Examination of an isolate of Potato leaf roll virus that does not induce visible symptoms in the greenhouse

Over the last 30 years the importance of Potato leaf roll virus (PLRV) in commercial potato and seed potato production has decreased considerably. Since PLRV is transmitted by aphids in a persistent manner it can be controlled by applying a systemic insecticide. However, the development of insecticide resistance in the main vectors of PLRV Myzus persicae, Aulacorthum solani, Rhopalosiphoninus latysiphon, Aphis fabae, A. nasturtii, A. frangulae and Macrosiphum euphorbiae, and the development of isolates of PLRV that do not induce visible symptoms in some potato cultivars may lead to a resurgence in the significance of PLRV. Isolates of this type were found repeatedly during growing-on tests in Lower Saxony, Germany. In this study we examined such a symptomless isolate. The visible symptoms induced by this isolate in different potato cultivars were compared with those induced by isolates causing typical symptoms of a PLRV infection. By using quantitative real-time PCR the quantifiable amount of viral RNA was determined. Under climate chamber conditions all the isolates tested induced similar symptoms and did not differ in viral RNA content. Complete sequences for the tested isolates were obtained and used in a phylogenetic analysis. All the PLRV isolates compared were very similar at the molecular level. Several motifs that could play a role in symptom expression were analyzed, but none of them were correlated with the absence of symptoms in potato plants during growing-on tests. The discrepancy between the observations recorded in the growing-on tests and our experiments are discussed.