VIRUS TRANSMISSION BY HOST-SPECIFIC STRAINS OF OLPIDIUM-BORNOVANUS AND OLPIDIUM-BRASSICAE

Zoospores of 12 isolates O. bornovanus from geographically diverse sites and representing the three host specific cucurbit strains were tested as vectors for seven viruses using watermelon bait plants and the in vitro acquisition method. All isolates of the cucumber, melon, and squash strains transmitted melon necrotic spot carmovirus (MNSV) and cucumber necrosis tombusvirus (CNV) but none transmitted petunia asteroid mosaic tombusvirus (PAMV) or tobacco necrosis necrovirus (TNV). The isolates varied as vectors of three other carmoviruses: cucumber leaf spot virus (CLSV); cucumber soil borne virus (CSBV); and squash necrosis virus (SqNV). All cucumber isolates transmitted CLSV and SqNV but not CSBV. Some of the melon isolates transmitted CLSV and SqNV but none transmitted CSBV. Two squash isolates transmitted CSBV and SqNV but not CLSV. Two isolates of O. brassicae transmitted only TNV and a third did not transmit any of the viruses. The species of bait plant sometimes affected transmission. The most efficient vector strains of O. bornovanus, as determined by reducing zoospores and virus in the inoculum, were the cucumber strain for CLSV; the cucumber strain for CNV if cucumber was the bait plant or melon strain if watermelon was the bait plant; and the squash strain for SqNV. The plurivorous strain of O. brassicae was the most efficient vector of TNV. Olpidium bornovanus is the first vector reported for CSBV and is confirmed as a vector of SqNV. It is proposed that all carmoviruses may have fungal vectors. Ligniera sp. did not transmit any of the viruses in one attempt.