From 1990 to 1997, experiments were conducted comparing the effect of swathing versus straight combining on seed infection by Alternaria species and on quality of harvested seed. This was done at several locations in Saskatchewan using a number of cultivars of Brassica rapa and B. napus. The level of A. brassicae in harvested seed was significantly higher in straight-combined than in swathed treatments. Green seed count and seed weight were not affected by treatment. Seed germination was reduced, but not always significantly, 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 the time for disease development because it reduces the time of ripening compared with straight combining.
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Duczek et al. (1999) studied this question.