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Extension > Garden > Yard and Garden > Vegetables > Diseases of cucurbits

Diseases of cucurbits

Michelle Grabowski, University of Minnesota, Extension Educator

Common diseases of cucurbit crops in Minnesota

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M. Grabowski, University of Minnesota

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M. Grabowski, University of Minnesota

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M. Grabowski, University of Minnesota

Seed rot and damping-off

Cucurbit seeds need warm soils to germinate and develop properly (65F at 2" depth). Seeds that are planted in cold, wet soils are at risk of seed rot and damping off. These diseases are caused by several different fungi that live in the soil including Rhizoctonia, Pythium and Fusarium. Damping off fungi infect and rot both seeds and young seedlings. Infected seeds may not emerge from the soil. Seedlings may emerge with soft brown water soaked areas on the cotyledons (seed leaves). Stems may be thin, wire-like and unable to support even the small seedling. Fine cobweb like fungal mycelia may be visible growing on wet potting media and infected seedlings. Infected plants are unlikely to grow into a mature plant.

Management

Seed rot and damping off can be avoided by following several cultural practices. Do not plant cucurbit seeds outdoors until soils have completely warmed to 65F at a 2" depth. Keep beds moist but not water logged. In areas of Minnesota where the weather will not provide these conditions, pumpkin, squash and some melon seeds can be started indoors.

Seed plants in pots that are at least 2 inches wide and deep. Transplant seedlings once they have 2-3 true leaves and soil temperatures have reached 65F. Take extra care not to disturb the plants root system or damage the plant when transplanting cucurbit seedlings. Some seed companies provide 'treated seed' which is coated with a layer of fungicide that will help to prevent seed rot.

 

Powdery mildew

Powdery mildew, caused primarily by the fungus Podosphaera xanthii, infects all cucurbits, including muskmelons, squash, cucumbers, gourds, watermelons, and pumpkins. In severe cases, powdery mildew can cause premature death of leaves, and reduce yield and fruit quality.

Identification

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M. Grabowski, University of Minnesota

Powdery mildew is first evident as pale yellow leaf spots. White powdery spots can form on both upper and lower leaf surfaces, and quickly expand into large blotches which ultimately can cover entire leaf, petiole, and stem surfaces. When the majority of the foliage is infected, the plant is weakened and the fruit ripens prematurely.

Important biology

Powdery mildew infections are favored by humid conditions with temperatures around 68-81F. In warm, dry conditions, new spores are produced and easily spread the disease. Symptoms of powdery mildew are often first noticed mid to late summer in Minnesota. The older mature leaves are more susceptible and will be infected first. Spores produced in leaf spots are blown by the wind to infect other leaves. Under favorable conditions, powdery mildew can spread very rapidly, often resulting in complete leaf coverage.

Although powdery mildew primarily infects leaves and vines, infections occasionally occur on cucumber or melon fruit. Squash fruit are not directly infected. Regardless of direct infection of the fruit, fewer and smaller fruit are produced on infected plants. Reduced fruit quality occurs due to increased sunscald, incomplete ripening, poor storability, and poor flavor.

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M. Grabowski, University of Minnesota

Densely planted vines, plants crowded by weeds, plants in shaded sites, and over fertilized plants are more likely to be infected with powdery mildew.

Management

Viruses

Infection with virus commonly occurs on all cucurbit crops in Minnesota. These diseases are caused by several different viruses including Cucumber Mosaic Virus (CMV), Squash Mosaic Virus (SqMV), Zucchini Yellow Mosaic Virus (ZYMV), Watermelon Mosaic Virus 2 (WMV-2) and Watermelon Mosaic Virus 1 (WMV-1) also known as Papaya Ringspot Virus (PRSV).

Identification

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M. Grabowski, University of Minnesota

It is difficult to distinguish between the different viruses by symptoms alone. Symptoms vary depending on the crop, variety, age of the plant at the time of infection and in some cases weather. In addition it is common to find plants infected with more than one virus at the same time, often resulting in combined severe symptoms. The best way to distinguish between the different viruses is through analysis of a sample at the University of Minnesota Plant Diagnostic Clinic.

