understood. Disease
outbreaks have been associated with high humidity and temperatures between
25–32°C (75–90°F). The pathogen has been reported to survive in crop residue in
the soil for up to three years. Conidia can be splash-dispersed by rain and overhead
irrigation or they can be carried by wind.
Rotate out of cucurbits for three years. Increase air circulation within fields by
reducing plant density, orient fields with prevailing winds, and avoid locations prone
to high humidity. Implementation of drip irrigation along with a preventative fungicide
spray program can help minimize or prevent outbreaks of Plectosporium blight.



Golovinomyces cichoracearum (synonym = Erysiphe cichoracearum)
Podosphaera xanthii (synonym = Sphaerotheca fuliginea)
All cucurbits are susceptible to powdery mildew. Symptoms appear as pale yellow
spots on stems, petioles, and leaves. Infection may occur on the upper and/or lower
leaf surface. As the spots enlarge, conidia are produced from affected tissue and the
spots take on a powdery appearance. Infected leaves gradually turn yellow and may
become brown and papery. In general, cucurbit fruits are not directly attacked by
powdery mildew fungi. However, due to the loss of plant foliage, fruit may be exposed
to direct sunlight, resulting in sunburn and reduced fruit quality. Fruit infection,
although rare, can occur on watermelon and cucumber.
The fungi that cause powdery mildew are obligate parasites, meaning that they
require a living host to survive. These fungi commonly overwinter on weeds and
are carried for long distances by air currents. Infection can take place without free
moisture on the plant surface, although high humidity (50–90% RH) is necessary.
Colonization, sporulation, and dispersal of conidia are favored by dry conditions.
Disease development is favored by vigorous plant growth, moderate temperatures,
low light and dew formation. Infection can occur between 10–32ºC (50–90ºF) with
an optimum temperature for infection between 20–27ºC (68–80°F).
Grow resistant varieties when available. Implement a preventative fungicide spray
program to delay infection and help lower disease incidence. In protected culture,
avoid practices which promote lush growth. Weed control and good sanitation
practices can also aid in controlling powdery mildew.


Cladosporium cucumerinum
Scab is most common on cucumber, but can also affect cantaloupe, pumpkin and
squash. All exposed portions of the plant and fruit can be infected. Leaf symptoms
appear as circular to angular, brownish, water-soaked spots with yellow margins.
Infected plants may have shortened internodes which can give the appearance of
virus infection. A gray to olive-colored sporulation can develop on infected tissue.
Infection appears on young fruit as water-soaked spots which develop into crater-like
depressions as the fruit mature. The crater-like depressions develop an irregular,
scab-like appearance as fruit age. Fruit lesions are commonly shallow and spongy.
Often a gummy brown substance appears on the scabby surface. Under humid
conditions, a mass of fungal spores (conidia) may develop on fruit.
Cladosporium cucumerinum survives in infected plant material. Conidia can be
dispersed by wind, insects, farming equipment, and workers. The disease develops
rapidly under cool [21–24ºC (70–75ºF)], moist conditions. Higher temperatures
inhibit disease development.
Grow scab-resistant cucumber varieties. Implement a preventative fungicide spray
program. Control volunteers and cucurbit weed hosts, which can serve as sources
of inoculum.


Sclerotinia sclerotiorum
All cucurbits are susceptible to sclerotina stem rot. This fungus is able to infect
stems at the soil level, as well as leaves and fruit above ground. The first sign of
disease is a white, cottony growth on infected tissue. As the disease progresses, the
plant gradually turns yellow and may die. When an infected stem is cut open, white
mycelium may be seen in the pith along with large (6–12 mm) black sclerotia. Upon
infection, fruit may be colonized by the white mycelium of Sclerotinia sclerotiorum
and quickly become soft and watery.
The fungus can survive in soil for many years as sclerotia. These overwintering
structures can infect plants by producing mycelium and forming apothecia, which
release ascospores. The disease develops under cool to moderate temperatures
and humid conditions. Prolonged periods of leaf wetness (12–24 hours) are optimal
for disease development. The fungus has an extremely wide host range of over
500 species of plants.
Implement good cultural practices such as a three- to five-year crop rotation
with non-hosts (corn, wheat, and sorghum), sanitation and deep plowing after
a crop to help reduce this disease. In addition, careful irrigation management may
minimize disease occurrence. Soil application of a biological control agent (i.e.,
Coniothyrium minitans) has been shown to reduce populations of viable sclerotia
in the top two centimeters of soil. Application of fungicides has been shown to
help control Sclerotinia stem rot. Soil fumigation is usually an effective method
of control in greenhouses.


