Studies on cereal mildew in Britain

Abstract

Overwintering cleistocarps of Erysiphe graminis do not contribute to spring outbreaks of cereal mildew in this country. Ascospores, shed from July to late September, are the source of infection for successive crops of winter wheat and barley, but conidia from infected late tillers are the chief source for winter oats. Ascospore differentiation only takes place under high moisture, following the completion of a ‘ripe-to-spore’ stage. Optimum conditions for this are located at the base of the straws and the stubbles are the main source of the ascospore shower. The intensity of the shower in any season is related to the environmental conditions during the growth of the crop and the degree of humidity at and following harvest. Direct infection of autumn-sown cereals may occur but more generally it derives from stubble volunteer seedlings infected by the ascospores. The timing of the ascospore shower and the extent to which volunteer seedlings develop on the stubbles determine the incidence of mildew in early-sown winter cereals, but susceptibility to disease decreases as the date of sowing is delayed. Although existing lesions continue growth and sporulation, during the winter there is little further spread of infection through the crops. The incidence of infection foci for the build-up of mildew in spring is thus dependent upon the climatic conditions prevailing during the previous year and the extent and timing of the autumn cultivations.