Seasonal and Diurnal Patterns of Spore Dispersal by Leptosphaeria maculans

Canola (Brassica napus L. and B. rapa L.), is one of the major oilseed crops grown worldwide. With significant expansion of acreage sown to canola during the past two decades, concern about blackleg, the most serious disease of canola, has increased (3,4). Blackleg disease is caused by Leptosphaeria maculans (Desmaz.) Ces. & De Not. (anamorph: Phoma lingam (Tode:Fr.) Desmaz.). The pathogen can attack all parts of the plant. Symptoms first appear on cotyledons and true leaves as dirty white to buff-colored lesions, round to irregular in shape, and dotted with numerous black pinhead-size pycnidia (19). Ascospores are the most important primary inoculum contributing to the disease (10). They can be liberated from infested stubble for at least 3 years (12,14,15). Pycnidiospores act as secondary inoculum after infection is established by ascospores and are believed to be spread by rain splash to other leaves and neighboring plants (1,14,19). Although the life cycle of the pathogen is well known, understanding of the epidemiology and weather factors affecting release and dispersal of spores is limited. Also, there are no field-scale studies on dispersal patterns of spores by L. maculans in western Canada. These factors limit our confidence in understanding the epidemiology of the blackleg pathogen and managing this disease. The objectives of this study were to investigate (i) seasonal and diurnal ascospore and pycnidiospore dispersal patterns; (ii) relationship of ascospore and pycnidiospore dispersal to temperature, relative humidity, wind, and rainfall; and (iii) disease gradients created from a point source in relation to spore dissemination distance.