Variation in Aggressiveness of Stagonospora nodorum Isolates in North Dakota

Stagonospora nodorum blotch (SNB), caused by Stagonospora nodorum, is an important disease in the northern Great Plains of the United States and in other wheat‐producing regions in the world. SNB can be managed by different strategies including the use of resistant cultivars. Genetic variation in the pathogen populations is one of the important factors in the development of durable resistant cultivars. Our main objective was to determine variation in aggressiveness/virulence in the 40 isolates of S. nodorum collected from various locations in North Dakota. To achieve this goal, we tested the isolates on two susceptible wheat cultivars (cvs ‘ND495’ and ‘Alsen’) and two resistant wheat cultivars (cvs ‘Erik’ and ‘Salamouni’) – two‐leaf‐stage seedlings under controlled conditions. Aggressiveness of each isolate was characterized by the two epidemiological parameters: percent necrotic leaf area (% NLA) and lesion type (LT) 8 days post‐inoculation. The isolates differed significantly (P ≤ 0.05) for % NLA and LT, and were grouped into three aggressiveness groups (AG): low, medium and highly aggressive. Four isolates (S50, S57, S66 and S89) induced 18–26% NLA and were included into the low aggressive group (AG 1). Three isolates (S15, S39 and S89) induced 57–59% NLA and were considered highly aggressive (AG 3). Thirty‐three isolates were medium aggressive (AG 2). No relationship between AG and mating types was observed. There were significant (P ≤ 0.05) differences in % NLA and LT among wheat cultivars. Significant wheat cultivars by isolates interaction was also demonstrated, suggesting evidence for the existence of host specificity in this system. Overall, our results indicate that S. nodorum isolates prevalent in North Dakota varied greatly in their aggressiveness and that AG 3 isolates can be utilized in breeding wheat for resistance to SNB.