Already wilting due to white mold, this high tunnel tomato plant will likely collapse before the fruit ripens.
PHOTO COURTESY OF JUDSON REID, CORNELL VEGETABLE PROGRAM.

Diseases caused by the white mold pathogens, Sclerotinia spp., attack an array of vegetable plants as well as canola, flowering bedding plants, legumes, stone fruits, sunflowers and tobacco. The pathogens thrive in cool, moist conditions.

The soilborne fungus S. sclerotiorum affects vegetables including beans, carrots, cole crops, peas, pumpkins and tomatoes. With such a range of hosts, crop rotation is an unlikely option. In addition, a legume cover crop should not be considered, further limiting cultural controls. What’s more, the fungus can live five years in the soil.

A white mold infection is characterized by white, cottony mycelial growth on the plants. In the presence of moisture, water-soaked lesions appear, enlarge, and quickly become colonized with the white fungal growth. Fruit and pod rot, stem rot, wilting leaves and a soft, watery decay lead to the plant’s collapse. With beans in particular, the blossoms are typically the first part to succumb to the white fungus. As the petals fall, any plant that contacts them becomes infected.

S. sclerotiorum produces hard, black, pellet-like fungal structures called sclerotia. After the plants decompose, these sclerotia fall to the soil, where they survive the winter. They will afflict susceptible crops when favorable environmental conditions return.

If the soil is very wet for about 10 days and the temperature reaches 50 to 80 degrees Fahrenheit, the sclerotia germinate tiny mushroom-like bodies called apothecia. These apothecia produce millions of airborne spores. When the relative humidity drops, the spores are forcibly ejected into the air. The wind carries the spores, with some landing on plant surfaces. The spores germinate when a film of water remains on the plant surface. At 75 degrees Fahrenheit, germination occurs in about 16 hours, but at lower temperatures germination takes two or three days.

Although rotation with vegetable crops is limited, rotation with corn and small grains should be beneficial. Numerous other cultural management controls can help minimize white mold losses. Follow practices that promote the drying of soil and plant surfaces. Avoid high plant populations. Well-spaced rows should be planted in the direction of prevailing winds to speed air drainage and facilitate moisture reduction on the soil and the foliage. Shady locations are undesirable. Well-drained soils help. Plants and soil in raised beds dry more quickly. Avoid fields surrounded by dense woods or even windbreaks that hamper air circulation, especially in small fields. Obviously, do not plant in fields with a history of white mold.

Irrigate only when necessary, preferably with drip irrigation. If overhead irrigation is unavoidable, reduce the number of times that leaves remain wet for 12 to 24 hours, and water early enough in the day for the sun to dry foliage before nightfall.

Judson Reid, extension vegetable specialist at Cornell University, has conducted considerable research with greenhouse and high tunnel vegetable production, especially tomatoes. He said, “I think there is a higher amount of white mold in soil-based greenhouses and high tunnels because of the higher humidity as well as increased pruning versus field-grown.”

Reid’s advice for pruning: “Keep wound size as small as possible to prevent providing an entry point for the pathogen.” He also suggested using clips instead of twisting twine around stems when trellising.

Plastic, straw and other mulches can provide a barrier to spore dispersal and serve to inhibit sclerotia germination.

Infection can occur during production or postharvest. Destroy any infected plants immediately, and handle carefully to avoid spreading the sclerotia. Destroy end-of-season plant debris quickly.

Since the sclerotia can persist in the top 1 or 2 inches of the soil for five years, adapting the tillage method can be complex. A moldboard plow may prevent germination, but subsequent plowing in another season can bring the sclerotia back to the soil surface. Any tillage operation can also contribute to sclerotia dispersal.

Fungicide effectiveness often depends on timing. For example, in bean production, application at peak bloom time is typically recommended. Environmental conditions play a significant role in disease development and severity. Many states or regions maintain disease forecasting systems, which report the occurrence of diseases and the weather patterns conducive to particular diseases. Local extension educators can advise growers of these services.

According to Amanda Gevens, University of Wisconsin assistant professor and extension plant pathologist, thiophanate-methyl (Topsin), boscalid (Endura) and iprodione (Meteor, Iprodione) have provided good control. She also noted that the newer reduced-risk fungicides – fluazinam (Omega), cyprodinil and fludioxonil (Switch), fluxapyroxad and pyraclostrobin (Priaxor), and penthiopyrad (Fontelis) – can help manage the disease.

White mold can be particularly damaging to all types of beans. Wisconsin leads in snap bean processing. The guide “Commercial Vegetable Production in Wisconsin” (http://learningstore.uwex.edu/assets/pdfs/A3422.PDF) indicates which products to use with edible pod and dried shell beans, and tabulates days to harvest and season application limits. It notes that vines treated with iprodione and thiophanate-methyl cannot be used for grazing or livestock feed.

Guides from the Fungicide Resistance Action Committee (FRAC) group fungicide chemistries by class, mode of action and resistance risk. For white mold control, the publication for the mid-Atlantic region lists Endura for cole crops and peas, and Fontelis for cole crops and high tunnel and greenhouse production. The guide is available at http://njveg.rutgers.edu/html/cp-FRAC.html.

The biological fungicide Contans contains the beneficial fungus Coniothyrium minitans. It can be applied in postharvest fields before incorporating the crop debris. Alternatively, it can be applied three to four months before planting to allow the active agent to reduce the sclerotia inoculum in the soil. It should be incorporated 1 to 2 inches deep in the soil. The field should not be plowed before seeding so as to prevent any untreated sclerotia in the lower soil levels from infesting the upper soil layer. The Contans label includes various vegetables, including carrots, celery, peas and tomatoes. With all pesticides, adhering to the label is imperative.

Dorothy Noble is a writer/researcher specializing in agriculture.