Potato virus Y (PVY) in seed potato certification

In the EU and many other countries worldwide, seed potatoesrequire certification to be marketed. In most of the Europeannational and international seed potato regulations, the tolerances for viruses are at present based on a combinationof virus incidence and symptom severity. Historically allsevere viruses were usually the origin of severe virus diseases,and mild viruses, in general, were seen to cause mild diseases.However, the situation has changed, especially for the most economically important virus, Potato virus Y (PVY), which isconsidered a severe virus. Investigation of PVY-infectedpotato leaf samples from Bavarian potato seed certification foundthat the “classical” O and the N strains, predominantly responsiblefor severe virus diseases, represented less than 1 % of the infections present. Ca. 99 % of the PVY infections werecomprised of the new recombinant strains PVYNTN and PVYNW. The ratio between these recombinants differed substantially. Differences in susceptibility of particular cultivars to certain PVY strains seem to be one reason for that. Therefore, the ratio of the recombinants may depend upon whichcultivars are favoured in different potato growing areas orcountries. In greenhouse tests, PVYNTN and PVYNW were associatedwith both severe and mild disease symptoms. In correspondingfield tests, the symptom severity differed considerably.Furthermore, there was no correlation detected between virusconcentration and symptom severity. These results will contribute to a worldwide discussion on the elimination of the criterion virus symptom severity from potato seed certification during the forthcoming months. The discussion follows amendments made to internationalseed potato regulation documents. National regulations must be aligned by the EU member states until January2016.