Virus infected leaves often have a mottling or mosaic pattern in shades of green and yellow. This mosaic can be very distinct and obvious or fairly subtle. Leaves are often distorted or deformed. They may be puckered, cupped under, have deep lobes, or appear thin and string-like. Young leaves often show the most severe symptoms and are frequently abnormally small. Growth on infected vines is typically stunted and in CMV infections, vines may wither completely.

The virus's affect on fruit varies depending on when the plant was infected. Early infections often result in no or very low fruit production. Later infection can result in fruit that is small, deformed and discolored. Fruit may have a mottled or mosaic pattern, ring spots or exhibit color break on all or part of the fruit. Melons infected with SqMV often lack netting at maturity.

Important biology

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M. Grabowski, University of Minnesota

All of the mosaic viruses can infect all of the cucurbit crops including melon, cucumber, pumpkin, summer and winter squash. In addition, many of the mosaic viruses can also infect common weeds. Squash mosaic virus will infect weeds in the Chenopodiaceae family like common lambsquarters, maple leaf goosefoot, Russian thistle and kochia. Watermelon mosaic virus infects legumes like clover. Cucumber mosaic virus can infect plants from over forty families, including vegetable crops like tomato, lettuce and spinach, flower crops like gladiolus, petunias, impatiens and rudbekia and a wide variety of weeds. All of the mosaic viruses can also infect weeds in the cucurbit family.

Most viruses that infect cucurbits are transferred by aphids with the exception of SqMV which is transmitted by both striped and spotted cucumber beetles. Aphids feed on virus infected plants (weeds or crop) and then transfer the virus when they feed on a new plant. Perennial weeds allow the virus to survive from season to season in a field. Viruses can also be transferred from plant to plant on the hands and tools of workers through infected sap. Squash mosaic virus can come in on infected seed. The other mosaic viruses are only rarely transferred on seed.

Once in the plant, viruses move systemically through the tissue infecting leaves, vines and fruit.

Management

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M. Grabowski, University of Minnesota

Common leaf spot and fruit rot diseases

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M. Grabowski, University of Minnesota

anthracnose-on-melon

M. Grabowski, University of Minnesota

Anthracnose on melon

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M. Grabowski, University of Minnesota

Anthracnose on watermelon

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M. Grabowski, University of Minnesota

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M. Grabowski, University of Minnesota

Anthracnose

Anthracnose is caused by the fungus, Colletotrichum orbiculare. This pathogen can attack all cucurbits but the most severe disease is seen on cucumbers, muskmelons, and watermelons.

Identification

All above ground plant parts can be infected. Symptoms vary depending on which cucurbit is infected.

Important biology

The Anthracnose fungus can survive in infected plant debris, in and on the seed. Spores are produced on infected leaves and fruit. These are easily spread by splashing rain, irrigation, on workers hands or equipment. The disease is favored by warm, moist environmental conditions and often is first seen mid to late season after the canopy closes. During wet summers, disease can be quite severe.

Management

Scab

Scab is caused by the fungus Cladosporium cucumerinum. The fungus infects cucumbers, melons, summer squash, pumpkins, and winter squash. Watermelon is very resistant to the disease and many varieties of cucumber that have resistance to scab are now available.

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C. Averre, NCSU Bugwood.org

Identification

Important biology

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M.A. Hansen, VAPISU Bugwood.org

Scab can be introduced into the field on infected seed or as spores carried on moist air currents. Fungal spores are then spread through the field by wind, insects, tools, and workers. Scab survives the winter in plant debris and once introduced, can reoccur season after season if management strategies are not implemented. Disease development is favored by cool (around 70F), moist weather.

Summer squash, pumpkins and cucumbers (varieties not specifically bred for disease resistance) are considered very susceptible to the disease. Fruit infections on these crops will profusely sporulate and rot will extend deep into the fruit.

Gourds and some winter squash (blue hubbard, buttercup) are moderately susceptible and will show symptoms on both leaves and fruit. Fruit infections, will not extend deep into the fruit however. Moderately resistant squash, like acorn and butternut, have few leaf spots and fruit infections will be raised tan corky spots instead of sunken craters. Fruit spots in these crops rarely produce fungal spores.

