Strains and distribution of Potato mop-top virus in Finland and other countries of the Baltic Sea region

The spraing symptoms caused by Potato mop-top virus (PMTV, genus Pomovirus) in potato tubers are an important quality problem in potato production in the Nordic countries. PMTV is very difficult to control because the virus remains infective in the resting spores of its vector, Spongospora subterranea (the powdery scab pathogen), in soil for many years and because PMTV-resistant cultivars are not available. Further problems are caused by the year-to-year variability in the portion of infected tubers which develop symptoms, and the similar symptoms caused by the nematode-transmitted Tobacco rattle virus. Hence, detection of PMTV in tubers cannot be based on observation of symptoms only. Since 2005, a coordinated action by the ten Baltic Sea region countries has aimed to determine the current geographic distribution of PMTV, improve diagnostic tools for detection of PMTV, and build up the necessary knowledge base for enhanced control of PMTV and the spraing disease. Because little was known about the genetic variability of PMTV in Finland, other Nordic countries and the few additional countries where PMTV occurs, sequences of PMTV isolates were characterized from 18 spraing-affected tubers grown in the field in Finland and from five symptomless tubers produced in a greenhouse in Latvia. The sequences were compared to ten PMTV isolates of which sequences were available from databases. The variability of the coat protein (CP) gene (98-100 %) and the readthrough (RT) region (97-100 %) in RNA2 was low. In contrast, the 8K gene located at the 3 - proximal part of RNA3 was highly variable, followed by a 3 -untranslated region (3 UTR) that was virtually identical in all isolates compared. Identity of the deduced 8K amino acid sequences was 88-100 % and sequence comparisons indicated existence of two readily distinguishable genetic strains. These results provided the first sequence information on PMTV isolates in Finland and Latvia, and the first report of occurrence of PMTV in Latvia, and the existence of two genetic strains of PMTV. Similar studies on the local PMTV isolates are on-going in Sweden and Norway. The data will be utilized in development of diagnostic tools, resistance breeding, and studies on interactions of PMTV with potato plants and the vector.