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Linear models were used to investigate the effects and interactions of treatments for the control of disease in multifactorial experiments on the dynamics of the take‐ al disease of wheat. Analyses were based on repeated measures of disease on second successive winter wheat crops in three successive se sons. The treatments were date of sowing, presence or absence of triadimenol‐containing seed treatment, autumn nitrogen and chloride applications, timing of early spring nitrogen and total spring nitrogen applications. The effects of treatments were investigated on line and quadratic trends of disease increase and on the integral amount of disease over time. Different disease variables were used to express disease relative to the total and diseased plant populations. The effects of treatments on absolute and relative numbers of diseased roots per plant were also analysed. During three se sons when disease was never severe, the effects of the treatments were shown to be variable. Late sowing consistently reduced disease during the early stages of the epidemic (in the winter months). The effect of late sowing in reducing disease, however, persisted through to the end of the growing se son in only one of the 3 years of the study. autumn nitrogen application and Baytan seed treatment (containing triadimenol) also reduced disease in some seasons. The presence of an effect for either was dependent on sowing date and was most evident on the proportion of diseased roots relative to the diseased plant rather than the total plant population.

Summary.Survival of Myzus persicae confined in clip‐cages on mature leaves of sugar beet declined as the plants aged. Death of aphids was often preceded by the appearance of a black deposit in the aphids' stomachs, which may have been the cause of death. Both the rate of death and the proportion of aphids dying with black deposits was significantly less when plants were infected with beet yellows virus or beet mild yellowing virus, by comparison with healthy plants. The implication of these phenomena on the onset of mature plant resistance is discussed.

Summary 0[ The incidence of virus yellows diseases in sugar beet crops in eastern England during 0854Ð85 was analysed in relation to numbers of migrating Myzus persicae\ the principal vector[ 1[ A non!linear model was _tted 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 o}spring trans! ferring the virus from infected to uninfected plants[ 2[ The model was _tted to the data assuming that "i# the epidemic commenced when the _rst migrating aphid was recorded\ "ii# the rate of primary infection was dependent on the total number of aphids migrating up to the end of June\ and "iii# the rate of secondary infection was dependent on the proportion of diseased plants in the crop[ 3[ 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 0854Ð85[ The shape of this function depicted a major change in the epidemiology of virus yellows since 0863Ð65[ Amongst several contributory factors\ this change coincided with the introduction and subsequent wide use of systemic granular pesticides[ 4[ Good _ts also depended on allowing the rates of disease progress to decay with time to accommodate e}ects of increasing host resistance with plant age to feeding aphids[ This meant that early epidemics rose faster than late epidemics\ which is consistent with general observations[ 5[ Given that the changes in the epidemiology of virus yellows over the years are associated with improvements in pest management practices\ the model presents a useful extension to disease forecasting by providing predictions of disease risk in the absence of pesticides [ Such predictions