WW-1172     2009  

Diseases of Cucurbits

Michelle Grabowski, University of Minnesota, Extension Educator

Copyright ©  2009  Regents of the University of Minnesota. All rights reserved.

Table of Contents

Common Diseases of Cucurbit Crops in Minnesota

Seed Rot and Damping-Off
Powdery Mildew
Viruses

Common Leaf Spot and Fruit Rot Diseases

Anthracnose
Scab
Gummy Stem Blight and Black Rot
Alternaria Leaf Blight
White Mold
Choanephora Rot

Less Common Diseases of Cucurbits in Minnesota

Bacterial Wilt
Angular Leaf Spot
Phytophthora Blight
Downy Mildew

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

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.

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

• Plant varieties with complete or partial resistance to powdery mildew.
• Apply fertilizer based on soil test results. Avoid over applying Nitrogen.
• Provide good air movement around plants through proper spacing, staking of plants and weed control.
• If susceptible varieties are being grown in an area where powdery mildew has resulted in yield loss in the past, fungicide may be necessary.
• Once a week examine 5 mature leaves for powdery mildew infection (in large plantings, repeat at 10 different locations in the field).
• Apply fungicides when a single spot of powdery mildew is first found.
• Home gardeners can apply sulfur products to both the upper and lower surface of the leaves.
• Commercial growers should refer to the Midwest Vegetable Production Guide for pesticide recommendations.

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

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

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

• Plant resistant or tolerant varieties whenever available. Resistance is virus specific and it is necessary to first determine which mosaic virus is causing disease in order to select appropriate viruses.
• Purchase clean seed from a reputable supplier. If saving seeds, do not collect seed from infected plants.
• Control weeds within and around the field. Especially focus control efforts on perennials weeds that may allow the virus to carry over from one season to the next.
• Manage aphids and cucumber beetles to keep populations low.
• If disease appears in a few plants, rogue and bury these plants to prevent further spread of the disease.
• Clean tools and workers hands with soap and water after working with infected plants. Reduce maintenance tasks that require handling of infected plants as much as possible.
• In greenhouse or hoop house production, use screening and other methods to exclude aphids. Eliminate weeds or other virus hosts within 350 feet of the greenhouse or hoop house. Avoid planting cucurbit field crops next to hoop house or greenhouse production areas.
• There are no pesticides that can be applied to reverse or limit the symptoms of viral infection.

Common Leaf Spot - Fruit Rot Diseases

Several different fungi can cause leaf spot and fruit rot diseases in cucurbits. These pathogens have very similar biology and therefore similar management practices.

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.

• Irregular brown leaf spots form on squash, melon and cucumber. The center of the leaf spot may drop out resulting in shot hole or ragged appearance. This is most common on cucumbers.
• Cucumber leaf spots often have a yellow halo.
• Watermelon leaf spots are smaller and dark brown to black.
• Sunken elongate stem infections can occur on cucumber and melon, but are not common on other cucurbits. Infections on melon often exude a reddish gum.
• Fruit infections are sunken black spots ¼ -1/2 inch across and ¼ inch deep. Spots may have fluffy white cotton like mycelia and sticky salmon colored spores during wet weather.

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

• Purchase clean seed from a reputable source. Do not save seed from infected plants.
• Some resistant varieties of cucumber are available.
• Rotate vegetables so 3 years go by before planting any member of the squash family in the same location.
• Use drip irrigation instead of overhead sprinklers if possible.
• Do not work in plants when wet.
• Remove and destroy infected vines at the end of the season in small gardens.
• In large fields till under crop residue at the end of the season.
• Several fungicides are registered for use against Anthracnose but may not provide adequate control if good coverage of fruit and leaves is not achieved. Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

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.

Identification  

• Leaf spots are white to gray and may have a yellow halo.
• As the infection ages, the center of leaf spot falls out. Leaves look shot holed or ragged.
• Cucumber, summer squash, zucchini, & pumpkin have sunken fruit spots that are covered with greenish black velvety fungal growth.
• Somewhat resistant winter squash have raised corky areas on infected fruit.
• The extent of fruit rot varies depending on resistance of variety.

