Transmission and the role of honeybees in field spread of blueberry shock ilarvirus, a pollen-borne virus of highbush blueberry

The rapid and random spread of blueberry shock ilarvirus (BlShV) in commerc ial plantings suggested that insects played a role in transmission. Transmi ssion from infected field plants to trap plants only occurred during bloom, indicating that flowers were the avenue for infection and flower-visiting insects including pollinators were involved. Trap plants readily became inf ected during bloom when a honeybee hive was included in cages with diseased field plants and trap plants. There was limited transmission in cages with out a hive and no transmission when healthy field bushes were caged with tr ap plants either with or without honeybees. Infective pollen survived withi n the hive for 1 but not 2 weeks. Western flower thrips did not transmit Bl ShV when they were allowed to feed on caged flowering trap plants in the pr esence of a source of infected pollen. Results of pollen washes, cell disru ption, and electron microscopy show that particles of BlShV were on, in, an d between the cells of the pollen tetrad. Pollen from infected plants germi nated as readily as pollen from healthy plants. BlShV was seedborne, but at a low level. All 42 cultivars tested were susceptible to BlShV when inocul ated by grafting. The primary mechanism of transmission appears to be the t ransfer of BlShV-contaminated pollen by honeybees from flowers on infected plants to flowers on healthy plants.