Potato virus Y (PVY) in Seed Potato Certification

In the EU and many other countries worldwide, seed potatoes require certification to be marketed. In most of the European national and international seed potato regulations, the tolerances for viruses are at present based on a combination of virus incidence and symptom severity. Historically all severe 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 is considered a severe virus. Investigation of PVY-infected potato leaf samples from Bavarian potato seed certification found that the ”classical“ O and the N strains, predominantly responsible for severe virus diseases, represented less than 1% of the infections present. Ca. 99% of the PVY infections were comprised 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 which cultivars are favoured in different potato growing areas or countries.