Occurrence of potato virus X strain group 1 in seed stocks of potato cultivars lacking resistance genes

Potato virus X (PVX) isolates were obtained from a simple seed potato production scheme or from ware potatoes produced by seed potatoes obtained from it. In this scheme, PVX infection is widespread in seed stocks and most of the potatoes grown lack PVX resistance genes. Thirteen PVX isolates were typed to strain group by inoculation to potato cultivars containing different combinations of hypersensitivity genes Nx and Nb. Six failed to overcome either gene and therefore belonged to strain group 1, four overcame Nb only and were placed in strain group 3 and three were mixtures of the two. All 13 isolates failed to overcome extreme resistance/immunity gene Rx. Naturally infected cultivars of genotype nx.nb contained strain group 1 alone or strain groups 1 and 3, while those of genotype nx:Nb contained only strain group 3. The widespread occurrence of strain group 1 contrasts with the predominant occurrence of strain group 3 in potatoes in the UK. However, it resembles the UK situation before sophisticated seed potato production schemes were introduced and before PVX hypersensitivity genes Nx and Nb were deliberately exploited in potato breeding. Prior infection with potato leafroll virus (PLRV) did not affect expression of hypersensitivity to PVX in inoculated leaves of an nx:Nb genotype. Copyright © 1995, Wiley Blackwell. All rights reserved