Variability of Leptosphaeria maculans in Relation to Blackleg of Oilseed Rape

Abstract

Virulent and avirulent strains of L. maculans isolated from rapeseed [Brassica napus, B. campestris] plants in Saskatchewan [Canada] were differentiated on the basis of cultural characteristics and virulence to rapeseed. Surveys of field populations showed that the avirulent strain was more prevalent. An isolate from cabbage [B. oleracea] in Wisconsin [USA] was identical to the virulent strain. Several isolates from rapeseed in Australia all were virulent on rapeseed but showed variability in culture. There was a significant correlation between disease rankings on 1 set of Brassica sp. seed lines tested against the Wisconsin isolate and an Australian isolate. Similar results were obtained with a 2nd set tested against the Wisconsin isolate and a field population of L. maculans in Australia. A strain of L. maculans also was detected on stinkweed (Thlaspi arvense L.) collected from fields in Saskatchewan. This strain could be distinguished from others by the differential reactions of rapeseed and stinkweed in pathogenicity tests. Blackleg was more severe in rapeseed field plots containing residues of rapeseed infected with blackleg than in those containing infected residues of stinkweed.