Genetic analysis of Pyrenopeziza brassicae, cause of light leaf spot of brassicas, in the European Union, Oceania, and North America

Light leaf spot (LLS), caused by Pyrenopeziza brassicae, is an important disease of Brassica napus (canola and oilseed rape) and B. oleracea (vegetable brassicas) in Europe (EU) as well as New Zealand and Australia (Oceania, OC). LLS was first reported in North America (NA) on B. juncea, B. napus, and B. rapa in six counties in western Oregon in 2014; and on B. juncea cover crops and wild B. rapa in three counties in northwestern Washington in 2016. Multi-locus sequence analysis (ITS ribosomal DNA, beta-tubulin, and elongation factor 1-α sequences) and comparison of the mating type genes (MAT1-1 and MAT1-2) grouped isolates from the EU (n = 28) and OC (n = 4) with the P. brassicae type specimen, IMI 204290, whereas isolates from NA (n = 16) represented a novel genotype. Sexual compatibility of NA and EU strains of complementary MAT1-1 and MAT1-2 genotypes is being determined to assess if NA isolates represent a distinct evolutionary lineage or a cryptic sibling species. Fungicide resistance has been documented in some EU populations of P. brassicae, but none of the NA isolates possessed amino acid substitutions E198A and L240F in the beta-tubulin sequences that confer resistance to benzimidazole fungicides; comparison of these sequences for the NA isolates revealed 100% identity to wild type EU P. brassicae isolates and the closely related fungus Rhynchosporium commune; and 98 and 99% identities to Sclerotinia sclerotiorum and Venturia inaequalis, respectively.