Sclerotinia stem rot, also known as white mold, is a widespread fungal disease affecting numerous crops globally. It significantly reduces crop yields and quality.
The Disease and Its Identification
Sclerotinia stem rot is caused by the fungus Sclerotinia sclerotiorum, which infects over 400 plant species. Initial symptoms include water-soaked lesions on leaves, stems, or pods. These lesions expand rapidly, causing affected plant parts to wilt, turn pale green or yellow, and eventually die.
A telltale characteristic is white, fluffy fungal growth (mycelium) on infected tissues, particularly noticeable under humid conditions. As the disease progresses, hard, black survival structures called sclerotia develop within or on plant tissues. These sclerotia resemble small, irregularly shaped pellets and can also be found inside hollow stems. Infected stems may become bleached and shred easily.
Disease Cycle and Environmental Factors
The fungus’s life cycle revolves around sclerotia, its overwintering survival structures, which can persist in the soil for several years. Under cool to moderate temperatures (around 15 to 21 degrees Celsius) and sufficient soil moisture, these sclerotia germinate.
Germinating sclerotia produce small, mushroom-like apothecia near the soil surface, which release millions of microscopic airborne ascospores. While ascospores can travel to nearby fields, they cannot directly infect healthy plant tissue. They require a nutrient source, such as senescing (aging or dying) flower petals, to colonize and initiate infection. The fungus then spreads from these colonized petals to healthy plant tissues like stems and pods.
Moisture plays a significant role in disease development. Prolonged periods of leaf wetness, high humidity, and consistent soil moisture are conducive to the germination of sclerotia, the production of ascospores, and the subsequent infection of plants. Dense crop canopies create a humid microclimate that traps moisture, further promoting disease progression. The fungus can also spread through infected crop residue or contaminated soil, and plant-to-plant contact can facilitate local spread within a dense canopy.
Management Strategies
Managing Sclerotinia stem rot requires an integrated approach to reduce disease incidence and impact. Crop rotation with non-host crops like cereals or grasses for several years helps reduce soil sclerotia. Deep plowing (over 10 cm) can delay sclerotia germination and apothecia production, though it may not eliminate them entirely.
Cultural practices include optimizing plant spacing and canopy density. Wider row spacing and appropriate plant populations improve air circulation within the canopy, reducing humidity and leaf wetness, which are conditions favorable for the fungus. Controlling broadleaf weeds within and around fields is also important, as many weeds can serve as alternate hosts for Sclerotinia sclerotiorum, allowing the fungus to persist even in non-host crop years. Cleaning farm equipment after working in infested fields prevents the mechanical spread of sclerotia to new areas.
While complete resistance is uncommon, some crop varieties offer partial resistance or tolerance. Biological control methods, using beneficial fungi and bacteria like Coniothyrium minitans and specific Bacillus and Pseudomonas strains, can reduce sclerotia populations or inhibit fungal growth. For instance, soil application of Coniothyrium minitans reduces viable sclerotia in the topsoil.
Chemical control, primarily through fungicides, is a significant tool. Fungicides are most effective when applied strategically, often during the flowering period of susceptible crops, as this is a critical window for infection. Proper coverage and understanding the fungicide’s persistence are important for efficacy. An integrated approach combining these strategies helps mitigate the impact of Sclerotinia stem rot.