Management

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M.Grabowski, University of Minnesota

Gummy stem blight and black rot

Gummy stem blight and black rot are both caused by the fungus Didymella bryoniae. Gummy stem blight describes the leaf and stem infection stages of the disease. Black rot describes the fruit rot stage. Fruit rot may develop in the field or after the plant has been harvested.

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Clemson University, Bugwood.org

Identification

Important biology

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Clemson University, Bugwood.org

The fungus, D. bryoniae, enters through wounds. Plants that are infested with cucumber beetles, aphids and powdery mildew have higher incidences of black rot and gummy stem blight than pest free plants due to minor wounding caused by these pests.

The black rot fungus can be brought into the field in infected seed and survives from season to season in crop debris. New spores are produced in spring in response to wet weather. Spores are then easily spread throughout the field on splashing water. Moisture on the leaves and high humidity favor disease.

Management

Alternaria leaf blight

Alternaria leaf blight is caused by the fungus Alternaria cucumerina. This disease is most problematic on melon but can also occur on cucumber, pumpkin and squash. Alternaria leaf blight does not commonly infect fruit but can reduce yield and quality through reduced plant vigor and sunscald of exposed fruit.

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M. Grabowski, University of Minnesota

Identification

Important biology

Alternaria cucumerina can be carried long distance on wind currents and can be spread within the field by splashing water. Wet rainy weather favors diseases, and damage can be very severe in warm, wet conditions. The fungus survives from season to season in plant debris.

Management

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M. Grabowski, University of Minnesota

White mold

White mold is caused by the fungus Sclerotinia sclerotiorum. This pathogen infects a wide variety of vegetable crops including beans, carrots, tomato, cabbage, and lettuce. In the cucurbit family pumpkins and some varieties of winter squash are most severely affected by the disease. This disease does not cause leaf spots but infects both stems and fruit.

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W. Brown, Colorado State University Bugwood.org

Indentification

Important biology

The white mold fungus form hard black resting structures (about the size of a raisin or smaller in cucurbits) called sclerotia that allow them to survive in the soil and plant debris for 5 or more years. These sclerotia produce small mushroom-like apothecia that release spores under cool wet weather conditions.

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P. Bachi, University of Kentucky, Bugwood.org

White mold frequently infects pumpkin through the blossom end of the fruit, especially when the blossom remains attached after pollination. Stem infections often start where cotyledons are fading or where the plant has been wounded.

Management

Choanephora rot

Choanephora Rot, also known as blossom end rot or wet rot, is a disease most commonly found on summer squash under wet conditions. This disease has been seen occasionally on other cucurbits including pumpkin and vegetable marrow.

Identification

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M. Grabowski, University of Minnesota

Important biology

Choanephora rot is caused by the fungus Choanephora cucurbitarum. The fungus survives from season to season in crop debris and is spread to new flowers by insects, splashing water, or wind. Infection most commonly occurs on flowers, although the fungi can also infect through wounds on the fruit. Infected flowers are soft, rotted and quickly become covered with first white then purplish black fungal growth. In female flowers, the infection progresses into the fruit and results in soft water rot of the blossom end of the squash. The fungus thrives in wet conditions.

Management

Less common diseases of cucurbits in Minnesota

Bacterial wilt

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H. F. Schwartz, Colorado State University, Bugwood.org

Bacterial wilt is caused by the bacterium Erwinia tracheiphila. This pathogen can cause severe losses in cucumbers and muskmelons; squash and pumpkins are less severely affected. Watermelon is not affected. Bacterial wilt does not occur every year in Minnesota.

Identification

Important biology

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C. Averre, NCSU Bugwood.org

The bacteria overwinter in the gut of the striped and spotted cucumber beetles. Not all beetles carry the bacteria. Beetles that feed on infected plants pick up the bacteria. They then move to new plants, creating wounds through feeding. The bacteria are on the mouth parts or in the fecal matter of the beetle and enter the plant through the feeding wounds.

