Identification of the telemorph Mycosphaerella graminicola in Manitoba by direct observation and molecular polymorphism
Hoorne, Charlotte Sara
Mycosphaerella graminicola (Fuckel) J. Schrot. in Cohn causes speckled leaf blotch (septoria leaf blotch) in wheat, a major leaf-spotting disease found in many wheat-growing regions. The sexual structures of M. graminicola were identified in wheat stubble during the summer of 2001 in Carman, Manitoba. Ascospores were isolated from pseudothecia and formed conidial colonies similar to the pycnidiospores of Septoria tritici Roberge in Desmaz., the asexual state, on agar medium. Wheat plants inoculated with conidia derived from ascospore isolates developed symptoms typical of speckled leaf blotch. The pathogen was reisolated and budding conidial colonies identical to those produced by the germination of the ascospores were obtained. This represents the first time that the teleomorph M. graminicola was identified by direct observation in Canada. Amplified fragment length polymorphism (AFLP) was the molecular marker system used to indirectly identify the sexual state, and the objective was to detect polymorphism among M. graminicola isolates due to sexual recombination. Isolates from Manitoba field locations at Carman and Minnedosa were collected using a hierarchical procedure and were divided into two separate sets. Set I consisted of 44 isolates representing the two field locations and different stations within each of the fields. Set II was a control test, with 42 isolates representing single spores and their respective pycnidia picked from different leaves and lesions at one station in each of Carman and Minnedosa. Both Set I and II AFLP banding patterns showed polymorphism among isolates and no distinct clustering of populations was observed in the dendrograms. Analysis of molecular variance (AMOVA) of Set I revealed that 99.02% (P<0.05) of the total genetic diversity was due to within population variation and 0.98% to between population variation. This uniform pattern of inoculum distribution within fields suggests that wind-blown ascospores are present in Manitoba and contribute significantly to speckled leaf blotch epidemics.