Pathogenic variability and prevalence of Avr genes in Leptosphaeria maculans populations from Alberta, Canada

Blackleg (Phoma stem canker), caused by the Leptosphaeria species complex L. maculans and L. biglobosa, is a severe and economically important disease in most oilseed production areas worldwide. In this study, the virulence structure of a population of 115 L. maculans isolates from six counties in Alberta, Canada, was determined on three differential canola cultivars, Westar, Quinta and Glacier. Based on the differential interactions as assessed on a 0-9 severity scale, the isolates were classified into four pathogenicity groups: PG-2, PG-T, PG-3, and PG-4. When the differential interactions were assessed by analysis of variance of lesion size, the isolates were classified as into PG-2, PG-T and PG-4, among which PG-4 was predominant comprising 59% of the isolates, followed by PG-T at 37% and PG-2 at 4%. The presence or absence of five avirulence genes, AvrLm1, AvrLm4-7, AvrLm6, AvrLmJ1 and AvrLm11, in each of the isolates was evaluated by PCR analysis. AvrLm1 was absent, while AvrLm4-7 and AvrLm6 were present, in all of the 115 isolates. AvrLmJ1 and AvrLm11 were present in most isolates but absent in three isolates collected from Ponoka County. AvrLm11 also was absent in another two isolates collected from Strathcona County. This study describes the current status of virulence in L. maculans population in Alberta, which may provide guidance for blackleg resistance breeding efforts and cultivar deployment.