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FUNGAL DISEASES 

ALTERNARIA LEAF BLIGHT
ANTHRACNOSE
BLACK ROOT OF CUCURBITS
CERCOSPORA LEAF SPOT
CHARCOAL ROT
DAMPING-OFF
DOWNY MILDEW
FUNGAL FRUIT ROTS
FUSARIUM CROWN AND FOOT ROT OF SQUASH
FUSARIUM ROOT AND STEM ROT OF CUCUMBERS
FUSARIUM WILT
GUMMY STEM BLIGHT
MONOSPORASCUS ROOT ROT AND VINE DECLINE
PHYTOPHTHORA CROWN AND ROOT ROT
PLECTOSPORIUM BLIGHT
POWDERY MILDEW
SCAB
SCLEROTINIA STEM ROT
SOUTHERN BLIGHT
TARGET LEAF SPOT
VERTICILLIUM WILT

ALTERNARIA LEAF BLIGHT

CAUSAL AGENT:
Alternaria cucumerina
DISTRIBUTION:
Worldwide
SYMPTOMS:
Alternaria leaf blight is a common disease on cantaloupe and of less importance on
cucumber, watermelon and squash. Symptoms first appear on the upper leaf surface
as small, circular, tan spots with white centers. These spots enlarge, turn light brown
and form a slight depression. Small leaf veins within the spots darken, resulting in a
netted appearance. As the spots enlarge on muskmelon and watermelon, concentric
rings develop that are visible only on the upper leaf surface, giving the spot a
target-like appearance. These circular spots can eventually affect the entire leaf.
Defoliation may occur, resulting in sunburn damage to the fruit and may lead to a
decrease in fruit soluble solids. Severely affected plants also are more susceptible
to heat and wind damage. Infected fruit develop circular, brown sunken lesions. Fruit
lesions may develop a dark olive to black-colored powdery mat on the fruit surface.
Undetected fruit infection at harvest can result in later losses in transit or storage.


CONDITIONS FOR DISEASE DEVELOPMENT:
Alternaria cucumerina survives in crop debris or on weeds and other cucurbit hosts. Disease
spread can occur with rain, irrigation, wind, cultivation, equipment and field workers.
This disease is favored by warm temperatures and moisture from dew, rain or overhead
irrigation. Infection can be initiated with two to eight hours of leaf wetness, but as the hours
of leaf wetness increase infection level increases. The frequency of rain and the length of
dew periods play a greater role in disease development than the volume of rain that falls.
CONTROL:
Implement a preventative fungicide spray program. Employ other cultural control measures
such as crop rotation (two years out of cucurbits), avoid overhead irrigation, thoroughly
incorporate crop debris following harvest and implement a hygiene program for personnel and equipment. For some crops (e.g., cucumber), resistant varieties are available.

ANTHRACNOSE

CAUSAL AGENT:
Colletotrichum orbiculare
DISTRIBUTION:
Worldwide
SYMPTOMS:
This disease is most commonly found on cucumber, melon and watermelon.
Symptoms on leaves begin as water-soaked spots which typically become yellowish in
appearance on cucumber and melon or dark brown to black on watermelon. These
spots eventually turn brown and may expand over the leaf surface. Foliar lesions are
not restricted by leaf veins and often have cracked centers. Infected petioles and stems
may develop shallow, elongated, tan lesions on melon but the lesions are less obvious
on cucumber. Stem lesions on melon can girdle the stem and cause plant wilting.
Infected fruit develop circular, sunken, blackish lesions where tiny fruiting bodies
(acervuli) may develop. Under humid conditions, the fruiting bodies produce conidia
which give the lesions a pinkish-salmon color, which is very characteristic of this
disease. When pedicels of young fruit become infected, the fruit may shrivel and abort.


CONDITIONS FOR DISEASE DEVELOPMENT:
Colletotrichum orbiculare can be associated with seed and infected crop debris.
Spread of this fungus can occur by splashing rain, overhead irrigation, insects, field
workers and equipment. Disease development is favored by warm, humid weather.
Optimum temperature for disease development is 24°C (75°F). Late infection of the
crop may result in fruit becoming unmarketable during storage, shipment or display.
CONTROL:
Implement a comprehensive preventative fungicide spray program. Employ other
cultural control measures, such as crop rotation (two years out of cucurbits), avoid
overhead irrigation, thoroughly incorporate crop debris following harvest and
implement a hygiene program for personnel and equipment. Use resistant varieties
when available.

