Molecular characterization of two potato virus S isolates from late-blight-resistant genotypes of potato (Solanum tuberosum)

Pest categorisation of potato virus S (non-EU isolates)

Following a request from the EU Commission, the Panel on Plant Health has addressed the pest categorisation of non‐EU isolates of potato virus S (PVS). The information currently available on geographical distribution, biology, epidemiology, potential entry pathways, potential additional impact compared to the current situation in the EU, and availability of control measures of non‐EU isolates of PVS has been evaluated with regard to the criteria to qualify as potential Union quarantine pest. Because non‐EU isolates of PVS are absent from the EU, they do not meet one of the requirements to be regulated as an RNQP (presence in the EU); as a consequence, the Panel decided not to evaluate the other RNQP criteria for these isolates. Populations of PVS can be subdivided into two strains: the ordinary strain (PVS‐O) with a worldwide distribution (including the EU), and the Andean strain (PVS‐A) which is absent from the EU or considered to have at most a limited distribution in the EU. Two additional divergent isolates (PVS‐A/PVS‐O recombinants and PVS‐arracacha) have also been categorised. Non‐EU isolates of PVS‐A are expected to have an additional impact as compared to the PVS isolates currently present in the EU, and therefore meet all the criteria to qualify as potential Union quarantine pests; the magnitude of the additional impact is, however, unknown. Non‐EU isolates of PVS‐A/PVS‐O recombinants and of PVS‐arracacha also meet these criteria, with the exception of the criterion regarding the potential additional consequences in the EU territory for which the Panel was unable to conclude. Non‐EU PVS‐O isolates are not expected to have an additional impact in the EU as compared to EU isolates and therefore do not meet the corresponding criterion.

The Biology and Phylogenetics of Potato virus S Isolates from the Andean Region of South America

Biological characteristics of 11 Potato virus S (PVS) isolates from three cultivated potato species (Solanum spp.) growing in five Andean countries and 1 from Scotland differed in virulence depending on isolate and host species. Nine isolates infected Chenopodium quinoa systemically but two others and the Scottish isolate remained restricted to inoculated leaves; therefore, they belonged to biologically defined strains PVS^A and PVS^O, respectively. When nine wild potato species were inoculated, most developed symptomless systemic infection but Solanum megistacrolobum developed systemic hypersensitive resistance (SHR) with one PVS^O and two PVS^A isolates. Andean potato cultivars developed mostly asymptomatic primary infection but predominantly symptomatic secondary infection. In both wild and cultivated potato plants, PVS^A and PVS^O elicited similar foliage symptoms. Following graft inoculation, all except two PVS^O isolates were detected in partially PVS-resistant cultivar Saco, while clone Snec 66/139-19 developed SHR with two isolates each of PVS^A and PVS^O. Myzus persicae transmitted all nine PVS^A isolates but none of the three PVS^O isolates. All 12 isolates were transmitted by plant-to-plant contact. In infective sap, all isolates had thermal inactivation points of 55 to 60°C. Longevities in vitro were 25 to 40 days with six PVS^A isolates but less than 21 days for the three PVS^O isolates. Dilution end points were 10^-3 for two PVS^O isolates but 10^-4 to 10^-6 with the other isolates. Complete new genome sequences were obtained from seven Andean PVS isolates; seven isolates from Africa, Australia, or Europe; and single isolates from S. muricatum and Arracacia xanthorhiza. These 17 new genomes and 23 from GenBank provided 40 unique sequences; however, 5 from Eurasia were recombinants. Phylogenetic analysis of the 35 nonrecombinants revealed three major lineages, two predominantly South American (SA) and evenly branched and one non-SA with a single long basal branch and many distal subdivisions. Using least squares dating and nucleotide sequences, the two nodes of the basal PVS trifurcation were dated at 1079 and 1055 Common Era (CE), the three midphylogeny nodes of the SA lineages at 1352, 1487, and 1537 CE, and the basal node to the non-SA lineage at 1837 CE. The Potato rough dwarf virus/Potato virus P (PVS/PRDV/PVP) cluster was sister to PVS and diverged 5,000 to 7,000 years ago. The non-SA PVS lineage contained 18 of 19 isolates from S. tuberosum subsp. tuberosum but the two SA lineages contained 6 from S. tuberosum subsp. andigena, 4 from S. phureja, 3 from S. tuberosum subsp. tuberosum, and 1 each from S. muricatum, S. curtilobum, and A. xanthorrhiza. This suggests that a potato-infecting proto-PVS/PRDV/PVP emerged in South America at least 5,000 years ago, became endemic, and diverged into a range of local Solanum spp. and other species, and one early lineage spread worldwide in potato. Preventing establishment of the SA lineages is advised for all countries still without them.

Strain Characterization of Potato virus S Isolates from Tasmania, Australia

Potato virus S (PVS) is prevalent within potato (Solanum tuberosum) production worldwide. Traditionally, PVS has been split into two strains, Ordinary (PVS^O) and Andean (PVS^A), based on reaction in herbaceous indicator species such as Chenopodium quinoa. However, recent research has identified further strain designations, such as PVS^O-CS (Ordinary and Chenopodium systemic). Forty-four isolates of PVS were collected from potato seed lines in different geographical regions within Tasmania, Australia. Isolates were initially characterized by reactions in C. quinoa. Nineteen isolates were characterized as PVS^O, based on the development of local lesions and serological detection in inoculated leaves only. Three isolates were identified as PVS^A-like, based on local lesion development in inoculated leaves, mild mottling or chlorotic spots on noninoculated leaves, and serological detection in both inoculated and noninoculated leaves. Thirteen isolates produced no symptoms, and were detected serologically in inoculated leaves only (PVS^O-like). Four isolates produced no symptoms but were detected serologically in both inoculated and noninoculated leaves (PVS^A-like). Five isolates produced symptoms in inoculated leaves only but were detected serologically in both inoculated and noninoculated leaves (also PVS^A-like). The ability of isolates to infect tomato has also been used as a criterion to assist in PVS strain differentiation. A subsample of isolates (n = 16) was unable to infect tomato 'Grosse Lisse'. Seventeen isolates representative of these groupings based on reactions in C. quinoa were also characterized by coat-protein sequencing. Phylogenetic comparisons suggested that all isolates were PVS^O rather than PVS^A. Therefore, whereas some of these PVS isolates were systemic in C. quinoa, findings from this study suggest that they were not PVS^A, and that only PVS^O and PVS^O-CS isolates are present in Tasmania. The implications of this finding for disease management are discussed.

Complete genome sequence of the first Andean strain of potato virus S from Brazil and evidence of recombination between PVS strains

An isolate of the Andean strain of potato virus S (PVS), named BB-AND, was detected for the first time in a Brazilian potato crop, fully sequenced and analyzed. A comparison of BB-AND with other PVS isolates (Andean and Ordinary) showed that BB-AND is quite distinct. The lowest amino acid sequence identity to the only other fully sequenced Andean isolate was found in ORF 1 (82%) and ORF 6 (87%). Recombination analysis showed that the isolate Vltava (AJ863510), from Germany, is a recombinant between PVS(O) and PVS(A) isolates, with the recombination event located between nucleotides 6125 and 8324.