Epidemiology of Leptosphaeria maculans in relation to forecasting stem canker severity on winter oilseed rape in the UK

In the UK, ascospores of Leptosphaeria maculans first infect leaves of oils eed rape in the autumn to cause phoma leaf spots, from which the fungus can grow to cause stem cankers in the spring. Yield losses due to early senesc ence and lodging result if the stem cankers become severe before harvest. T he risk of severe stem canker epidemics needs to be forecast in the autumn when the pathogen is still in the leaves, since early infections cause the greatest yield losses and fungicides have limited curative activity. Curren tly the most effective way to forecast severe stem canker is to monitor the onset of phoma leaf spotting in winter oilseed rape crops, although this d oes not allow much time in which to apply a fungicide. Early warnings of ri sks of severe stem canker epidemics could be provided at the beginning of t he season through regional forecasts based on disease survey and weather da ta, with options for input of crop-specific information and for updating fo recasts during the winter. The accuracy of such forecasts could be improved by including factors relating to the maturation of ascospores in pseudothe cia, the release of ascospores and the occurrence of infection conditions, as they affect the onset, intensity and duration of the phoma leaf spotting phase. Accurate forecasting of severe stem canker epidemics can improve di sease control and optimise fungicide use.