Studies on effects of crop rotation and tillage on blackleg disease (Leptosphaeria maculans) in canola (Brassica napus), dispersal paterns of L. maculans spores, and effects of temperature and relative humidity on infection of canola cotyledons
Leptosphaeria maculans (anamorph: Phoma lingam), the causal agent of blackleg of canola, is an important disease affecting the crop across western Canada. Effects of cropping practices on blackleg disease of canola were studied in the Department of Plant Science, and Carman Research Station, University of Manitoba, Manitoba from 1999 to 2002. The disease could be significantly reduced when canola was rotated with wheat and flax, and was grown on tilled plots. Tillage showed a significant effect on decreasing the disease when it was performed with a single-crop rotation; however, it did not with a two-crop rotation. A non-host crop grown in the previous year before canola effectively decreased the inoculum level (amount of spores released) and disease. Survival of blackleg pathogen on canola stubble significantly decreased within nine months. With increase of soil depth, viability of the pathogen significantly decreased. The pathogen had more difficulty in surviving in clay than in loam and sand. There was a significant seasonal dispersal pattern of ascospores by L. maculans from the middle of June to the end of July, a dispersal pattern of pycnidiospores from the middle to end of July or beginning of August, and a diurnal dispersal pattern of ascospores and pycnidiospores from 9 pm to 4 am. The optimum temperature for ascospore and pycnidiospore release were 13-18 oC and relative humidity > 80% respectively. Peak ascospore and pycnidiospore dispersal were associated with more than 2-mm rain events. Peak pycnidiospore dispersal could occur at the same hour as rain events...