Vectoring of Botrytis cinerea (Persoon: Freis) to kiwifruit (Actinidia deliciosa) flowers by honey bees (Apis mellifera Linnaeus)

Abstract

Botrytis cinerea Persoon ex Fries causes stem-end rot of kiwifruit (Actinidia deliciosa) in storage. The honey bee (Apis mellifera) may influence the epidemiology of B. cinerea in kiwifruit orchards by acting as a vector of viable B. cinerea propagules to kiwifruit flowers during foraging. The ability of honey bees to vector viable B. cinerea propagules to kiwifruit flowers was assessed. Methods were developed for collecting foraging honey bees directly from flowers and isolating viable B. cinerea propagules from the external surfaces of bees. Eighty seven percent of honey bees foraging on kiwifruit flowers carried viable B. cinerea propagules. The mean number of propagules carried by bees increased from early to late kiwifruit flowering. Bees carried similar numbers of propagules when foraging on staminate and pistillate kiwifruit flowers. Honey bees which visited manuka (Leptospermum spp.) flowers in close proximity to the experimental kiwifruit orchard site were also contaminated with B. cinerea. This demonstrated the potential for foraging bees to transfer inoculum to kiwifruit flowers from external sources. Honey bees acquired large numbers of dry B. cinerea spores (mean c. 4 x 103) while foraging on kiwifruit flowers that had been artificially contaminated with the fungus. The acquisition of spores from artificially inoculated flowers provided evidence that honey bees may pick up spores from naturally infested kiwifruit flowers during pollen collection. Measurement of the dispersal of B. cinerea propagules between kiwifruit flowers showed that honey bee dispersal is an important mechanism for the spread of inoculum over short distances. The dispersal of inoculum by honey bees followed a negative exponential gradient with distance from the inoculum source. Dispersal of B. cinerea spores by honey bees was considerably greater than dispersal by wind over short distances. Application of B. cinerea spores to fruit at petal fall stage increased the number of B. cinerea propagules on the surface of fruit during the season. This indicated that inoculum spread to flowers by honey bees could influence the contamination of fruit later in the season. High natural levels of B. cinerea on fruit surfaces were found, with a general increase in the number of viable propagules per fruit from petal fall stage to mature fruit stage. The implications of these results for B. cinerea epidemiology in kiwifruit orchards are discussed and requirements for further research are identified.