Epidemiology of Potato virus S and potato virus X in seed potato in Tasmania, Australia

Potato virus S (PVS) and Potato virus X (PVX) are common viruses of potato (Solanum tuberosum L.) and are transmitted by plant-to-plant contact. Some PVS strains are aphid transmissible. Both viruses can reportedly cause up to 10-15% yield loss in potato crops. This project investigated the epidemiology of PVS and PVX in Tasmanian seed potatoes and their effect on yield. Surveys for PVS and PVX were undertaken in seed potato crops in 2002/2003 and 2003/2004. During 2002/2003, PVS and PVX were detected in 66.7% of 225 crops and 12.9% of 232 crops respectively, with a mean incidence of 17.9% and 0.3% respectively. In 107 seed crops surveyed during 2003/2004, PVS and PVX occurred in 42.1% and 4.7% crops respectively with a mean incidence of 8.1% and 1.0% respectively. PVS was more prevalent and occurred at greater incidence in the north-east of Tasmania, while PVX was restricted to the north-west. Three years of field trials with cv. Russet Burbank showed a significant (P<0.001) negative linear relationship between incidence of PVS and processing yield. Regression analysis predicted reductions in processing yield of 5.6, 6.3 and 10.1 t/ha over three years as a result of complete infection with PVS. Virus spread was assessed by regular sampling during the growing season in four commercial fields of seed potatoes cv. Russet Burbank. PVS incidence did not increase in two fields, but increased by 5.2% between 31 and 107 days after planting (DAP), and 25.5% between 30 and 105 DAP in each of the other fields. Known aphid vectors of PVS were not detected on sticky traps at any field. PVX incidence increased by 10.1% between 31 and 107 DAP in one field. The other fields were free of detectable PVX early in the season, but one had trace infection (0.1%) at 129 DAP. Spatial analysis detected aggregation of PVS infected plants early in the season in two fields, suggesting virus transmission occurred during planting or seed-cutting. Overseas studies suggest both viruses are readily transmitted by seed cutting, but several Tasmanian studies show limited seed-cutting transmission of PVS. Increased PVS incidence late in the season in one field was associated with aggregation of PVS infected plants along, but not across rows, suggesting mechanical transmission of the virus. PVS was detected infrequently, and PVX was not detected, in weeds from four potato fields, suggesting weeds were not a major source of inoculum Fifty-two isolates of PVS were characterised as PVS° and three isolates as PVSA based on symptom expression and capacity for systemic invasion of indicator host Chenopodium quinoa. Subsequent analysis of 21 PVS isolates by RT-PCR-RFLP, including isolates identified as PVSA-like on C. quinoa, demonstrated RFLP patterns predicted for PVS°. Results suggest that the differentiation of PVS into two strains may need revision, and a need for a more in-depth study of the phylogeny and biological properties of PVS isolates from a wider geographic area, to better understand strain relationships. Thirteen PVS isolates latently infected Solanum laciniatum Ait., following mechanical inoculation. This is the first record of S. laciniatum as a host of PVS.