Disease forecasting and the role of pollinators in the spread of blossom blight of seed alfalfa in Southern Alberta

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Reich, Jonathan
University of Lethbridge. Faculty of Arts and Science
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Lethbridge, Alta. : University of Lethbridge, Dept. of Geography
Blossom blight of seed alfalfa (Medicago sativa L.), caused by fungal pathogens Botrytis cinerea Pers.:Fr. and Sclerotinia sclerotiorum (Lib.) de Bary, can contribute to significant yield declines in moist growing seasons. In a greenhouse study that examined the occurrence of pollen infection by B. cinerea, pollen infection was observed at very low levels (<1% of pollen infected) in suspension-inoculated treatments. When leafcutter bees (Megachile rotundata), hypothesized vectors of B. cinerea spores, were collected from seed alfalfa fields, the bees tested positive for B. cinerea but their pollen load did not. Frequency of bees carrying B. cinerea spores increased over the growing season, with 96-100% of bees from the final collections (~August 20) testing positive for B. cinerea. These patterns closely follow patterns of observed airborne B. cinerea spore concentrations as detected by a quantitative polymerase chain reaction (qPCR) assay using aerosol samples collected from seed alfalfa fields. A sensitive, species-specific primer and probe for S. sclerotiorum was designed for use in the same qPCR assay. Discharge of S. sclerotiorum in one field closely followed the discharge of B. cinerea spores, with several larger (>200 ascospores per day) discharge days observed beginning in mid-July and continuing to the end of August. In the other two fields surveyed, discharge of S. sclerotiorum was concentrated in one or two large events. It is concluded that pollen infection and transmission of blossom blight pathogens by leafcutter bees are not large contributors to the spread of blossom blight in southern Alberta. In addition, using qPCR to quantify airborne inoculum levels will likely be useful in developing a disease forecasting model for blossom blight of seed alfalfa.
disease forecasting , pollinators , leafcutter bees , fungal pathogens , seed alfalfa , blossom blight