Sclerotinia

Infected stems and pods develop bleached, straw-coloured lesions covered in white, cottony fungal growth under humid conditions. Hard, black, irregularly shaped sclerotia form inside hollowed stems or on the outer surface, and heavily infected stems can lodge or die prematurely.

Sclerotia surviving in soil for extended periods germinate under cool, moist conditions to produce small, mushroom-like apothecia that release airborne ascospores. These spores commonly infect senescing flower petals, which then fall onto leaves and stems and provide the nutrient source the fungus needs to invade healthy tissue.

• Sclerotia can survive in soil for a number of years even without a susceptible host
• Petal-mediated infection is a well-documented pathway in crops such as oilseed rape and soybean
• Dense canopies retain the moisture needed for apothecia development and infection

Sclerotinia sclerotiorum has a very wide host range across many broadleaf crops and weeds. Oilseed rape, soybean, sunflower, and common bean are among the economically important hosts, and a history of any of these crops in rotation can build soil sclerotia levels for the others.

• Cool, wet weather during and immediately after flowering
• Dense canopies that restrict airflow and retain moisture at the base of the plant
• A history of susceptible broadleaf hosts in rotation, raising soil sclerotia levels
• Excess nitrogen promoting lush, dense canopy growth
• Irrigation timed close to flowering in already humid conditions

Because infection is closely tied to flowering and canopy moisture, prevention emphasises canopy management, rotation, and monitoring around the flowering period.

• Use wider row spacing or seeding rates that improve airflow through the canopy
• Use longer rotations away from other susceptible broadleaf hosts
• Avoid excess nitrogen that promotes dense, humid canopies
• Monitor weather-based risk information or petal testing where available around flowering