Seasonal dynamics of Botrytis cinerea and Sclerotinia sclerotiorum in seed alfalfa fields of Southern Alberta

Blossom blight of seed alfalfa (Medicago sativa L.) is caused by the fungal pathogens Botrytis cinerea Pers.:Fr. (Bc) and Sclerotinia sclerotiorum (Lib.) de Bary (Ss). Nineteen commercial seed alfalfa fields in southern Alberta were surveyed over three growing seasons (2013–2015) to determine the seasonal occurrence of Bc and Ss. Each field was surveyed four times during July and August in each year. Disease incidence and severity ratings were assigned in the field, and floret and pod sampleswere plated on semi-selective media in the lab. In 2014 and 2015, aerosol samples were collected by three spore samplers installed in three fields. Aerosol samples
were analysed in a real-time quantitative polymerase chain reaction (qPCR) assay to quantify the daily spore concentrations of Bc and Ss. In all 3 years, blossom blight symptoms were present at trace levels. Plated samples revealed greater variability in pathogen incidence among fields, although seasonal trends were similar between years despite considerable differences in macroclimatic variables. Spore discharge for both pathogens did not follow the same seasonal trends as plated samples. Overall, these results suggest that management and microclimatic factors (e.g. irrigation) may play a more important role than macroclimatic factors in the development of blossom blight of alfalfa in southern Alberta.