Important Biology

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

• Purchase clean seed from a reputable source. Do not save seed from infected plants.
• Resistant varieties of cucumber are available and are the best management option for that crop.
• Rotate vegetables so 2 or more years go by before planting any member of the squash family in the same location.
• Use drip irrigation instead of overhead sprinklers if possible.
• Do not work in plants when wet.
• Remove and destroy infected fruit and vines at the end of the season in small gardens.
• In large fields till under crop residue at the end of the season to speed up decomposition.
• Several fungicides are registered for use against Scab. Preventative sprays are effective but are only necessary in fields with a history of scab. Squash sprays should start at bloom. Melon and pumpkin sprays should start when the vines begin to run.
• Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

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.

Identification

• Leaf symptoms start as browning at leaf edges.
• In pumpkin, cucumber and summer squash leaf infections develop into a brown wedge extending into the leaf tissue
•  In melon browning progresses into brown discoloration between veins or into circular tan to brown spots.
• Yellow halos may occur around leaf spots and older spots are often dry and cracked.
• Winter squash do not develop leaf or stem blight but may be infected with black rot.
• Tan lesions develop on infected stems, often close to the crown of the plant.  These lesions typically crack open and exude gummy-amber colored fluid.
• Raised black pimple like spore producing structures and dark gummy exudates are common within stem infections.
• Fruit rot initially looks water soaked and eventually turns completely black.
• Raised black pimple like spore producing structures are often seen in the black areas of fruit infections.
• Fruit infections may develop in the field or in storage depending on the crop.

Important Biology

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

• Buy clean seed from a reputable source. If saving seed, do not collect seed from infected plants.
• Rotate vegetables so 2 or more years go by before planting any member of the squash family in the same location.
• Use drip irrigation instead of overhead sprinklers if possible.
• Remove and destroy infected fruit and vines at the end of the season in small gardens.
• In large fields, till in infected plant debris at the end of the season to speed up decomposition.
• Use powdery mildew resistant varieties or spray to control powdery mildew.
• Use integrated pest management practices to control cucumber beetles and other insect pests.
• Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

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.

Identification

• Mature leaves near the crown of the plant are often infected first.
• Leaf spots start as small brown spots, often with a yellow halo, and grow into irregular brown spots (up to ¾”).
• Leaf spots sometimes develop a target-like pattern of rings.
• Severely infected leaves turn brown, curl upward, wither and die.
• Fruit are not commonly infected but can suffer from sunscald due to leaf loss.

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

• Rotate vegetables so 3 or more years go by before planting any member of the squash family in the same location.
• Use drip irrigation instead of overhead sprinklers if possible.
• Do not work in plants when wet.
• Remove and destroy infected plants at the end of the season in small gardens.
• In large fields till under crop residue at the end of the season.
• Several fungicides are registered for use against Alternaria Leaf Blight. Preventative sprays are effective but are only necessary in fields with a history of Alternaria Leaf Blight.  
• Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

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.

Identification

• Infected fruit and stems first appear water soaked.
• Fluffy white cottony fungal growth is commonly seen on infected areas.
• Small hard, black raisin-like fungal structures, known as sclerotia, eventually develop; embedded in the cottony mold.
• Fruit are often infected through the blossom end and become rotted and watery. Sclerotia may be found inside these rotted fruit.
• Rot may occur in the field or post harvest

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.

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

• No resistant varieties are available, but plants with an open growth habit have less disease than plants with dense leaf coverage.
• Use wide row spacing, control weeds and plant rows parallel to prevailing wind to promote rapid drying of plant leaves.
• If only a few plants are infected remove them from the field before sclerotia form and bury them deeply.
• Do not compost infected fruit or other plant material.
• Crop rotation to corn or other grasses to limit or reduce pathogen in the soil.

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

• Flowers are covered with first white then purplish black fungal growth
• The blossom end of the squash is soft, rotted and covered in fluffy purplish black fungal growth’

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

• Fungicides are ineffective against Choanephora rot because new susceptible flowers open every day.
• Rotate out of cucurbits.
• Avoid overhead irrigation.
• Space plants to provide adequate air movement in the field to quickly dry flowers and fruit.
• Raised plant beds and plastic mulch may be of help to limit fruit contact with moist soil and reduce moisture in the lower plant canopy.

Less Common Diseases of Cucurbits in Minnesota

The following diseases are less common in Minnesota. Many occur only in years where weather conditions favor disease and are then not seen again for several years. Other diseases only occur in certain fields where the disease has been introduced.

