Forecasting light leaf spot on winter oilseed rape

Abstract

The aim of this four year project was to improve strategies for control of light leaf spot by developing accurate systems for forecasting the severity of the disease and by optimising the efficacy of fungicide applications.

Seven 'light leaf spot regions' were identified (using existing disease survey and weather data), and now form the basis of a regional light leaf spot risk forecast. A model was developed to identify which crops were most at risk in any season, by assessing the influence of cultural and weather factors on light leaf spot epidemiology. This model is used to forecast, in the autumn, the risk of severe light leaf spot occurring in the spring. The forecast has been issued each autumn since 1996 (with updates in the spring), and so growers have been aware of impending, high risk seasons in their regions. Each year, predicted disease severity has been compared with actual disease severity, and the model modified in the light of new information (including results from controlled environment, epidemiology studies). As a result, the forecast has become progressively more accurate. In 1998, the forecast was made available on a web-site, which also contains other relevant information on the disease. The web-site is now interactive, and growers can in-put information on sowing date, cultivar and use of autumn fungicides, in order to customise the risk forecast for their own crops.

A series of nine field experiments showed that large yield increases can result from well-timed use of fungicides in the autumn. Growers have been advised of these findings and there has been a shift from spring to autumn spraying in recent years. Cultivar susceptibility to light leaf spot was shown to be very important in determining yield loss and yield benefit from use of fungicides and this information has been incorporated into the forecast of crop risk.

A molecular diagnostic technique for Pyrenopeziza brassicae, for use as a research tool, has been successfully developed, and work has begun on an immunodiagnostic technique, which could be used by growers to help them detect light leaf spot in winter oilseed rape crops.