Blossom blight of alfalfa: Distribution in western Canada and impact on seed yield
B.D. Gossen¹, J.D. Holley², L.M. Harrison³ and S.R. Smith^{4 1}AAFC Research Centre, Saskatoon, SK, Canada, S7N 0X2; ²Crop Diversification Centre, Brooks, AB, T0J 0J0; ³ Alberta Agriculture, Fairview, AB, T0H 1L0; ⁴Dep. Plant Science, University of Manitoba, Winnipeg, MB, R3T 2N2. Epidemics of blossom blight [Botrytis cinerea, Sclerotinia sclerotiorum] on alfalfa occurred across the Canadian prairies in 1993, in scattered areas in 1994, and in northern areas of Alberta and Saskatchewan in 1995 - 96. In 1997, severe drought prevailed across the region during flowering, but there were a few areas that received timely showers and they had moderate amounts of blossom blight. In 1995 to 1997, surveys were carried out across the region to assess disease severity; grower-cooperators plated flowers onto agar media at regular intervals during flowering and sent them to a central laboratory for assessment. Botrytis cinerea was the most common pathogen at most sites, but S. sclerotiorum predominated at several sites in southern Alberta. Blossom blight epidemics were consistently associated with prolonged cool, wet weather during flowering. Seed production under wet conditions is affected by reduced flower production and reduced pollinator activity, as well as by disease. As a result, the inconspicuous symptoms associated with moderate levels of blossom blight infection are easily overlooked. A test kit, designed to assist growers in assessing levels of infection in their fields, is being assessed by a limited number of growers in 1998, and should be available for general use in 1999. To determine the impact of blossom blight on seed yield, fungicide application during flowering was examined at 9 sites across western Canada in 1995, 13 sites in 1996, and 8 sites in 1997. Where blossom blight was severe, and levels stayed high for several weeks, seed yield in the controls was very low (occasionally less than 100 kg/ha). Application of benomyl consistently reduced flower infection, and increased seed yields by 25-100% at sites where blossom blight levels were moderate to severe. Application of chlorothalonil did not reduce flower infection, but yield increases at some sites were equal to those of benomyl. Similar results were obtained in a limited number of trials that included mancozeb. Iprodione and vinclozolin did not improve seed yields. The impact of fungicide application on foliar disease severity was generally small. We conclude that blossom blight occurs throughout the region, and that it causes significant yield loss in some years.