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 PVY NTN and PVY N W. 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.In greenhouse tests, PVY NTN and PVY N W were associated with both severe and mild disease symptoms. In corresponding field tests, the symptom severity differed considerably. Furthermore, there was no correlation detected between virus concentration 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 international seed potato regulation documents. National regulations must be aligned by the EU member states until January 2016.

Testing for potato viruses is globally very important to prevent a critical shortage of potato supply. In most countries, testing is obligated by law. In Germany, seed potatoes are monitored for six viruses: PLRV, PVY, PVM, PVA, PVX and PVS. They can cause up to 90% loss of potato tubers in the field. Common methods currently used for testing are ELISA and conventional real-time PCR, but both are very time-consuming, and the former needs a high capacity of green houses and human resources, the latter elaborate RNA extraction steps. Recently, we proposed a new method called real-time DiRT-PCR which enables us to test for PLRV, PVY and PVS along with an internal control in three duplex real-time PCR reactions directly on diluted tuber sap. In this study, we describe the first TaqMan® assay for PVM published so far and embed it into a multiplex system to detect the remaining viruses. We are now able to sensitively test for the presence of six viruses in two multiplex reactions using the real-time DiRT-PCR without RNA purification.

Potato viruses PLRV, PVY, PVM, PVA, PVX and PVS can cause up to 90% loss of potato harvest. Therefore, they are monitored by law in many countries using DAS-ELISA or conventional real-time RT-qPCR. Previously, we developed a multiplex real-time DiRT-PCR (Direct reverse transcript – polymerase chain reaction), which works directly on diluted tuber sap and thus saves time and chemical processing for RNA extraction or time and space in the glasshouse. So far, this method only ran on sap of single tubers which is not practical for routine testing. We are now able to sensitively test for the presence of six viruses in two multiplex reactions using the real-time DiRT-PCR on pooled samples of ten tubers. Here we show that there is an “almost perfect” agreement (Gwet’s AC1 index) comparing this multiplex real-time DiRT-PCR on pooled samples with DAS-ELISA and a commercial RT-qPCR kit with a rapid extraction method. The multiplex real-time DiRT-PCR is now ready to be used for routine testing.