Sclerotinia stem rot, commonly known as white mould, is caused by the soil-borne fungus Sclerotinia sclerotiorum. In canola, yield losses due to sclerotinia stem rot vary from year to year as the disease is greatly influenced by weather. Moisture is the main determining factor affecting sclerotinia outbreaks. Sclerotinia can be a limiting factor in pulses grown in moister regions and wet years. Sclerotinia infection in pulses leads to stem and pod rot, crop lodging, and reduced seed fill.
Appearance: Initial symptoms of sclerotinia appear as soft, water-soaked white to grey lesions on leaves and stems. As lesions advance they may resemble a “bull’s eye” pattern of concentric rings of rotting tissue. White mould growth is usually evident on infected plant tissue. Plant parts above the affected area will often turn pale green or yellow, wilt, and eventually die. Mature lesions are bleached and are easily shredded, resulting in premature ripening and lodging. Sclerotinia infection may be observed as individual plants scattered throughout the field, or in patches in the field where moisture was greatest. Lodged crops are more susceptible to sclerotinia mould development.
Disease Cycle: The sclerotinia fungus spends most of its life cycle in the soil as a hard-walled resting structure called a sclerotium. Sclerotia are resilient to adverse conditions and can survive in soil or plant tissue for three or more years.
In the spring, sclerotia can germinate resulting in the formation of apothecia. Apothecia are small mushroom-like structures that function for about 5 to 10 days and during that time can release as many as two million tiny air-borne spores called ascospores. The majority of ascospores are deposited within 100 to 150 metres from their source, but spores that are picked up by wind currents can be dispersed for several kilometres. Ascospores land on flower petals, which they use as a food source to establish infection. The infected petals drop down into the crop canopy and infect healthy leaf and stem tissue. Lesion development is favoured by humid conditions and by temperatures between 20°C and 25°C. Dry conditions will impede or stop infection and lesion development. Sclerotinia lesions are usually accompanied by a white mould growth. Advanced lesions become bleached and brittle, leading to shredding and stem collapse.
Sclerotia develop within or on diseased plant tissue and are returned to the soil with crop residue or are harvested with the seed, thus completing the disease cycle.
Disease Control: Crop rotation is not always effective because of the pathogen’s large host range and its ability to survive for years in the soil as sclerotia. Also, air-borne ascospores can blow in from infected broad-leaf residue from nearby fields, which further reduces the impact of a diverse crop rotation. Regardless, sclerotinia stem rot disease levels are greatest if broad-leaf crops are grown consecutively. Aim for a minimum of one year, preferably three years, between susceptible crops to reduce the buildup of sclerotia in the soil. There are currently no canola varieties with resistance to sclerotinia stem rot. Breeding for resistance is underway but advancements have been slow as the host-pathogen interaction is very complex. However, some varieties may avoid infection due to their physiology. For example, apetalous canola varieties (petals are absent) have been developed. Hybrid varieties with strong stems and a branching habit are less prone to lodging even if they are infected by sclerotinia. Seed treatments are not effective at controlling sclerotinia diseases. Foliar fungicides are an option for sclerotinia stem rot management.
Petal Test Kit for Sclerotinia in Canola: Discovery Seed Labs offers growers the ability to purchase a do-it-yourself petal test kit for testing the infestation of canola petals with Sclerotinia. The petal test involves the collecting of canola petals, placing of the petals into media containing plates, scoring the culture plates by identifying colonies of Sclerotinia, and estimating the risk of disease develoment from the results. An electronic copy of the manual is available below.