White mold, with a focus on vegetables

Encyclopedia Article

Introduction and 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.

Signs and symptoms

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. 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.


Symptoms of white mold in pepper. Note discoloration in the stem (collar)

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 you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states 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 plant fomr the field below and collecting seedling at https://youtu.be/T3S2PZ8s8T8

Management

Plant health maintenance


Note white mycelia (threads of the fungus), present during humid weather or
 form after incubation in moist chamber for a week

Maintaining good airflow throughout the plant canopy can help prevent an environment conducive for development of 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 /orsclerotia 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. thi fungus has windborne ascospores, that can blow from surrounding soybeans.

Sanitation


S.  sclerotiorum overintering bodies (sclerotia) form in tissue 

Since sclerotia can survive in the soil for years, 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 of the 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 can vary depending on the plant’s growing season . Follow label directions and being cautious about pre-harvest intervals. For commercial vegetable see recommendation at  Midwest Vegetable Production Guide for Commercial Growers

S. sclerotiorum in culture, white micelia and black sclerotia
S. sclerotiorum in culture, white mycelia and black sclerotia

 

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 has the potential 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