Distribution and diversity of Potato mop-top virus in Sweden

During the last decades, spraing disease caused by Potato mop-top virus (PMTV) has become a severe problem in the potato production throughout the Nordic countries. In this thesis, it was shown that PMTV is widely distributed throughout Sweden, Norway and Denmark, and has now become prevalent in the potato-production areas within these countries. In Sweden, PMTV was found to be present further north than observed in an earlier study. Compared to the Nordic countries, the occurrence of PMTV in Poland, the Baltic countries and northwestern Russia is still very limited. Molecular characterization and phylogenetic relationship analyses were carried out for 43 new PMTV sequences originating from potato tubers and field soil from Sweden, Denmark and the USA. The results showed a limited diversity of PMTV based on RNA-CP and RNA-TGB. No strict geographical grouping was obtained for isolates from Europe, Asia and North America indicating that PMTV has spread through infected seed tubers. Weeds, as alternative hosts of plants viruses and their vectors, are an important factor in the epidemiology of plant virus diseases. PMTV was detected in the roots of the weed Chenopodium album, which had been naturally infected with the virus in the field. C. album may together with weeds of the family Solanaceae be an important factor for maintaining PMTV in a field. Sequence analysis revealed a close relationship between the PMTV isolates from C. album and other characterised isolates suggesting that the same genotypes of PMTV are infecting potato and C. album. The presence in Sweden of three other potato-infecting viruses associated with necrotic symptoms in tubers was demonstrated: Tobacco rattle virus (TRV), Tobacco necrosis virus (TNV) and Potato virus Y (PVY). TRV has become prevalent in potato fields located from southern and up to the central part of Sweden. Analyses of tubers with typical necrotic symptoms of the ABC disease confirmed infection with TNV. In addition, two different necrotic strains of PVY, PVYNTN and PVYNW, were detected in symptomatic potatoes. The presence of TNV and PVYNTN/PVYNW in potato plants was to our knowledge confirmed for the first time in Sweden