Adaptability and virulence specificity in Australian strains of blackleg [Leptosphaeria maculans (Desm.) Ces. et De Not.] on different host genotypes of rapeseed (Brassica napus L.)

Abstract

Five clone-lines of rapeseed (Brassica napus L.) were artificially inoculated with 24 isolates of blackleg (Leptosphaeria maculans (Desm.) Ces. et De Not.) from 12 sites around Australia to determine the proportion of virulence genes in L. maculans which interacted with genes for specific resistance in B. napus. Resistance/susceptibility was evaluated both as external lesion size (%G) and internal lesion area (%II) of individual plants of the clone-lines. Using two different regression techniques (stability analysis) involving levels of infection caused by the isolates on the clone-lines, isolates were ranked according to levels of specific virulence. There were differences in stability between isolates in their capacity to exhibit consistent virulence over the range of genotypes used. Using deviations from regression as an indicator of the proportion of genes for specific- and non-specific virulence, it was observed that a third of all isolates used possessed significant levels of specific virulence. The significance of this finding in relation to glasshouse screening programs for blackleg resistance is discussed.