Leptosphaeria maculans populations of North Dakota are becoming more diverse

Blackleg, caused by Leptosphaeria maculans is the most important fungal disease affecting canola (Brassica napus) production in North Dakota. The ability of this pathogen to produce physiological races and overcome the protection offered by single resistance genes limits the implementation of longer lasting management strategies. Races in L. maculans are determined by the presence of up to 14 avirulence (AvrLm) genes. Ten of these genes have already been sequenced. The objective of this study was to characterize the changes in avirulence gene prevalence in L. maculans isolates collected from north eastern North Dakota canola fields in 2004, 2014, and 2024. A total of 132 isolates were used for this study. The isolates had their genomic DNA extracted using commercial kits. Polymerase chain reaction assays were conducted on these isolates using primers specific to ten avirulence genes. Assays for each gene were conducted twice. The results of the study indicate that in 2004, five races were detected with race 2-3-4-7-5-6 representing 87% of the samples. In 2014, two races were identified and one of them, race 1-2-3-4-7-5-6-9-10a-14, represented 98% of the isolates. In contrast, in 2024, 11 races were identified, with the two most prevalent, 1-2-3-4-7-6-9-10a-10b-11 and 1-2-3-4-7-5-9-10a-10b-11, accounting for 31% and 36% of the isolates, respectively. Information generated through this study will help implement a program for resistance gene rotation that will help alleviate the selection pressure placed on the pathogen by the constant use of the same resistance genes.