Epidemiology and the development of risk assessment models for the management of tomato spotted wilt virus (TSWV) in potatoes

Tomato spotted wilt virus (TSVVV) occurs sporadically in potato (Solanum tuberosum) crops in Australia, sometimes causing severe losses. Very little is known about the disease in potato as evidenced from the literature review (Chapter 1). Part of the problem stems from the sporadic nature of the disease in potato, which makes it hard to find a consistent data set at the desired level of aggregation, both for disease incidence and intensity as the variables to be explained and for explanatory variables. This makes it difficult to formulate empirical descriptions of risk factors and statistical exploration of interactions of different variables that develop into functional complexes causing the epidemics. Surveys were conducted from June 2001 to March 2004 in commercial potato fields in the southern States of Australia. Data sets from these surveys were used to develop dynamic mathematical models, defining spatio-temporal oscillations, in both population structure and demography, of the only two thrips vector species, Thdps tabad and Frankliniella schulei, found in potato crops (Chapter 2). The Gompertz and Exponential curves best described the relationships between the vector and the regional weather variables. In general, the optimum daily minimum air temperature (°C) required by T. tabaci population to start rising within potato crops ranged between 10°C and 12.0°C and daily maximum air temperature (°C) range was 14.5°C and 22°C. The observed versus predicted estimates for the daily relative humidity at 3pm in Tasmania had low percentage variance (r2) values for T. taboo: The general optimum daily minimum air temperature (°C) range for F schultzei populations to start rising was between 4°C and 6°C and daily maximum air temperature (°C) range of 20°C and 22°C. Less than favourable daily relative humidity at 3pm of more than 40% depresses F. schultzei populations in both Victoria and South Australia. Both T. tabaci and F. schultzei populations are depressed by precipitation as indicated by three models across the sampling sites. The models can be used as a hazard prediction to orient integrated TSWV disease management. To test the significance of the potato cultivars being grown in affecting the epidemiology of TSVVV, 27 cultivars were evaluated for two seasons in both glasshouse and natural field conditions. Variations in susceptibility to infection were exhibited by potato cultivars and were conditioned by the age of the plant at the time of infection as measured by symptom expression, shoot and tuber infections in both mechanical and thrips transmissions (Chapter 3). For both foliar and tuber infections, more cultivars exhibited symptoms when infected late than early. TSVVV infections between cultivars were not significantly different (P>0.05), except in tubers from mechanically inoculated plants during 2002/2003. Foliar infections in a number of cultivars did not translate into virus translocation to tubers except in Atlantic, Bintje, Fontenot, Kennebec, Kipfler, McCains 1 and Shepody. Thrips had a high feeding preference for the cultivar Royal Blue, but this did not translate into high TSVVV infections. The cultivar Bismarck, which despite being highly susceptible to foliar and tuber TSVVV infections through mechanical inoculations, exhibited strikingly robust field resistance to thrips feeding and consequently TSVVV infection in the field. The study suggests that the risk of TSVVV epidemics depends on when and to what extent the plants develop resistance in relation to the age of the crop. Certification guidelines for tolerance levels of TSVVV infection in seed potatoes in Australia should reflect this on a cultivar specific basis. The independent and interactive effects of temperature with plant age at the time of inoculation on viral movement restriction, symptom expression and foliar and tuber infections were assessed and quantified in potato cv. Shepody (susceptible) and Russet Burbank (tolerant) under controlled conditions (Chapter 4). Plants exposed immediately after inoculation or after 72 hours to four different temperature regimes at two different stages of growth (early versus late) showed variations in symptom expressions and virus movement patterns. In inoculated plants immediately exposed to different temperature treatments, foliar infections and symptoms were generally restricted to lower and middle leaves of the canopies and auxiliary shoots. More leaves of early-inoculated plants expressed symptoms than in late inoculations. In early-inoculated cv. Shepody plants continuously exposed to 22°C, virus migration reached top canopies two days earlier (5 days after inoculation) and induced TSVVV symptoms than similar plants under continuous 16 °C. In both cultivars, plants were more resistant to TSVVV infections when inoculated late and immediately exposed to 22 °C for 48 hours, inoculated and then transferred to 16°C. Under these conditions, some infections were detected in lower inoculated leaves of cv. Russet Burbank plants at 12 days after inoculation. When plants had a 72 hours delayed exposure to different temperature regimes, the number of plants with detectable TSVVV in shoots, number of leaves on each plant and total leaf areas with symptoms were in the middle and top canopies. The virus was detected only in early-inoculated plants. Most plants did not exhibit TSVVV symptoms and the virus was not detectable by ELISA, except in Shepody plants exposed to 16°C continuously and 22°C for 48 hours, inoculated and then transferred to 16°C. In these plants the virus reached the middle and top canopies where it was detected in cvs. Russet Burbank and Shepody at 28 and 43 days after inoculation, respectively. The underlying mechanism by which temperature influences these virus movements and symptom expressions in TSVVV infections is still unclear. Results from this study would be useful in the rational selection of suitable conditions when screening potato cultivars for TSVVV resistance by breeding programs and production in appropriate climatic conditions. In summary, the results from the above studies, collectively, contribute to uncover some ecological relationships and patterns of both TSWV and its vector thrips that can be integrated with plausible mechanisms to explain the epidemiology of the virus in potato crops and advance the rationale for future research in this direction.