Sclerotinia Stem Rot

Sclerotinia stem rot or white mold is caused by the fungus Sclerotinia sclerotiorum. It can be very destructive during periods of wet weather. The sclerotinia fungus survives up to five or six years in the soil in the form of hard black fungus bodies called sclerotia. Whenever there is wet weather for a week or two, with moist soil, the sclerotia germinate to produce tiny mushroom-like bodies called apothecia. These apothecia are only to 3/16 inch across yet they produce millions of airborne spores. The spores infect the cast dead canola blossoms during periods of wet weather. Variation in the percentage of infected plants is due to differences in the quantity of infectious spores, plant population, crop height and vigor, rainfall, soil moisture, temperature and humidity. Even after plants are infected, the severity of stem rot symptoms and the resulting effect on yield will vary according to temperature, rainfall, crop density and especially the stage of crop growth at the time of infection.

Infections that start on the dead blossoms spread to adjacent tissues, resulting in dead branches or dead plants, causing the plants to lodge. The rotted stems usually have a bleached appearance. Sclerotinia infections can be serious on canola if cool wet weather occurs in the last two weeks of June and continues into early July when blossoming occurs.

In rare cases half the potential yield of a crop may be lost to Sclerotinia. Yield losses reflect the yield reduction per infected plant and the percentage of infected plants in a crop. In general, when conditions for the disease are favorable and infections occur throughout the flowering period, yield reduction per infected plant can equal 50% (0.5). In this situation, if 25% (0.25) of the plants in a crop are infected and the yield potential is 40 bushels per acre, the bushel loss would be 0.5 X 0.25 X 40 = 5 bushels per acre. However, if infections are delayed until late bloom or if dry weather set in after early bloom, yield reduction per infected plant may be as little as 10% (0.1). Again using the example of a crop with 25% (0.25) infected plants, the bushel loss would be 0.1 X 0.25 X 40 = only 1 bushel per acre. These types of calculations may be very important when deciding whether to spray to control the disease.