BLACK ROOT OF CUCURBITS

CAUSAL AGENTS:
Phomopsis sclerotioides
DISTRIBUTION:
Asia, Europe and Canada
SYMPTOMS:
Black root of cucurbit is an important soil-borne pathogen that attacks cucumber,
although melon and bottle gourd are also susceptible. Young plants are stunted and
wilted. Roots are underdeveloped and rotted, exhibiting a blackened appearance due
to the formation of pseudosclerotia. Leaf senescence increases on affected plants,
resulting in appreciable yield losses. Foliar symptoms can appear similar to symptoms
caused by vascular wilt fungi (e.g., Fusarium, Verticillium).
CONDITIONS FOR DISEASE DEVELOPMENT:
Infection is favored by temperatures below 20°C (68°F). However, as temperatures
become warmer and/or water requirements increase, disease progression also
increases. Phomopsis scerotioides survival in soil is believed to be by means of
pseudostromata and pseudosclerotia. The potential for infection becomes greater
in fields where cucurbits have been grown year after year.
CONTROL:
Crop rotation has not been shown to be an effective control measure for black root of
cucurbits due to the longevity of pseudosclerotia in soil. Soil fumigation and/or steam
sterilization can help reduce fungal populations in soil, but grafting onto a squash
rootstock and/or moving production out of soil and into an artificial substrate offer the best options for control.

CERCOSPORA LEAF SPOT

CAUSAL AGENT:
Cercospora citrullina
DISTRIBUTION:
Worldwide
SYMPTOMS:
Cercospora leaf spot occurs on all cucurbits but is most common on watermelon,
cantaloupe, and cucumber. This disease is usually found only on the foliage, but if the
environment is suitable, symptoms may also occur on petioles and stems. The fungus
is not known to infect fruit. On watermelon, leaf spots manifest on young leaves as
small grey or white spots with black margins. Larger leaf spots which are circular to
irregularly circular develop on other cucurbits. The centers of these leaf spots are tan
to light brown becoming transparent and brittle with time. Lesions with surrounding
chlorotic halos may coalesce and turn leaves yellow. Although defoliation from the
disease may reduce fruit size and quality, serious economic losses are rare.
CONDITIONS FOR DISEASE DEVELOPMENT:
Conidia of Cercospora citrullina become airborne and may be carried great distances on
moist winds. Infection requires free moisture and is favored by temperatures of 26–32°C
(80–90°F). Cercospora citrullina survives on crop debris, volunteers and cucurbit weeds.
CONTROL:
Incorporate cucurbit debris into the soil to hasten its breakdown and /or remove
pruning debris entirely from the field. Rotate out of cucurbits for two to three years
and establish a fungicide spray program to help control this disease.

 

 

CHARCOAL ROT

CAUSAL AGENT:
Macrophomina phaseolina
DISTRIBUTION:
Worldwide
SYMPTOMS:
This soil-borne fungus can attack roots, stems or fruit in contact with the soil. On
seedlings, black, sunken cankers may appear on hypocotyls at the time of emergence.
These cankers may develop a concentric ring pattern, stunt affected plants and cause
wilt. When older plants are attacked, runners and crown leaves may turn yellow and
die. Typically, a water-soaked lesion will occur at the soil level and extend several
centimeters up the stem. Brown, water-soaked lesions are also symptomatic of fruit
infection. Amber-colored droplets of exudate may form within the affected area.
Eventually, the lesion dries up, turns light tan and microsclerotia form.
CONDITIONS FOR DISEASE DEVELOPMENT:
Macrophomina phaseolina is seed-borne and can be seed-transmitted. Infection and
disease development are favored by high temperatures. High soil salinity, drought
stress and heavy fruit load can predispose plants to infection. Microsclerotia in
infected host tissue and in soil are the primary propagules and survival structures.
Microsclerotia reside in the top 0–20 cm of soil and are able to survive from 2–15
years, depending on environmental conditions.
CONTROL:
Manage irrigation to avoid drought stress. If soil salinity is high, leach to reduce salt
buildup. Drip irrigation may result in higher soil salinity compared to furrow irrigation
if salinity of the irrigation water is moderate to high. Destroy or deep-plow all plant
debris at the end of the season. A three-year rotation out of cucurbits to a non-host
species may be beneficial. However, this strategy is not as effective at controlling
Macrophomina phaseolina as it is with other pathogens due to its wide host range
and the longevity of the microsclerotia.

