Epidemiology of Tomato spotted wilt virus (TSWV) in vegetables relative to thrips population dynamicsRiley D Entomology Department, University of Georgia, P.O. Box 748, Tifton, Georgia USA. dgr@uga.edu The epidemiology of thrips-vectored plant disease includes the study of the causes of the disease, analyzing disease occurrence and distribution, and understanding the factors that govern it¨s spread. The ultimate goal of epidemiological studies is to control disease in host crop populations. For thrips- vectored Tospovirus, epidemics require the interaction of the thrips vector, the pathogen host plant and the pathogen. In Georgia, the temporal and spatial dynamics of Tomato spotted wilt virus (TSWV) in Solanaceous vegetable crops, such as tomato, appear to be related to the dynamics of the two main thrips vectors species, Frankliniella fusca (Pergande) and the flower thrips, Frankliniella occidentalis (Hinds). The dynamics of thrips vectors in weed hosts during the winter have been reported to be associated with disease incidence in the Spring cropping season. In studies where weed host of TSWV and thrips were maintained adjacent to a tomato field, elevated levels of TSWV were observed in the tomato field compared to fields without the weed source for TSWV. The dominant thrips vector species in weed host studies in the early Spring appear to be F. fusca. Also, early colonizers of the foliage of tomato plant were observed to be predominantly F. fusca prior to flower formation. Since early inoculation of TSWV results in severe yield reduction, yield loss correlates well with F. fusca populations. In most studies, F. occidentalis was the dominant TSWV vector species on the plant once flowers are present on the crop. In addition, late season incidence of TSWV is better correlated with the incidence of F. occidentalis than F. fusca, suggesting that F. occidentalis is the more effective vector later in the tomato growing season. Frankliniella occidentalis has also been shown to be more positively affected by pollen deposition on leaf tissue in terms of settling and oviposition than F. fusca. We suspect that ambient pollen dynamics in the Southeastern USA directly affects population dynamics of flower thrips and indirectly affects the epidemiology of TSWV. Another factor that could be important in the epidemiology of TSWV is thrips feeding preference. Studies suggest that F. fusca prefers young tomato plant foliage compared to older leaf tissue and avoids imidacloprid- treated leaf tissue. On the other hand, F. occidentalis is less affected by tomato plant age and imidacloprid treatment tends to enhance feeding. Also, imidacloprid treatment and pollen deposition in peanut enhances settling by F. occidentalis on leaf tissue. A two-vector model is proposed to explain some of the observed trends in TSWV epidemiology in the Georgia.
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