Potential role of pollen and pollinators in the spread of blossom blight of seed alfalfa caused by Botrytis cinerea

Abstract

The fungal pathogen Botrytis cinerea Pers.: Fr. infects flowers of many plant species, including alfalfa (Medicago sativa L.). In southern Alberta, blossom blight, caused in part by B. cinerea, reduces yields of seed alfalfa in cool, wet growing seasons. Previous studies have suggested that B. cinerea uses pollen to begin infection, and that the fungal-infected pollen may be vectored by alfalfa leafcutter bees (Megachile rotundata Fab.). A longitudinal greenhouse study was performed to investigate the role of pollen in the infection process of alfalfa florets by B. cinerea. The effects of pollination (pollinated vs unpollinated) and inoculation method (dry vs aqueous suspension) on floret and pollen infection was tested. Florets were harvested at 0, 24, 48 and 94 h after inoculation. Pollinated florets that were dry inoculated had significantly greater levels of infection (84%) than similarly inoculated unpollinated florets (38%, P < 0.001); however no pollen infection was observed. In contrast, 90% of suspension-inoculated florets were infected and a small (<1%) but significant (P < 0.0001) level of infected pollen was observed in these samples. Leafcutter bees were also collected from seed alfalfa fields in 2013 and 2014 to estimate the prevalence of B. cinerea on pollen. Pollen removed from field bees showed no growth by B. cinerea; however, the bees that were directly plated on agar medium exhibited an increased load of B. cinerea as the growing season progressed. Thus, under typical field conditions, alfalfa pollen is not likely to be a significant factor contributing to the establishment and spread of B. cinerea.