White mold, with a focus on vegetables

Need to know: 

• Fungus has a wide range of hosts including tomato, pepper, beans, crucifers/cole crops. 
• White, cottony mycelial growth can be observed on infected plants. 
• Optimal conditions are temperatures between 50-70 degrees F and moisture. 
• Manage by scouting for disease, sanitation, and practicing good plant health maintenance.  

Overview

White mold is caused by the fungal pathogen Sclerotinia sclerotiorum and can be devastating to a wide range of plant hosts. This fungus is a generalist, meaning that it has a wide host range. Hosts for S. sclerotiorum range from the large-scale production soybean to gardener-cultivated vegetables including tomato, pepper, beans, crucifers/cole crops (cabbage, Brussels sprouts, etc.) and more.

Symptoms of white mold

The primary symptom seen on plants is an expanding discolored (bleached-to-greyish) lesion on the affected plant part. Lesions formed on stems can result in girdling of the stem and subsequent colapse of the plant above the point of infection. Infected petioles can result in defoliation. Fruits and pods that are infected by the fungus will be unmarketable, unsalvageable, and inedible, reducing yields and profit.

Signs of white mold

This fungus produces white, cottony mycelial (fungal) growth on plant parts, mostly stems, petioles, crowns, or fruits/pods, under conducive weather conditions. Furthermore, as the fungal growth progresses, small (less than 1-2 cm), irregularly shaped, black fungal masses called sclerotia can be found on or in the plant.

Disease cycle

Sclerotinia sclerotiorum overwinters as sclerotia in the soil for many years. Under ideal conditions, extended periods of cool temperatures (50-70ºF), and moisture, sclerotia within 2 cm of the soil surface germinate to form sexual fruiting structures called apothecia. Apothecia release ascospores, the infection unit of this fungus.  The ascospores are disseminated by the wind and germinate when they land on an ideal host in a conducive environment. Disease development by S. sclerotiorum  is optimal during cool (less than 80ºF) and wet/humid weather.

Type of sample needed for confirmation/diagnosis

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if your plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each state's diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa, please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us.

Samples suspected of white mold should be submitted, including plant parts that show symptoms of concern. Photos of the affected plants can help diagnosticians get a clearer picture of the distribution of the disease. The sample should be mailed early in the week, no later than Tuesday, to ensure arrival in the same week. Otherwise, the sample may rot, or the pathogen may die during transit.  See our videos on collecting full plants from the field below and collecting seedlings at https://youtu.be/T3S2PZ8s8T8

Management

Plant health maintenance

Maintaining good airflow throughout the plant canopy can help prevent an environment conducive to developing disease caused by S. sclerotiorum. This can be achieved by sufficient row spacing and pruning (when applicable). Additionally, consideration towards watering practices, if any, can help prevent disease development. When flowers on plants are senescing (natural aging) before fruit formation, plants should be watered before noon during the growing season, which allows plants to dry before nightfall and prevents extended periods of wetness on tissues that are easiest to colonize by ascospores.

Scouting for disease

Scouting for this disease in a backyard garden or the field can be as simple as examining plants exhibiting blighting symptoms for discolored lesions or areas near the crown or the collar of the plant. Additionally, white, cottony mycelial growth and /or sclerotia are maybe present on or around the lesion.

In larger-scale crop production, such as pepper to tomato fields, white mold is not expected to occur in a uniform distribution throughout the field. The disease incidence will be found in an aggregated pattern throughout the field since S. sclerotiorum is a soilborne fungus. This fungus has windborne ascospores that can blow from surrounding soybeans.

Sanitation

Since sclerotia can survive in the soil for years, the removal of infected plant debris can help prevent the future prevalence of the disease by removing or reducing the inoculum source. If plant debris cannot be removed, incorporation of plant debris into the soil at depths greater than 3 cm (1-1.2 inches) can also help, as sclerotia do not produce apothecia at depths greater than 2 cm.

Another sanitation tactic that can help prevent white mold disease development is the management of alternative weedy hosts (i.e., weed management). Since S. sclerotiorum is a generalist, many uneconomical hosts include weedy species. By using sufficient weed management practices, reservoirs for the fungus are minimized, thus reducing the initial numbers and source of the pathogen.

Chemical management

Fungicides will not work once you observe disease symptoms; a preventative, not curative, measure for S. sclerotiorum infection. Keep in mind pollinator health; read this article. Repeated applications may be necessary and vary depending on the plant’s growing season. Follow label directions and be cautious about pre-harvest intervals. For commercial vegetables see recommendations in Midwest Vegetable Production Guide for Commercial Growers

A biological control agent consisting of spores of the mycoparasitic fungus Coniothyrium minitans can reduce S. sclerotiorum inoculum when incorporated into the soil. This agent can lessen the severity of the disease in areas of high disease incidence.

Related articles  White Mold on Tomato and White Mold on Ornamentals

For full information on white mold management in soybean visit White Mold - Crop Protection Network.

By Chelsea Harbach and Lina Rodriguez Salamanca (former plant disease diagnostician at the PIDC)

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective in managing plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.