Sclerotinina stem rot is caused by the pathogen Sclerotinia sclerotiorum. This pathogen has a wide host range and is known to infect numerous species of plants (canola, sunflower, soybean, flax, etc.). Disease outbreaks can be particularly severe under conditions of zero crop rotation or if rotations include several susceptible plant species. Yield losses can reach 20% when infections occur early in the flowering period.
The disease survives primarily as sclerotia in the soil. These sclerotia germinate in the soil to form apothecia which release air-borne spores, the primary source of infection in canola. Infections typically occur at flowering from these air-borne spores, usually during wet, humid conditions in July; however, symptoms do not show up until several weeks after flowering begins. Stem and leaf axils become discolored with a soft, watery rot. Leaf axils are most likely to become infected because spore-infected flower petals become lodged there. Pale-grey to white lesions will develop on the stem and branches of the plants. Stems become bleached and dried out, tending to shred and crack. Black fruiting bodies (sclerotia) will be found within the infected plant stems.
Spraying fungicide to prevent the formation of Sclerotinia stem rot is highly recommended in canola fields in Canada. Scout for the tan colored apothecia of stem rot which are shaped like a golf tee, and can be found growing at the soil surface or from decaying canola. The use of provincial disease risk maps can also be valuable as a predictive tool for sclerotinia infection levels and for planning fungicide applications.