Development and Control of Botrytis Cinerea on Alfalfa Flowers
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Blossom blight of alfalfa was first identified on the Canadian prairies in 1993. It Is caused by two common fungal pathogens, Botrytis cinerea and Sclerotinia sclerotiorum. This disease develops rapidly under wet and cool weather conditions, and can result in serious losses in alfalfa seed yield. Strategies for the management of the disease are needed. This study was conducted to develop a quick method to assess the incidence of the two pathogens in alfalfa flowers in the field, to screen cultivars for resistance to infection by B. cinerea and to quantify the relationship between environmental conditions and infection incidence. Various amendments to potato dextrose agar (PDA) and pretreatments in the dark vs. a 14-h photoperiod were explored to develop a semi-selective medium for rapid identification of the causal agents. Amendment with alfalfa leaves in PDA and pretreatment in the dark promoted sporulation of B. cinerea and sclerotial formation of S. sclerotiorum. A combination of alfalfa leaves and lactic acid in PDA (AL-PDA) or pimaricin, chloramphenicol and alfalfa leaves in PDA (PCL-PDA) resulted in the best suppression of contamination in field samples. AL-PDA also promoted sporulation of B. cinerea and sclerotial formation of S. sclerotiorum. The reaction of 12 alfalfa cultivars to infection by B. cinerea was evaluated in several tests, including detached flower and whole plant tests in growth cabinets and field tests. There were consistent differences among cultivars in their susceptibility to infection by B. cinerea. Cvs. DK 135, OAC Minto and Iroquois were generally less susceptible than cvs. Apollo II, Algonquin and Heinrichs. Upward-facing flowers in cv. Vernal were less susceptible (16%) than downward-facing flowers (86%). Purple flowers were less susceptible than white flowers in cv. Iroquois, but not in cvs. Apollo II and AC Nordica. The impact of wetness duration and temperature on infection of alfalfa flowers by B. cinerea was examined under controlled conditions in a split-plot design. The main-plot treatments were different temperatures (10°, 15°, 20°, 25°, 30°C) and the subplot treatments were surface wetness duration (0, 4, 8, 12, 16, 20, 24, 48 h). The incidence of infection of alfalfa flowers was very low at 0, 4 and 8 h of wetness for all temperatures. Incidence increased sharply after 12 h at 20°C, 16 h at 15°C and 20 h at 10° and 25°C. Infection at 30°C was very low. The optimum conditions for infection were between 15° and 20°C, with a minimum of 12 to 16 h of surface wetness.