Extreme resistance to potato virus V in clones of Solanum tuberosum that are also resistant to potato viruses Y and A:: evidence for a locus conferring broad-spectrum potyvirus resistance

Extreme resistance to the potato V potyvirus (PVV) was found in four potato cultivars that contain Ry genes from Solanum stoloniferum. When plants of these cultivars, were inoculated by grafting in shoot tips from PVV-infected tomato plants, necrotic symptoms developed in some cultivars, although a full hypersensitive reaction was not elicited, while other cultivars were symptomless. PVV replication was not detected in any of the inoculated plants by ELISA, an infectivity assay of leaf extracts by manual inoculation to Nicotiana benthamiana indicator plants, or by 'return grafting' of shoot tips taken from newly developed shoots of the potato plants to virus-free indicator plants of tomato. These methods readily detected PVV infection in inoculated plants of cv 'Flourball', which does not contain an Ry gene and is susceptible, and in cvs 'Maris Piper' and 'Dr Macintosh', which contain gene Nv conditioning a hypersensitive reaction to inoculation. One of the Ry-containing cultivars, 'Barbara', has been previously shown to contain two genes that control extreme resistance, defined as no viral replication in intact plants, to the potyviruses potato viruses Y and A (PVY and PVA). These genes are: Ry(sto), which conditions resistance to PVY and PVA, and gene Ra, which conditions resistance to PVA only. It was found that in genotypes from a progeny of the cross 'Barbara' (Ry(sto)/Ra) x 'Flourball' (ry/ra), extreme resistance to PVV segregated with gene Ry(sto). It is proposed that either gene Ry(sto) conditions broad-spectrum extreme resistance to the distinct potyviruses PVY, PVA, and PVV or that Ry(sto) represents a family of genetically closely linked genes each controlling resistance to a specific virus.