White mold, or sclerotinia, is a plant disease caused by fungal infection. The fungus Sclerotinia sclerotiorum is the most common culprit, but some other Sclerotinia species also infect plants. This disease is also sometimes called stem rot, stalk rot, wilt, or head rot. The fungus is most common in temperate climates, though it can grow in a wide variety of ecological environments. Sclerotinia sclerotiorum is one of the most successful plant pathogens, and it affects some of our most important food crops.
Roughly 408 different broadleaf plant species, including soybeans, sunflowers, peanuts, lentils, chickpeas, and canola, can be infected with white mold. Many weeds, including thistle and wild mustard, are vectors for the disease. Sclerotinia can be introduced into a healthy crop in many ways. In addition to being spread by infected weeds and other plants, it can be spread into adjacent crops through soil, wind, and irrigation water or rainwater.
Sclerotinia sclerotiorum has an overwintering structure called the sclerotium, which consists of a protective black rind covering a light-colored medulla. The sclerotium remains in the soil during the winter and germinates in the spring through one of two methods. It may form small mushrooms that release spores into the air to infect plants, or it may form a mycelium, the vegetative portion of a fungus, that directly affects the roots of nearby plants.
This fungus is a necrotrophic organism, meaning that it feeds on dead or decaying tissue. When it infects plants, it kills plant tissue before the mycelium begins growing on the plant. A common symptom of an infection is water-soaked spots on the plant, which eventually become covered with a white, cottony layer and become soft and slimy.
Some infected plants, on the other hand, develop dry lesions around the base that cause the plant to die. The plant's leaves also dry out, becoming yellow, then brown, before they die. Plants with white mold typically wilt and collapse or drop their leaves, spreading more of the fungus into the soil and causing nearby plants to become infected.
White mold is very difficult to control. It is not known how long Sclerotinia sclerotiorum can remain active in a field, but moisture and heat in the soil both contribute to its growth. Some methods for controlling the spread of the mold include the introduction of fungal parasites into the soil, the use of fungicides, crop rotation, and sanitation practices.