VARIATION IN PATHOGENICITY OF LEPTOSPHAERIA-MACULANS ON BRASSICA SPP BASED ON COTYLEDON AND STEM REACTIONS

Variation in pathogenicity among single ascospore isolates of Leptosphaeria maculans obtained from Manitoba, Saskatchewan, and Western Australia was investigated on a range of brassica genotypes based on the disease reaction elicited on the cotyledon and in the stem. Based on the cotyledon reaction on Quinta and Glacier, isolates from Western Australia were more pathogenic than those from Manitoba and Saskatchewan. Isolates from Manitoba and Saskatchewan could be differentiated on Brassica napus genotypes R83-14.DH47, R83-14.DH26, and Karat, and on B. juncea accession UM3132. Based on stem reaction, isolates from Western Australia were more pathogenic on R83-14.DH47. Isolates from Manitoba and Saskatchewan could be differentiated on B. napus genotypes Global and Karat, and on B. juncea genotype UM3132. Cotyledon reactions were compared with stem reactions for each combination of isolate and genotype. A resistant cotyledon reaction was associated with a resistant stem reaction. A susceptible cotyledon reaction was generally associated with a susceptible or moderately susceptible stem reaction. Exceptions were the resistant stem reactions for R83-14.DH26, and Global, indicating adult plant resistance. For both cotyledon and stem reaction, isolates of L. maculans obtained from a single field varied in pathogenicity. Isolates from Western Australia were more pathogenic than those from Saskatchewan and Manitoba. Isolates from Manitoba and Saskatchewan had a similar pathogenicity spectrum.