Effect of swathing on alternaria black spot in Brassica rapa canola in Saskatchewan

From 1990 to 1997, experiments were conducted comparing the effect of swath ing versus straight combining on seed infection by Alternaria species and o n quality of harvested seed. This was done at several locations in Saskatch ewan using a number of cultivars of Brassica rapa and B. napus. The level o f A, brassicae in harvested seed was significantly higher in straight-combi ned than in swathed treatments. Green seed count and seed weight were not a ffected by treatment. Seed germination was reduced, but not always signific antly, with straight combining compared to swathing. This data support the recommendation for swathing at the optimal time to reduce alternaria black spot when the potential for disease development is high. Swathing reduces t he time for disease development because it reduces the time of ripening com pared with straight combining.