INHERITANCE OF RESISTANCE TO POTATO VIRUS-Y AND VIRUS-A IN PROGENY OBTAINED FROM POTATO CULTIVARS CONTAINING GENE RY - EVIDENCE FOR A NEW GENE FOR EXTREME RESISTANCE TO PVA

Extreme resistance in cultivated potato (Solanum tuberosum) to potato viruses Y and A (PW and PVA) conditioned by the presence of Ry genes i ntroduced from Solanum stoloniferum was described by Cockerham (1970). Cockerham detailed a number of genes which controlled a variety of re actions, including extreme resistance to both viruses (i.e. little or no visible reaction of plants and no viral replication following graft and manual inoculation) controlled by gene Ry(sto). In the present st udy, cvs 'Pirola' and 'Barbara', which contain a Ry gene, were found t o have extreme resistance to PW isolates from the ordinary (PVYO), vei nal necrosis (PVYN) and potato tuber necrotic ringspot (PVYNTN) subgro ups, and PVA. The inheritance of this phenotype was examined in seedli ng progenies obtained by crossing 'Barbara' and 'Pirola' with suscepti ble cultivars. Segregation data for resistance to PVY and PVA in a pro geny involving cv 'Pirola' best fitted a genetical model of one gene c ontrolling extreme resistance to both PVY and PVA, although the possib ility that there are two genes, each controlling resistance to one vir us but closely linked, cannot be excluded. Segregation data from proge nies involving cv 'Barbara' best fitted a genetical model in which the re are two independent genes, one controlling extreme resistance to PV A and PVY and a second gene controlling extreme resistance to PVA but not to PW. This previously unrecognised gene conferring extreme resist ance to PVA only, should be given the notation Ra in keeping with nome nclature used for other resistance genes.