Molecular and phenotypic characterization of populations of Leptosphaeria maculans and induced resistance to blackleg disease of canola by Leptosphaeria biglobosa

Blackleg of canola (Brassica napus L. and B. rapa L.) is a disease complex of at least two fungal species: Leptosphaeria biglobosa n.sp., weakly virulent and assigned to the pathogenicity group 1 (PG-1) and L. maculans (Desmaz.) Ces. & De Not, highly virulent comprising PG-2, PG-3, or PG-4 isolates. In greenhouse tests, disease reduction was observed both on cotyledons of two Brassica napus cvs, Westar and Invigor 2153 when PG-1 was either pre- or co-inoculated with PG-2, PG-3, and PG-4 and on six-leaf stage plants of Westar when the lower leaves were treated with either PG-1 or salicylic acid (SA). The activity of defense-related enzymes was greatly enhanced when Westar cotyledons were inoculated first with PG-1 followed by PG-2. Field experiments in 2003 and 2004 showed decreased blackleg severity in plants inoculated with PG-1 alone or prior to PG-2. Pathogenicity tests on a total of 512 isolates collected worldwide or from a single field during a four-year period (2001 to 2004) on three differential cvs. Westar. Glacier and Quinta indicated that PG structure in Western Canada and U.S.A. had remained unchanged from 1984 to 2001. They were either PG-1 or PG-2, whereas isolates from Brazil, Australia and UK were mainly PG-3 or PG-4. In 2002 and 2003, the presence of PG-3, PGT and PG-4 were detected for the first time across the Western Prairies of Canada and North Central U.S.A. Co-existence of five PG types in a single field in Manitoba, Canada suggested the possibility of the occurrence of sexual recombination in the pathogen. Genetic diversity and population structure among six geographic populations were determined using the sequence-related amplified polymorphism (SRAP) technique. According to the number of polymorphic loci, values of heterozygosity (H) and rate of gene flow, the populations from Western Canada and North Dakota showed a higher genetic diversity than populations from Australia, United Kingdom. Avirulent PG-1 isolates were clustered into a separate lineage from PG-2, PG-3, PG-4 and PGT populations. The analysis of molecular variance revealed that high genetic variation was found among isolates within populations regardless of the origin and pathogenicity.