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Abstract
The influence of tillage regime on the decomposition of shoot and root residues of canola (Brassica napus, Brassica rapa) infested with the virulent strain of Leptosphaeria maculans and disease levels was evaluated from 1993 to 1997, near Beaverlodge, Alberta. Fall mold-board ploughing in October 1993 was most effective at reducing the amount of canola residue, especially for the first 2 years. Residue retention was also greater for zero tillage than for conventional tillage. After 3 years, residue amounts were low for all tillage treatments. Initially shoot residues were more abundant than root residues, but the rate of decomposition for shoot residues was higher than for root residues. Soil disturbance as a result of tillage and seeding operations appeared to bring some residue, especially root residues, to the soil surface, resulting in a nonlinear relationship between residue weight and time. Low levels of blackleg were found in the canola grown in 1997 and were not significantly different among tillage treatments. A rotation including barley, field peas, and wheat for 3 years following canola helped to eliminate potential disease sources under all tillage systems used in this study.