Light leaf spot and white leaf spot of Brassicaceae in Washington State. Carmody, S.M. MS thesis 1017

Pyrenopeziza brassicae, cause of light leaf spot of brassicas, was first found in the USA in 2014 in the Willamette Valley of Oregon. Neopseudocercosporella capsellae, cause of white leaf spot of brassicas, occurred rarely in the Pacific Northwest prior to being found across the Willamette Valley in 2014. In this study, a 2016 survey of northwestern Washington, a primary region of biennial brassica vegetable seed production for the USA, revealed both diseases to be present in mustard (Brassica juncea) cover crops and on bird’s rape mustard (B. rapa) weeds, but not in cabbage (B. oleracea var. capitata) seed crops. Sexual crossing tests, pathogenicity tests, and DNA phylogenetic analyses (latter of the internal transcribed spacer region of ribosomal DNA, ? tubulin gene, translation elongation factor 1-alpha gene, and mating type genes (MAT1-3 and MAT1-2); and multi-locus sequence analysis of the first three sequences) of P. brassicae isolates from the USA, European Union, New Zealand, and United Kingdom revealed that isolates from the USA likely represent a new species of Pyrenopeziza, hereafter referred to as P. cf. brassicae. P. cf. brassicae was demonstrated to be seedborne and seed transmitted on cabbage and mustard. Incubating infested seed on NP-10 agar medium at 4oC, followed by microscopic examination of the seed revealed P. cf. brassicae to be present on 12.50 to 19.75% of a mustard seed lot and <0.50% of a cabbage seed lot. Planting the infested mustard seed in a greenhouse resulted in a seed transmission rate of 0.1 to 5.3%. Seed treatment trials revealed that chlorine (1.2% NaOCl for 10, 20, 30, and 40 minutes), hot water (50oC for 15 and 30 minutes), steam (62.8, 65.6, 68.3, and 71.1oC for 90 seconds), and 10 fungicide seed treatments all reduced the incidence of mustard seed infected with P. cf. brassicae to <5%, and reduced seed transmission of the fungus from 3.4% for non-treated seed to <1%. Hot water and most of the steam treatments eradicated the pathogen from seed, but the hottest steam treatment was phytotoxic. The most efficacious fungicide seed treatments contained benzimidazole, a demethylation inhibitor, and/or strobilurin active ingredients.