Sclerotinia sclerotiorum is a destructive plant pathogen, commonly known as white mold. It impacts a vast array of plant species globally. It can lead to significant economic losses, with yield reductions often exceeding 20-35%, and reaching 80-100% in favorable environments. Its widespread distribution, broad host range, and management challenges contribute to the substantial damage it causes to various crops.
Understanding the Fungus
Sclerotinia sclerotiorum is classified as a necrotrophic fungus, meaning it actively kills host plant tissues to acquire nutrients. This pathogen has an exceptionally broad host range, infecting over 400 plant species. Common hosts include many vegetables like beans, carrots, lettuce, and tomatoes, as well as field crops such as soybeans, sunflowers, and canola, and various ornamental plants.
The life cycle of S. sclerotiorum centers around its survival structures called sclerotia. These small, black, irregularly shaped bodies allow the fungus to survive in soil for extended periods. Under cool, moist conditions (10-20°C) and adequate moisture, sclerotia germinate carpogenically. This germination produces small, cup-shaped, mushroom-like structures known as apothecia.
Apothecia release ascospores into the air, which are dispersed by wind. These ascospores are the primary source of initial infection, landing on susceptible plant tissues, such as senescing flowers or damaged areas. Once an infection is established, the fungus rapidly grows, forming a white, cottony fungal mass called mycelia on the infected plant parts. This mycelial growth leads to soft rot and decay, and as the plant tissue dies, new sclerotia form within the infected areas, completing the disease cycle.
Recognizing the Symptoms
The initial signs of Sclerotinia sclerotiorum infection appear as dark green, water-soaked lesions on stems, leaves, or pods. These lesions are common in areas of high humidity or where plant tissues have been damaged. As the disease progresses, these water-soaked spots quickly expand, causing wilting of the affected plant parts. If the fungus girdles the stem, the entire plant may wilt and eventually die.
A distinctive characteristic of white mold is white, cottony fungal growth, or mycelia, on infected plant tissues. This growth is especially noticeable during humid conditions. A definitive diagnostic feature is the formation of black, hard sclerotia, which develop within the white fungal growth or inside infected stems and pods. These sclerotia vary in shape and size. The disease can affect various plant parts, including stems, leaves, flowers, pods, and fruits, often resulting in a soft, watery rot.
Managing White Mold
Managing Sclerotinia sclerotiorum involves a combination of strategies to reduce fungal populations and minimize infection. Cultural practices are foundational for prevention. Crop rotation with non-host crops, such as corn, small grains, or grasses, helps to reduce the number of sclerotia in the soil. Removing and destroying infected plant debris and weeds, which can also host the pathogen, is another important sanitation measure.
Improving air circulation within the plant canopy is also beneficial. This can be achieved through proper plant spacing, pruning, and avoiding overhead irrigation, to reduce humidity around the plants. Ensuring good soil drainage can also create conditions less favorable for sclerotia germination. Regarding tillage, deep plowing can bury sclerotia deeper in the soil, potentially reducing their viability, while shallow tillage might bring them closer to the surface where they can germinate.
While complete immunity to S. sclerotiorum is uncommon, some plant varieties exhibit partial resistance or tolerance. Partially resistant germplasm has been identified in canola and certain wild Cicer accessions, which can be incorporated into breeding programs to enhance resistance. Utilizing these partially resistant cultivars as part of an integrated management plan can help reduce yield losses and lower inoculum levels over time.
Chemical control, through fungicides, can be an effective management tool, though timing is crucial. Fungicides are applied preventatively, often at flowering, to protect susceptible tissues from ascospore infection. In canola, fungicides are applied between 20% and 50% bloom, with earlier applications often yielding better economic returns. Following label instructions precisely for application rates and intervals is important for optimal efficacy and to minimize the risk of fungicide resistance development.
Biological control methods offer another approach, utilizing beneficial microorganisms that can parasitize or compete with S. sclerotiorum. Coniothyrium minitans and Trichoderma species have shown promise in degrading sclerotia in the soil. Coniothyrium minitans can parasitize sclerotia and produce antifungal substances, effectively reducing disease incidence. Trichoderma species can also exhibit hyperparasitic activity against mycelia and sclerotia, leading to their degradation.
For smaller areas, soil solarization can be a viable method. This technique involves covering moist soil with clear plastic sheeting during hot summer periods to trap solar radiation and significantly raise soil temperatures. Temperatures can reach 48.9°C at shallow depths (e.g., 5.1 cm), which can kill a high percentage of sclerotia. The effectiveness of solarization increases with longer durations and higher temperatures, with summer applications being more impactful than spring or fall.