Modelling the incidence of virus yellows in sugar beet in the UK in relation to numbers of migrating Myzus persicae.
The incidence of virus yellows diseases (caused by beet yellowing virus [beet yellows closterovirus] and/or beet mild yellowing luteovirus) in sugar beet crops in eastern England during 1965-96 was analysed in relation to numbers of migrating M. persicae. A non-linear model was fitted to the data incorporating dual routes of infection: primary infection, arising from winged immigrant aphids carrying the virus, and secondary infection, arising from local dispersion of their wingless offspring transferring the virus from infected to uninfected plants. A good description of virus yellows incidence in August depended on allowing the rate parameters for primary and secondary infection to decrease sigmoidally over the years during 1965-96. The shape of this function depicted a major change in the epidemiology of virus yellows since 1974-76. Amongst several contributory factors, this change coincided with the introduction and subsequent wide use of systemic granular pesticides. Good fits also depended on allowing the rates of disease progress to decay with time to accommodate effects of increasing host resistance with plant age to feeding aphids. Given that the changes in the epidemiology of virus yellows over the years are associated with improvements in pest management practices, it is suggested that the model presents a useful extension to disease forecasting by providing predictions of disease risk in the absence of pesticides.