OCCURRENCE AND IMPACT OF BLACKLEG ON OILSEED RAPE IN ONTARIO

The occurrence in Ontario of blackleg on oilseed rape (Brassica napus), caused by Leptosphaeria maculans, and impact of the disease on seed yield of the crop are documented for 189 winter rape fields during the period 1986-1989 and for 67 spring rape fields in 1988 and 1989. Leaf lesions were the most common symptom at crop flowering, whereas crown cankers predominated at harvest. All of 161 isolates of L. maculans obtained from crowns and stems contained the fast isozyme of glucose phosphate isomerase characteristic of the highly virulent strain of the fungus. Prevalence, incidence, and severity of crown cankers were greatest in Bruce and Huron counties and were greater in winter rape than in spring rape. Over the four-year period, the annual prevalence of fields affected by crown cankers ranged from 60% to 100% in winter rape and from 27% to 31% in spring rape. Severity of crown canker at harvest (X) was closely related to loss in seed yield (Y%) of winter rape by equations of the form log10(Y+1) = a + bX. Mean annual losses in seed yield ranged from 1.1% to 7.5% in winter rape and from 0.3% to 1.6% in spring rape. Maximum estimated yield losses in fields examined were 29.2% in winter rape and 8.8% in spring rape. Reduction in 1000-seed weight accounted for 73% and 34% of the loss in seed yield in winter rape in 1986 and 1987. A highly virulent strain of L. maculans is widespread in the major oilseed rape production area of Ontario and can cause appreciable losses in seed yield, particularly in winter rape.