White mold is a widespread plant disease that impacts numerous crops globally. It can significantly reduce agricultural yields, especially under specific environmental conditions. Understanding its symptoms and life cycle is important for effective management strategies.
Identifying Sclerotinia White Mold
The initial signs of Sclerotinia white mold often appear as water-soaked lesions on stems, leaves, or pods. These lesions quickly expand, becoming soft and rotten, particularly in moist conditions. As the disease progresses, a dense, white, cottony fungal growth, known as mycelium, becomes visible on affected plant tissues. This white growth is a clear indicator of the disease.
Within this white growth, or sometimes inside infected stems and pods, small, hard, black structures begin to form. These structures, called sclerotia, resemble small rodent droppings and are a definitive diagnostic feature of Sclerotinia white mold. Infected stems can also become bleached and stringy, leading to wilting, lodging, and eventually plant death.
How Sclerotinia White Mold Develops and Spreads
The fungus responsible for white mold, Sclerotinia sclerotiorum, survives in the soil for extended periods, often several years, as hard, black sclerotia. These structures serve as resting bodies, enduring unfavorable conditions until suitable environmental cues trigger their germination. When conditions become favorable—cool temperatures between 15 to 21 degrees Celsius, high humidity, and moist soil—the sclerotia germinate.
Sclerotia can germinate in two ways: by producing mycelium directly, which can infect plants at or below the soil line, or, more commonly, by developing small, mushroom-like fruiting bodies called apothecia. These apothecia release microscopic airborne spores called ascospores. These ascospores are then dispersed by air currents and primarily infect plants through senescing (aging or dying) flower petals, or other non-living plant tissues. Once the ascospores colonize these tissues, the fungus produces mycelium that invades healthy plant parts, spreading internally and causing rot. The pathogen has a broad host range, affecting over 400 plant species, including many broadleaf crops, vegetables, and ornamental plants, but not corn or small grains.
Managing Sclerotinia White Mold
Effective management of Sclerotinia white mold involves an integrated approach combining multiple strategies. Cultural practices play a significant role in minimizing disease development. Crop rotation with non-host crops, such as small grains or corn, for 3-5 years can help reduce sclerotia in the soil. This prevents new sclerotia from forming if no susceptible hosts are present.
Sanitation is another important cultural control method. Removing infected plant debris after harvest reduces the amount of sclerotia returned to the soil, lowering the inoculum for future seasons. Proper plant spacing and avoiding overly dense canopies improve air circulation, which reduces humidity and leaf wetness—conditions that favor fungal growth. Optimizing irrigation methods, such as using drip irrigation instead of overhead watering, helps keep foliage dry.
Biological control offers an environmentally friendly option, utilizing beneficial organisms to suppress the pathogen. The fungus Coniothyrium minitans, for example, is commercially available and can parasitize sclerotia in the soil, reducing their viability. This biological agent is most effective when applied several months before disease onset, allowing sufficient time for it to colonize and degrade the sclerotia.
Chemical control, using fungicides, can also be part of a comprehensive management plan. Fungicides are more effective as a preventive measure, applied before or at the onset of flowering when plants are most susceptible to infection. While some crop varieties may exhibit partial resistance or tolerance, highly resistant commercial varieties are not widely available for all crops, making integrated strategies particularly valuable.