Diseases of winter oil-seed rape: occurrence, effects and control

SUMMARY The results of observations begun in 1973 and of disease surveys carried out on winter oil-seed rape crops on ten farms in Hertfordshire and Bedfordshire between 1975 and 1978 are reported. The major diseases encountered in the surveys, namely downy mildew ( Peronospora parasitica (FT.) Tull.), canker ( Leptosphaeria maculans (Desm.) Ces. & de Not.), light leafspot or scorch ( Pyrenopeziza brassicae Sutton & Rawlinson) and grey mould ( Botrytis cinerea Fr.) are discussed in relation to diseases of rape grown elsewhere and the results of experiments on disease control reported. Symptoms of virus infection and root disease were rare. Early (before stem extension) use of two applications of benomyl (1·2 kg a.i./ha) decreased the incidence and severity of light leaf spot and canker and increased yield by up to 33%. Benomyl plus thiram seed treatment (5 and l·5ga.i./kg) was less effective than a benomyl foliar spray. In field experiments differences in severity of light leaf spot and canker explained a substantial part of variation in yield but downy mildew did notaccount for further variation.