 

DAMPING-OFF

CAUSAL AGENTS:
Pythium spp., Rhizoctonia solani, Acremonium spp., Fusarium equiseti, and
other fungi.
DISTRIBUTION:
Worldwide
SYMPTOMS:
Pre-emergence damping-off: Seeds may rot before germinating or seedlings
may die prior to emergence.
Post-emergence damping-off: Young seedlings develop a rot at the crown; later,
the tissue becomes soft and constricted and the plants wilt and fall over.
Pythium spp.: Seedlings turn dull green and cotyledons droop. Water-soaked lesions
develop on the hypocotyls at the soil line and seedlings wilt and collapse. Seedlings
may also rot in the soil before emergence.
Rhizoctonia solani: This fungus can infect seed, preventing germination. Symptoms
on young seedlings are similar to those caused by Pythium species. On older
seedlings, a depressed tan to reddish-brown dry lesion may be observed on the
hypocotyl.
Acremonium spp.: Symptoms develop 7–10 days after seedlings emerge. Infection
begins where the seed coat remains attached to the hypocotyl. This area turns a light
yellow-brown color. Within two to three days a dry red-brown rot develops, which may
lead to seedling death. Surviving seedlings remain stunted.
Fusarium equiseti: A dry, reddish-brown rot occurs on the hypocotyl. The fungus
causes both pre- and post-emergence damping-off.
Thielaviopsis basicola: Lesions begin as grey to reddish areas that almost
immediately turn coal black in color. In wet soil, a frosty coating may cover parts
of the black lesion.
CONDITIONS FOR DISEASE DEVELOPMENT:
Damping-off is generally most severe under conditions of high soil moisture and/
or compaction, overcrowding, poor ventilation and cool, damp, cloudy weather. In
addition, Acremonium root rot is favored by deep planting. Fusarium equiseti attacks
melons which have been seeded into cool, moist soil which later crusts around
or over the hypocotyls. Seedlings are most susceptible to damping-off prior to
emergence or within the first week after emergence. In greenhouses, incompletely
pasteurized soil is common source of damping-off fungi and overwatering commonly
exacerbates damping-off.
CONTROL:
Open Field: In addition to the greenhouse measures described above, avoid soil
compaction, prepare high beds to obtain better drainage and avoid long irrigation
periods. Acremonium root rot can also be minimized by shallow planting in dry soil
followed by irrigation.
Protected Culture: Ensure that substrate/soil consists of components which favor
drainage and aeration. Use a reputable substrate/soil supplier. Implement sanitation
measures for supplies and equipment. Manage irrigation practices to avoid long
periods of high soil moisture. Use high quality seed to help reduce damping-off.
Fungicidal soil drenches and seed treatments are available that help manage
damping-off. The use of a biological control agent (e.g., Trichoderma harzianum) has
been shown to be effective in controlling damping-off pathogens in various cucurbits.

 

DOWNY MILDEW

CAUSAL AGENT:
Pseudoperonospora cubensis
DISTRIBUTION:
Worldwide
SYMPTOMS:
Symptoms initially appear as small chlorotic lesions on older leaves, later appearing
on the younger leaves. The margins of these lesions are generally irregular on most
cucurbit species. However, on cucumber, lesion margins are defined by the leaf veins
which give an angular appearance to the lesions. When leaf surfaces remain wet for
extended periods, water-soaked lesions develop on the undersides of leaves. These
lesions can appear similar to those caused by Pseudomonas syringae pv. lachrymans.
In humid environments, sporangia form on the underside of leaves, giving the appearance
of a whitish-gray to purple fine downy growth. Lesions eventually coalesce
and become necrotic, but may continue to expand until the entire leaf dies. Severe
infection results in defoliation, stunting of plants and poor fruit development.
CONDITIONS FOR DISEASE DEVELOPMENT:
Survival of Pseudoperonospora cubensis between growing seasons is dependent on
living cucurbit hosts. Sporangia may be transmitted considerable distances between
fields by wind. Within fields sporangia are spread by air currents, splashing water,
workers and/or equipment. Fog, dew and frequent rain favor disease development,
which can be rapid when temperatures are moderate to warm. High temperatures
[>35°C (>95°F)] are not favorable for disease development, but disease development
may progress if night temperatures are cool [15–20°C (59–68°F)].
CONTROL:
Provide adequate spacing between plants to reduce canopy density. Grow varieties
with genetic resistance to Pseudoperonospora cubensis. Implement a preventative
fungicide spray program. Regional disease forecasting models have been used
successfully to predict onset of symptoms and to time spray applications for effective control of Pseudoperonospora cubensis.

 

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