Epidemiology / Épidémiologie Effects of crop rotation and tillage on blackleg disease of canola

The effects of a 4-year crop rotation and two tillage systems on blackleg (Leptosphaeria maculans) disease of canola were determined in a field experiment carried out from 1999 through 2002 at Carman, Manitoba. Crops in rotation were canola (Brassica napus) (C), wheat (Triticum aestivum) (W), and flax (Linum usitatissimum) (F). Rotations were done under conventional tillage (T) or zero-till (Z) systems. Diseased stem incidence and severity were assessed. Diseased stem incidence and severity in 2001 and 2002 were significantly lower when canola was rotated with wheat, and rotated with wheat and flax under both tillage systems. The CCCCT rotation had a higher disease incidence than CWCCT, CWFCT, and CCWCT rotations, which were not significantly different from each other. Zerotill plots had significantly greater diseased stem incidence and severity than tilled plots, with or without rotation. Tillage reduced the disease when it was performed with a simple-crop rotation; however, the effect was reduced with a two-crop rotation. The results suggest that an appropriate rotation and tillage strategy could be effective in reducing the severity of blackleg of canola in farmers’ fields.