Constant rank of virulence and resistance in the Leptosphaeria maculans - Brassica napus pathosystem

The reactions of five double haploid lines of spring oilseed rape (Brassica napus) to 98 isolates of Leptosphaeria maculans, cause of blackleg, were tested in the greenhouse. The average of lesion lengths measured 3, 4, 5, 6, and 7 weeks after inoculation of stems of 5-week-old plants was used as a measure of virulence of isolates and of resistance of lines. Pearson and Spearman analyses showed that lesion lengths produced by isolates on pairs of lines were significantly correlated. This indicates that the relative virulence of an isolate was similar on all lines and the relative resistance of lines was similar for each isolate. It is concluded that field tests in which canola lines are exposed to natural inoculum of the fungus will generally provide reliable information on the resistance of the plant to naturally occurring populations of the fungus across a wider range of locations.