Comparative studies of light leafspot (Pyrenopeziza brassicae) epidemics on the growth and yield of winter oilseed rape

Comparisons of epidemics of light leafspot of differing duration and time of initiation were made in two experiments using a single cultivar of Brassica napus. Fungicide was applied before introduction of disease to prevent infection or some time after inoculation to stop further disease development. In the first experiment, substantial reductions in green leaf area and total plant dry-matter were found at flowering when disease was introduced in the autumn or in January. Plant dry weight at maturity was also greatly reduced in these treatments. The detrimental effect of an epidemic initiated in the autumn was avoided to a large extent if fungicide application began in February. Epidemics initiated in March had only small effects on final dry-matter yield. Seed yield was negatively correlated with the length of the epidemic. In a second experiment, early epidemics initiated in the autumn were halted after different time intervals. Commencing fungicide application even as early as December failed to prevent some loss of dry weight at flowering. At maturity, however, dry weight and seed yield were reduced significantly when fungicide application was delayed until February. Failure to control the disease resulted in a 46% loss of seed yield.