Bacterial Wilt

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

• Leaves first appear dull green, wilt during the day, and recover at night.
• Leaves eventually yellow and brown at the margins, completely wither and die.
• Wilt progression varies by crop. Cucumbers and melons wilt and die rapidly. Pumpkins take up to 2 weeks to completely wilt. Summer squash may continue to produce several for several weeks even when infected.
• Wilt progresses down the vine until entire vine is wilted or killed.
• Striped or spotted cucumber beetles will be present in the garden.
• If infected vines are cut close to the crown of the plant and the cross sections pressed together, thread like strands of bacterial ooze can be seen when the two halves are gently pulled apart again.

Important Biology

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

• Managing cucumber beetles provides the most effective control of bacterial wilt.
• If disease appears in a few plants, rogue and bury these plants to prevent further spread of the disease.
• Pesticides will not help in managing a cucurbit plant infected with this bacterial disease.

Angular Leaf Spot

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

• Leaves develop small, angular, brown or straw-colored spots with a yellow halo.
• Leaf spots dry and drop out, leaving irregularly shaped holes in the leaves.
• Water soaked to tan small circular spots on fruit.
• Bacterial soft rot often develops after fruit spots and rot the entire fruit.
• Sticky drops of whitish liquid form on the underside of the leaf when wet, dry to a crust when dry.

Important Biology

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

• Many resistant varieties are available for cucumbers and are the best form of control for this crop.
• Buy clean seed from a reputable source. If saving seed, do not collect seed from infected plants.
• Rotate vegetables so 2 or more years go by before planting any member of the squash family in the same location.
• Use drip irrigation instead of overhead sprinklers if possible.
• Do not work in plants when leaves are wet.
• Remove and destroy infected fruit and vines at the end of the season in small gardens.
• In large fields, till in infected plant debris at the end of the season to speed up decomposition.
• Copper can slow the spread of disease if the infection is caught early. Sprays are not effective once disease is severe. Sprays do not need to be continued if dry weather persists beyond 2 weeks.

Phytophthora Blight

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

• Large irregular brown spots form on leaves.
• Stem and leaf petiole lesions are light to dark brown, water soaked and irregular.
• Leaves wilt and the entire plant may collapse if root and crown rot occurs. Infected roots and crowns are black. Plants pull up easily from the soil due to root loss.
• Fruit develop soft, water-soaked rot. Infection may start where the fruit contacts the soil, where the stem connects to the fruit or as a random circular spot.
• Infected fruit are soft, easily punctured and often collapse.
• Infected fruit are covered with white powder sugar like fungal growth.

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

• Plant in well drained fields. Used raised beds to improve drainage. Do not work in fields when soils are wet to avoid compacting soil.
• Avoid planting susceptible crops in fields with a history of disease.
• Rotate out of cucurbit and solanaceous crops for a minimum of 3 years.
• Remove and destroy infected fruit and vines in small gardens.
• In large fields, till in a small area of infected plants plus a border of healthy plants. Clean equipment thoroughly afterwards.
• Clean tools after working with Phytophthora infected plants.
• Fungicide sprays can help to prevent disease. Commercial growers should refer to the Midwest Vegetable Production Guide for specific fungicide recommendations.

Downy Mildew

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

• Pale green to yellow spots form on upper surface of leaves, and later turn brown.
• Leaf spots are angular bounded by leaf veins. This is most distinct in cucumber.
• Dark purplish grey fuzz forms on underside of the leaf in high humidity.
• In wet or very humid conditions, disease progresses rapidly. Leaf spots grow together and entire leaves turn brown. Often appearing as if they were killed by frost.
• On watermelons, an exaggerated upward leaf curling is common.

Important Biology

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

• Plant resistant varieties when available. Cucumber varieties with moderate to high level of resistance are available, moderate to low levels of resistance is available for melons. Some resistance available in pumpkins and winter squash but this tends to be specific for certain pathotypes of Downy Mildew and may not be effective in all cases.
• Use drip irrigation and wide row spacing to promote leaf drying and encourage good air movement around the plants.
• Monitor plants for symptoms of disease especially from August through harvest.
• If the disease is found in a home garden, plants should be immediately removed and destroyed to prevent the spread to other plants.
• Fungicides are effective if applied before disease becomes severe. Both contact and systemic fungicides are registered for use against downy mildew. Systemics are more effective if weather conditions are conducive to disease and the host is very susceptible. However using the same systemic fungicide repeatedly can result in fungicide resistance. It is important to rotate systemic fungicides or tank mix with a contact to avoid this.

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

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