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

Final report for GW16-055

GW16-055 (project overview)
Project Type: Graduate Student
Funds awarded in 2016: $15,675.00
Projected End Date: 04/30/2017
Grant Recipient: Washington State University
Region: Western
State: Washington
Graduate Student:
Shannon Carmody
Email
Principal Investigator:
Dr. Lindsey du Toit
Email
Washington State University
Expand All

Project Information

Summary:

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 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.

Project Objectives:

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.

Cooperators

Click linked name(s) to expand/collapse or show everyone's info
  • Shannon Carmody
  • Prof. Lindsey du Toit (Researcher)
  • Dr. Cynthia Ocamb (Researcher)
  • Prof. Jonathan West (Researcher)
  • Dr. Kevin King (Researcher)

Research

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

12 Consultations
2 Curricula, factsheets or educational tools
1 Published press articles, newsletters
2 Tours
6 Webinars / talks / presentations
2 Workshop field days

Participation Summary:

50 Farmers participated
150 Ag professionals participated
Education and outreach methods and analyses:

Abstracts for 2 posters presented at 2017 APS meeting:

  • Carmody, S.M., and du Toit, L.J. 2017. Seed treatments to eradicate Pyrenopeziza brassicae from infected mustard (Brassica juncea) seed. Poster 295-P, 2017 APS Annual Meeting, 5-9 Aug. 2017, San Antonio, TX.
  • Carmody, S.M., King, K.M., Claassen, B.J., Fraaije, B.A., West, J.S., Ocamb, C.M., and du Toit, L.J. 2017. Genetic analysis of Pyrenopeziza brassicae, cause of light leaf spot of brassicas, in the European Union, Oceania, and North America. Poster 536-P, 2017 APS Annual Meeting, 5-9 Aug. 2017, San Antonio, TX.

Abstract for oral presentation at 2016 APS PD meeting:

  • Carmody, S.M., Ocamb, C.M., and du Toit, L.J. 2016. Potential seed transmission of Pyrenopeziza brassicae and Mycosphaerella capsellae in brassicas in the Pacific Northwest USA. Paper presented at American Phytopathological Society (APS) Pacific Division Meeting, 28-30 Jun. 2016, La Conner, WA. Phytopathology 106:S4.196 (Abstr.)

Newsletter:

  • Carmody, S., and du Toit, L.J. 2016. Light leaf spot and white leaf spot – two new fungal diseases of brassicas in the PNW. Tilth Producers’ Quarterly 26(4):5,18,20.

Presentations at grower/commodity meetings:

  • du Toit, L.J. and Carmody, S.M. 2017. Brassica light leaf spot, white leaf spot, and black leg; onion neck rot; and bacterial blight of carrot. Invited presentation to Bejo Seeds production team, 7 Nov. 2017, Mount Vernon, WA. (30 people)
  • du Toit, L.J., and Carmody, S. 2017. Light leaf spot and white leaf spot on brassica crops in western Washington. Wilbur Ellis Growers’ Meeting, 2 Feb. 2017, La Conner, WA. (100 people)
  • Carmody, S.M. 2017. Light leaf spot and white leaf spot on brassica crops in western Washington. Puget Sound Seed Growers' Association Annual Meeting, 13 Jan. 2017, Mount Vernon, WA. (75 people)
  • Carmody, S.M. 2016. Light leaf spot and white leaf spot on brassica crops in western Washington. Puget Sound Seed Growers' Association Annual Meeting, 29 Jan. 2016, Mount Vernon, WA. (50 people)
  • American Seed Trade Association Vegetable Technical Subcommittee Meetings: Gave updates on WSDA Crucifer Quarantine and black leg, white leaf spot, and light leaf spot situation in the Pacific Northwest. 6 Jan., 12 Apr., & 8 Jun. 2016 (via conference call, 25 people at each meeting); prepared presentation on these topics for ASTA Vegetable & Flower Conference, Monterey, CA given by Philip Brown, Sakata America, 2 Feb. 2016. (75 people)

  • Diagnosing Plant Problems. S.M. Carmody prepared and presented a 4-hour plant diagnosis workshop (slides, handouts, and samples) for the "Growing Veterans" program and beginning Latino farmers in Skagit Valley, WA on 7 Nov. 2015, in collaboration with VIVA Farms. (25 people)

  • du Toit, L.J. Organic Seed Alliance Research Field Day, 14 Oct. 2015, Chimacum, WA. Discussion on black leg of crucifers, risk management, and pending WSDA quarantine regulation. (25 people)

  • du Toit, L.J. Black leg, light leaf spot, and white leaf spot of crucifers in the Pacific Northwest: Lessons learned from the 2014 Willamette Valley epidemic. Douglas Co. and Okanogan Co. Growers Meeting. Invited to help growers avoid introducing seedborne pathogens on crucifer cover crop and canola seed. 3 Mar. 2015, Waterville, WA. (25 people)

Presentations at field days:

  • du Toit, L.J., and Carmody, S.M. 2017. Light leaf spot and white leaf spot on brassica crops in western Washington. WSU Mount Vernon NWREC Field Day, 13 Jul. 2017, Mount Vernon, WA. (120 people)
  • Carmody, S.M. 2016. Light leaf spot and white leaf spot on brassica crops in western Washington. WSU Mount Vernon NWREC Field Day, 13 Jul. 2016, Mount Vernon, WA. (120 people)
  • Carmody, S.M. 2015. Light leaf spot and white leaf spot on brassica crops in western Washington. WSU Mount Vernon NWREC Field Day, Jul. 2015, Mount Vernon, WA. (100 people)

 

50 Farmers intend/plan to change their practice(s)
25 Farmers changed or adopted a practice

Information Products

Return to Project Overview
Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.