“…Cockerham (1955) classi®ed different isolates of PVX into four groups based on phenotype to two endogenous resistance genes in potato. PVX-CP (group 2; Orman et al, 1990), PVX-X3 and PVX-UK3 (group 3;Huisman et al, 1988;Kavanagh et al, 1992), PVX-HB (group 4; Querci et al, 1993), and a Russian isolate that we have designated PVX-R (Skryabin et al, 1988) were compared to our isolate (Hemenway et al, 1988) denoted as PVX-WA. PVX-WA is likely to be a group 3 strain based on amino acid similarities, but has not been tested on the appropriate host plants.…”