Strain group specific and virus specific hypersensitive reactions to infection with potyviruses in potato cultivars

When 12 potato cultivars were inoculated with isolates (one each) of potato virus Y (PVY) ordinary (Y^o), C (Y^c) and tobacco veinal necrosis (Yⁿ) strain groups, potato virus A (PVA) and potato virus V (PVV), none of them responded hypersensitively to Yⁿ. However, with Y^o, Y^c, PVA and PW specific hypersensitive reactions developed depending on isolate-cultivar combination which were all independent of each other. When field isolates of PVY thought to be Y^oor Y^cwere inoculated to the same 12 cultivars, two did not fit into either strain group giving hypersensitive reactions in only two cultivars instead of seven with Y^oor eight with Y^c. These two isolates may represent a previously unreported PVY strain group (Y^z). When Y^owas graft-inoculated to seedlings of the cross Desiree × Maris Piper (hypersensitive × non-hypersensitive for Y^o), the segregation ratio obtained for non-hypersensitive:hypersensitive reactions was close to 1:1 suggesting that a single dominant gene (Ny_tbr) determining Y^ospecific hypersensitivity may be present in cv. Desiree (simplex condition). In tests using PVV and Desiree × Maris Piper (non-hypersensitive × hypersensitive for PVV) seedlings, the segregation ratio obtained was close to 1:5 indicating that a single dominant gene (Nv) determining PVV specific hypersensitivity may be present in cv. Maris Piper (duplex condition). Cultivars Corine, Pirola and clone G5457(4) which each carry one of the extreme resistance genes (Ry) from Solanum stoloniferum were graft-inoculated with Yⁿ, Y^o, Y^c, PVV and PVA. G5457(4) gave a strong localised hypersensitive reaction in all instances, while cv. Pirola did so with all except PVA to which it was immune. In cv. Corine a severe localised hypersensitive reaction developed with PVA, generalised hypersensitivity with PVV but an immune response with the three PVY strain groups. Large-scale grafting of Yⁿto plants of cvs Corine and Pirola gave no evidence of selection of a strain which overcomes Ry genes.