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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 commercial plantings suggested that insects played a role in transmission. Transmission 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 infected during bloom when a honeybee hive was included in cages with diseased field plants and trap plants. There was limited transmission in cages without a hive and no transmission when healthy field bushes were caged with trap plants either with or without honeybees. Infective pollen survived within the hive for 1 but not 2 weeks. Western flower thrips did not transmit BlShV when they were allowed to feed on caged flowering trap plants in the presence of a source of infected pollen. Results of pollen washes, cell disruption, and electron microscopy show that particles of BlShV were on, in, and between the cells of the pollen tetrad. Pollen from infected plants germinated as readily as pollen from healthy plants. BlShV was seedborne, but at a low level. All 42 cultivars tested were susceptible to BlShV when inoculated by grafting. The primary mechanism of transmission appears to be the transfer of BlShV-contaminated pollen by honeybees from flowers on infected plants to flowers on healthy plants.