The bacteria multiply rapidly within the plant and plug the vascular tissue resulting in wilting of the vines. Once a plant is infected with bacterial wilt, there is no way to control the disease. The bacteria cannot be transmitted in seed, do not survive in soil, and only survives in plant debris for a short period of time. It cannot overwinter in Minnesota in plant debris.

Management

Angular leaf spot

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Clemson University, Bugwood.org

Angular leaf spot is a bacterial disease caused by Pseudomonas syringae pv. lachrymans. Although angular leaf spot can occur on any of the cucurbit crops, cucumber is less commonly affected due to the availability of resistant varieties.

Identification

Important biology

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C. Averre, NCSU Bugwood.org

Angular leaf spot thrives in warm humid conditions. The bacteria can infect all cucurbit crops and will infect all above ground parts of the plant including leaves, fruit and vines. When fruit are infected, the bacteria moves deep into the fruit and infects the seed.

The disease can be introduced into a field through contaminated seed. When humidity is high, a drop of clear to white sticky bacterial ooze forms on infections. These bacteria are moved from plant to plant on the hands and tools of workers, by insects, or by splashing water. The pathogen can survive in plant debris for over 2 years.

Management

Phytophthora blight

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P. Bachi, University of Kentucky, Bugwood.org

Phytophthora blight is caused by Phytophthora capsici. This pathogen can infect all cucurbit crops as well as peppers, tomatoes, eggplants, and infrequently beans. Infection is most common in squash and pumpkin. Phytophthora capsici infects every part of the plant including roots, crowns, leaves, vines and fruit. Phytophthora has only been reported in a few fields in Minnesota.

Identification

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H. F. Schwartz, Colorado State University, Bugwood.org

Important biology

Phytophthora capsici is an oomycete, often referred to as a water mold. Oomycetes are not true fungi, but are more closely related to certain kinds of algae. Phytophthora has swimming spores known as zoospores that can swim through films of water and saturated soils to locate a new host plant. As a result, new infections often appear in the direction in which water drainage occurs. Phytophthora thrives in warm (75-85F) wet conditions. Disease is commonly seen first in low lying poorly drained areas of the field, but can spread throughout the field if environmental conditions are right. Spores can be blown on windblown rain or carried in soil stuck to equipment that was used in an infested area.

Phytophthora overwinters in soil and plant debris. There are two different mating types of Phytophthora capsici. If only one mating type is present in a field, the pathogen can survive for 2 years. If both mating types are present, the fungus will create oospores, a hard walled resting structure that can survive 5 or more years. It is unknown if both mating types occur in Minnesota.

Management

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P. Bachi, University of Kentucky, Bugwood.org

Downy mildew

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G. Holmes, Valent Corp. Bugwood.org

Downy Mildew, caused by Pseudoperonospora cubensis, is an oomycete that is not a true fungus and is often referred to as a water mold due to the fact that it thrives in wet or very humid conditions. Downy Mildew can infect all cucurbits including cucumber, melon, pumpkin and squash. Although it can be a problem in field, hoop house and greenhouse conditions, it has not yet been reported in Minnesota.

Identification

Important biology

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Clemson University, Bugwood.org

Downy mildew does not create viable oospores (thick walled resting structures) that would allow it to survive in Minnesota's harsh winter. It cannot survive on plant debris and only grows on living plant tissue. This means that in order for downy mildew to occur in MN, spores must be blown in on air currents. In eastern states with similar conditions, downy mildew does not arrive until the end of the growing season, often in August. In recent years, however, downy mildew has been making its way to the Midwest earlier in the season. This early infection is possibly due to a change in the pathogen's biology or due to greenhouse production of cucurbits that is allowing the pathogen to overwinter on living plants.

Downy mildew can start an infection in a wide range of temperatures (41-86F) but is most severe from 59-68F. The pathogen needs moisture on the leaf surface in order to germinate and start a new infection. Under humid conditions downy mildew rapidly reproduces and spreads, resulting in severe crop damage. The pathogen can move on air currents, splashing water and on the tools and hands of workers.

There are several pathotypes or strains of downy mildew that attack different crops. It is not uncommon to see a healthy pumpkin field alongside a severely diseased cucumber field for this reason.

Management

Commercial growers should read and follow all application instructions in the Midwest Vegetable Production Guide.

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