Resistance to infection and translocation of Tomato spotted wilt virus in potatoes

Tomato spotted wilt virus (TSWV) was detected by ELISA within freshly harvested infected tubers of all cultivars tested, but had an erratic distribution which could affect detection reliability. The lack of detectable virus within many tuber eye samples was reflected in production of healthy shoots from infected tubers, some of which did eventually succumb to TSWV infection after 1-4 weeks' growth, and observations suggested this may be enhanced by virus multiplication in diseased shoots produced from the mother tuber. Breeding populations of Thrips tabaci were found within a field exposure trial, and trapping confirmed this as the only known TSWV vector present. In this trial, relying on natural thrips transmission, there was no significant variability in susceptibility of the cultivars tested to initial foliar infection. However, in a glasshouse trial using mechanical inoculation, significant cultivar differences in the rate of foliar systemic infection were observed. Substantial cultivar differences were also found in the efficiency of TSWV translocation from infected plant to tuber, and from infected tuber to progeny plants. Tuber infection rate was further affected by the age of plants at infection, and by the proportion of stems of each plant showing symptoms of TSWV infection. Two apparent resistance mechanisms operated to restrict TSWV translocation to tubers. The first showed strong but incomplete resistance at all ages of challenge; the second showed initial susceptibility to tuber infection but, in challenges after tuber initiation, showed a strong resistance phenotype. Expression of tuber necrosis also varied with cultivar and was affected by plant age at infection. Malformed tubers and secondary growth were also found following foliar TSWV infection, but were not necessarily associated with tuber infection. Current certification tolerance levels for TSWV infection in seed potatoes in Tasmania overestimate the risk of TSWV infection. Given that disease risk varies markedly with cultivar, perhaps certification tolerance levels should reflect this on a cultivar-specific basis.