LAUSR

Abstract Surveys of alfalfa (Medicago sativa L.) seed production fields on the Canadian Prairies from 1993 to 2017 demonstrated that epidemics of blossom blight caused by Botrytis cinerea Pers. and Sclerotinia sclerotiorum (Lib.) de Bary developed almost every year at some location(s). Blossom blight symptoms were prominent when epidemics were severe, but more difficult to identify when present at low levels, which likely contributes to the infrequent reporting of blossom blight in alfalfa seed production. Epidemics were observed across the entire region, from the Peace River region of northern Alberta to southern Manitoba, consistently associated with relatively cool, wet weather conditions. The widespread nature of blossom blight on alfalfa in the years following its initial diagnosis in Canada in 1993, together with the ubiquitous distribution of the pathogens involved on other crops, indicated that blossom blight was almost certainly present on alfalfa seed crops on the Canadian Prairies for many years before the initial identification. It appears likely that blossom blight continues to cause yield loss at some locations in most years.