Seasonal and Diurnal Patterns of Spore Dispersal by Leptosphaeria maculans from Canola Stubble in Relation to Environmental Conditions

Abstract

Seasonal and diurnal patterns of spore dispersal by Leptosphaeria maculans, which causes blackleg disease of canola, were studied in two consecutive field seasons using a 7-day Burkard spore sampler and rotorod impaction spore samplers. Ascospores of L. maculans were trapped from mid-June to the end of July, whereas pycnidiospores were trapped from mid-July until the end of July or early August. Ascospores and pycnidiospores were trapped between 9:00 P.M. and 4:00 A.M., when air temperatures were 13 to 18°C and relative humidity was >80%. Peak ascospore and pycnidiospore dispersal was associated with rain events. Peak ascospore dispersal was found to occur several hours after rainfall ≥2 mm, and ascospore dispersal continued for approximately 3 days after such events. Peak pycnidiospore dispersal occurred during the same hours as rain events. More ascospores and pycnidiospores were carried in the direction of prevailing winds than in other directions. To the south of the inoculated area, the gradients of disease incidence and stem disease severity were -19.2 and -0.8 m-1, respectively. Disease incidence and stem severity declined by 50% 12.5 and 5.5 m from the inoculated area, respectively. To the north of the inoculated area, the gradients of disease incidence and stem severity were -21.5 and -0.7 m-1, respectively. Disease incidence and stem severity declined by 50% 14.0 and 5.2 m from the inoculated area, respectively. In 2001, ascospores and pycnidiospores were trapped within 25 m of the inoculated area, whereas pycnidiospores were trapped up to 45 m from the inoculated area.