Seed Transmission and Management of White Leaf Spot and Light Leaf Spot Pathogens in Brassicas in the Pacific Northwest

View the project final report

• 2016 annual report

Commodities

• Agronomic: canola, mustard, radish (oilseed, daikon, forage), rapeseed, Turnip cover crops
• Vegetables: broccoli, brussel sprouts, cabbages, cauliflower, radishes (culinary), rutabagas, turnips
• Additional Plants: Brassica weeds, brassica cover crops

Practices

• Crop Production: cover crops, cropping systems, postharvest treatment, plant diseases, seed pathology
• Education and Training: extension, workshop
• Pest Management: chemical control, economic threshold, eradication, field monitoring/scouting, integrated pest management, physical control
• Production Systems: organic agriculture

Carmody and du Toit - WSARE Annual Report 1-07-2017

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, β 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 4^oC, 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 (50^oC for 15 and 30 minutes), steam (62.8, 65.6, 68.3, and 71.1^oC 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.

The following research objectives were addressed:
1. Assess the potential for Pyrenopeziza brassicae (cause of light leaf spot) and Neopseudocercosporella capsellae (cause of white leaf spot) to be seedborne in brassica crops, and to understand if these pathogens can be introduced into new regions on infected seed. This was addressed by:

a. Establishing whether P. brassicae and/or N. capsellae can infest or infect brassica seed; and
b. Establishing whether brassica seed infected with P. brassicae and/or N. capsellae can lead to seed transmission.

2. Survey brassica crops, brassica weeds, and brassica seed lots to establish if the light leaf spot pathogen and/or white leaf spot pathogen are present in the primary region of biennial brassica vegetable seed production in northwestern Washington. This entailed:

a. Confirming pathogenicity of isolates collected from infected plants in this region, using Koch’s postulates; and
b. Determining differences between isolates collected in the PNW USA and isolates collected from other countries where P. brassicae and N. capsellae have established, such as the UK and New Zealand.

3. Identify seed treatments effective at eradicating P. brassicae and/or N. capsellae from infected brassica seed. This was accomplished by:
a. Evaluating the efficacy of organic seed treatments such as hot water, steam, and 1.2% NaOCl;
b. Evaluating fungicide seed treatments representing different Fungicide Resistance Action Committee (FRAC) groups to identify products effective at preventing seed transmission of P. brassicae and/or N. capsellae.

The research was aimed at developing a better understanding of the prevalence of light leaf spot and white leaf spot in northwestern Washington, and helping brassica growers and the brassica seed industry understand if the seedborne phase of either pathogens is of concern. This research is expected to provide brassica growers and the brassica seed industry with tools to detect the pathogens on infected seed, and to eradicate the pathogen from infected seed by treating infected seed lots using organic and/or conventional treatments.