Some aspects of the epidemology of dark leaf and pod spot (Alternaria brassicae L.) on winter oilseed rape

Dark leaf and pod spot(Alternaria brassicae) can be a serious disease of oilseed rape (Davies, 1986). In this study a detailed examination of the development of dark leaf spot on winter oilseed rape was made during three seasons (1986-1988) in Hampshire, U.K.. Disease area diagrams for dark leaf and pod spot were constructed and different measures of disease compared. Taylor's Power Law was fitted to the data for each variable, and transformations and the sample numbers required for different levels of accuracy were derived. Relationships between different variables showed that incidence-severity relationships and selection of the plant parts assessed could be useful in reducing the time required for assessment. Disease progress on the leaf canopy was described using simple graphical methods and contour mapping. The latter provided a successful way of representing the vertical progression of the disease through the leaf canopy during the season. Although disease intensities differed in each epidemic the distribution pattern of disease was similar. Disease was already widely spread in the crop during the `rosette' stage and continued to develop slowly during the winter. During stem extension the disease was confined to the basal main stem leaves even though the leaf canopy was well spread vertically and branch leaves surrounded the mainstem leaves at the crop base. As the leaf canopy matured the disease moved up the plant so that prior to complete leaf loss the vertical distribution of the disease resembled that of the leaf canopy. On the pod canopy disease development in each epidemic showed a similar development pattern which was adequately described by the logistic function. In each epidemic a relatively long lag phase was followed by disease development which coincided with heavy rainfall. Plant traps were used in 1987 and 1988 to assess the levels of infection occurring during the season. In 1988 infection on the plant traps was highly correlated with spore numbers caught (R2= 78%) whereas in 1987 no clear relationship could be derived. Infection on the plant traps were almost always associated with rain induced wetness but attempts to relate infection with measures of relative humidity, rainfall and wetness using multilinear regression were unsuccessful. In 1987 and 1988 levels of inoculum were measured using spore traps. Different patterns of spore capture occurred in each year and no clear relationships between measures of relative humidity, rainfall and wetness were developed using multilinear regression.