Sclerotium rolfsii (teleomorph: Athelia rolfsii)
Southern blight is commonly observed on cantaloupe, squash and watermelon. The
first symptom of the disease is a mid-day wilting of the plant. Leaves turn yellow and
within a few days the plant collapses leading to plant death. The rapid collapse of the
plant is due to girdling of the stem at the soil surface, and the entire root system is
often completely rotted. The fungus develops a white mycelium, which may be fan
shaped, over the surface of the stem. Light brown bodies (sclerotia) may be observed
embedded within the white mycelium. These sclerotia turn dark brown with age. The
fungus also infects fruit in contact with infested soil, developing sunken, yellow spots
which decay and collapse. Large amounts of white mycelium and sclerotia can form
on the fruit as it decays.
Sclerotia, which are the overwintering structure, allow Sclerotium rolfsii to survive for
many years in the soil. Sclerotia are spread by movement of soil or by surface water.
The disease is favored by high temperatures [27–32ºC (80–90ºF)] and high soil moisture.
A good sanitation program is generally the most effective control measure. Remove
and burn infected plants to prevent inoculum build-up. Deep plow crop residue to
reduce inoculum levels. Rotate to non-host crops (corn and small grains) for a period
of three to five years. Good water management can help reduce soil moisture, which
in turn reduces the amount of germinating sclerotia in the soil. Fumigation can also
provide control. Some fungicides have proved effective in controlling this disease.


Corynespora cassiicola
The disease can be found on all cucurbits, although it is most common on
cucumber. The first symptoms appear on older leaves as angular, yellow spots. In the
open field, these spots enlarge and become circular with light brown centers and dark
brown borders. Later these large spots become gray and drop out, giving a shot-hole
or shredded appearance to the leaf. Under greenhouse conditions spots have light
centers with rings of olive green tissue and yellow borders. Eventually, defoliation can
occur. Spots on stems and petioles are more elongated, which helps to distinguish this
disease from other diseases, which include anthracnose, downy mildew or angular
leaf spot. Early infection at the blossom end results in darkened, shriveled fruit. Root
and flower infection can also occur.
The fungus can exist on infected plant residue for at least two years, or on weed
hosts. It is dispersed by air currents. Warm temperatures [25–35ºC (77–95ºF)] and
long days are best for disease development, although infection takes place under
humid and moderately cool temperatures [21–26ºC (70–80ºF)]. Also, fluctuating
daily temperatures appear to favor disease development.
Use resistant varieties when available. Implement a preventative fungicide spray
program. In protected culture, good sanitation practices in and around structures will help avoid future infestations.


Verticillium dahliae
Verticillium albo-atrum
This disease affects all cucurbits. In general, symptom expression occurs at or
following fruit set. Crown leaves initially wilt and take on a faded green color. As
symptoms progress, leaf margins develop “V”-shaped chlorotic lesions, which
eventually collapse and turn necrotic. Wilting may progress along runners, which can
result in plant death. Brown discoloration of root and stem tissues at the crown of the
plant is visible in longitudinal section. Symptoms may be confused with other vascular
wilt pathogens (e.g., Fusarium).
These fungi have a broad host range and can survive in soil as microsclerotia for
many years. Infection takes place through the roots and disease development is
favored by cool soil temperatures [21–24ºC (70–75ºF)]. However, wilting is generally
observed during warm, dry periods when plants are under stress (e.g., fruit set).
Soil fumigation and solarization have been the only methods shown to effectively
reduce the incidence of Verticillium wilt. Avoid infested fields. Implement good cultural
practices, including proper disposal of plant debris, deep plowing and a minimum of a
three-year crop rotation with non-susceptible hosts (e.g., monocots). Avoid following
highly susceptible crops (e.g., cotton, potato or tomato) with cucurbits or other
verticillium wilt-susceptible crops. When possible, delay planting until the soil is warm.
In protected culture, grafting onto a resistant root stock may also help provide control.