For growers coping with white mold, caused by the fungus Sclerotinia sclerotiorum, the symptoms can look like the common name: a gauzy white coating, particularly when humid conditions prevail. But they can also appear as brown lesions on stems and stalks. Symptoms vary depending on environmental conditions and the crop. The defining symptom of white mold is the presence of black sclerotia, the survival bodies of the fungus, which ultimately germinate. These are very small, ranging in size from 1/4 to 1/2 inch and are found on stems or leaf surfaces.
White mold is also commonly known as “drop.” This name comes from another prominent symptom seen in many infected crops, such as cabbage or lettuce: the outer leaves drop, then the crown rots. Drop can be caused by other Sclerotinia species, not only spp. sclerotiorum, confusing things.
White mold has a large range of hosts impacting potatoes, tomatoes and beans like soybeans, cole crops and Cucurbita. Because white mold affects so many vegetables, and the sclerotia can live in the soil for many years, long rotations that avoid successive susceptible crops are a standard management practice.
Corn and grains are not susceptible and can be used in rotations. But proper rotation is difficult. Because white mold is also transmitted via airborne spores, infections can spread from nearby fields, even with the best rotation. Broadleaf weeds are also host to white mold, so weed control in and around fields is important.
Other methods to prevent infection include using ground coverings on soil to prevent germination of sclerotia; not planting in wet soils; maintaining adequate air flow across fields to promote drying of plants and to prevent humid microclimates; and removing and disposing of crop residues.
Plowing under residue will increase the inoculum and should be avoided. When deeply buried, the sclerotia will remain in the soil, without germination, for many years. If brought to the surface, they can germinate. Irrigating infected fields immediately post-harvest may kill sclerotia, reducing the infestation. Sclerotia on the soil surface are harmed by freezing and thawing or alternating wet and dry periods.
Chemical control can occur at seeding, transplant and thinning, Rutgers Cooperative Extension Specialist Andrew Wyenandt said. Fungicides with FRAC (Fungicide Resistance Action Committee) Code 7 are effective. Biological control, using another fungus – one which destroys the sclerotia – is also effective.
“The fungus needs time to attack,” Wyenandt said, and must be applied three to four months prior to planting.
Coniothyrium minitans, the fungus which parasitizes the white mold fungus, is also used to control drop caused by Sclerotinia minor. It does not have a wide range of hosts beyond Sclerotinia spp. It is diluted in water, applied to soil, and mixed into the top 2 inches. Alternatively, it can be applied to crop debris after harvest. It is approved for certified organic production.
Soil temperatures between 50 and 80 degrees Fahrenheit, along with wet conditions persisting for 10 days, are optimal for Sclerotinia sclerotiorum growth. Apothecia form from the sclerotia when conditions are conducive, and then release acospores into the air. The spores require wet plant surfaces to cause infection, but if conditions are not conducive, the spores – like the sclerotia in the soil – are patient. They can wait around on the plant, until the conditions- free water and conducive temperatures are met. Flowers are susceptible to infection as they provide nutrients needed for the fungus to thrive, as do any damaged plant tissues.
Sclerotia can, in some crops such as lettuce or cabbage, also infect a plant directly at the soil line. In other crops, such as beans, a lush canopy promotes white mold growth, and reduces the harvest. Stem infections in tomatoes are common, and cause the plants to collapse and die.
Dan Egel, Extension plant pathologist, Southwest Purdue Agricultural Center, focuses on diseases of greenhouse tomatoes, including white mold. Egel noted that white mold is a common problem in greenhouse or high tunnel tomato production, where high humidity and continuous planting of the same crop annually are risk factors.
The white mold spores can infect dying tomato flowers during cooler temperatures and take root in the fruit or the stem of the plant. Good air circulation in the greenhouse via pruning, decreasing planting density and increasing ventilation, can reduce the spread of infection. This is true for tomatoes growing in the field as well.
White mold does not spread directly from plant to plant, Wyenandt said. One plant can be infected, while its immediate neighbors are fine. Across the field, another plant may be infected.
“That’s because the sclerotia on the soil are germinating and affecting the plants in a random pattern,” he said. “One plant may have it and the one next to it may be fully healthy.”
Post-harvest, fruits infected with white mold will rot in storage. Control of white mold, which can affect many crops and cause widespread damage when conditions are right for survival of the fungus, requires a multifaceted approach. Growers can control cultivation factors to reduce the environmental susceptibility and should appropriately treat infected soils to lessen the survival of Sclerotinia sclerotiorum, the white mold fungus.