Black leg (Leptosphaeria maculans (Desm.) Ces. de Not.) of rapeseed in Victoria: sources of infection and relationships between inoculum, environmental factors and disease severity

Abstract

In established rapeseed-growing districts, ascospores discharged from crop residues were shown to be the major source of infection for black leg disease. Infection from seed was of relatively minor importance. The potential amount of ascospore inoculum on crop residues was related to severity of the disease in the original field. Rainfall of more than 1.0 mm was required for large ascospore discharges, but smaller ones occurred in light rain or dew. Discharge from the same crop residues increased greatly between April and May, remained at a high level until August, and then gradually declined until by January very few spores were discharged. In April, discharge resumed and the same cycle was repeated. The amount of crop residue surviving in fields from the previous year declined by 90% during the growing season. No relationship was apparent between the amount of rainfall during the period when infection occurred and the disease severity in crops sown at different times. Stem canker development was less severe in plants grown at 8° than in those grown at 12° and 15°C.