2016 26.4 Light Leaf Spot and White Leaf Spot
Two New Brassica Fungal Diseases in the PNW
Mild winters make the coastal Pacific Northwest an ideal place to grow brassica crops. However, the mild, wet winter conditions also are ideal for some of the diseases that can afflict these species. We grow many kinds of brassica crops in the Pacific Northwest—in recent years there have been more than 21,000 acres of brassica vegetable crops and over 30,000 acres of brassica cover crops in Washington state annually. Various types of brassica crops can be found in fields year-round—over winter to vernalize cabbage and cauliflower seed crops, turnips and radishes grown through the summer, and fall plantings of mustard cover crops and vegetables to take advantage of cool temperatures and fall/winter rain. In addition to cultivated brassica crops, wild and volunteer brassicas of different genera and species abound in fields and along roadsides.
In all, the Brassica genus comprises 37 species, six of which are commonly grown and consumed for the buds, stems, leaves, flower, and seeds. The “Triangle of U” Theory, proposed in 1935 by Woo Jang-Choon, explains the relationships among these distinct, though closely related species. Three species—B. oleracea, B. rapa, and B. nigra, can be crossed to create the remainder of six economically important species: B. napus, B. juncea, and B. carinata. It is this close relationship among Brassica species that renders them all susceptible to a similar range of diseases. Add the year-round presence of brassica plants and this ideal climate for many of the pathogens, and it’s no surprise there is ample opportunity for establishment and proliferation of brassica-infecting plant pathogens.
Fungal diseases of brassicas, such as ringspot (caused by Mycosphaerella brassicicola), blackleg (Phoma lingam), Alternaria black spot (Alternaria brassicae and A. brassicicola), downy mildew (Peronospora destructor), and white mold (Sclerotinia sclerotiorum) have caused losses in the Pacific Northwest for almost as long as brassica crops have been cultivated here. In 2014, Dr. Cynthia Ocamb from Oregon State University added two new fungal diseases to that list: light leaf spot (caused by the fungus Pyrenopeziza brassicae, also known by the asexual stage, Cylindrosporium concentricum) and white leaf spot (caused by the fungus Mycosphaerella capsellae, which has the asexual stage, Pseudocercosporella capsellae). In a 2014 survey of brassica crops in the Willamette Valley, Dr. Ocamb detected light leaf spot in 24 of 61 field sites, which was very surprising for a disease that had never been reported before in North America; and white leaf spot in 17 of 61 sites, which is rarely found in the Pacific Northwest (occasionally a problem in the southeast U.S. and southern California).
In April 2016, we found light leaf spot to be widespread on wildbird’s rape mustard (B. rapa) and mustard cover crops (B. juncea) in Skagit, Whatcom, and Snohomish Counties of Washington state. White leaf spot also was detected in some B. juncea cover crops and on bird’s rape mustard in Skagit and Whatcom Counties. Neither disease has previously been found in Washington state. Both pathogens can infect a wide range of genera and species in Brassicaceae, though their impact on Pacific Northwest brassica crops remains to be determined. In the U.K., light leaf spot is the main cause of yield losses in winter canola. In the E.U., Canada, Australia, and the southeast U.S., white leaf spot can impact the quality and value of fresh market brassica produce.
Light Leaf Spot: Symptoms and Disease Development
Light leaf spot can affect any above ground part of a brassica plant including leaves, flower buds, stems, and pods. Symptoms commonly occur on older leaves first and on both sides of the leaves (see photo, below). The most common leaf symptom is bright yellow spots that can range from less than a ¼ inch to several inches in diameter as lesions coalesce. Lesions initially are light yellow with a diffuse margin, and turn yellow or brown in the center.
Infected leaf veins usually turn brown. Brown stem lesions with black speckling on the margin increase in length as the plant grows. Symptoms can vary depending on the host species. Infected cauliflower inflorescences develop discolored tan or brown areas. In canola crops, infection first is evident as white pustules (from the fruiting bodies called acervuli) on the leaves, pods, and stems before the infected areas develop into necrotic (dead) lesions.
A light leaf spot outbreak in climates like this typically starts in the fall, when air-borne spores serve as the primary inoculum released from the fungal fruiting bodies that develop on brassica plant residues. These spores can be spread long distances by wind.
Secondary inoculum, in the form of the asexual spores, is spread over shorter distances by splash-dispersal during rain or irrigation. These spores are an important source of inoculum within brassica crops or between brassica crops in close proximity, and/or brassica weeds. Wet, cool conditions (50-60°F) are ideal for development of light leaf spot.
White Leaf Spot: Symptoms and Disease Development
Like light leaf spot, white leaf spot can affect any above ground part of the brassica plant. Symptoms typically start as small, pin hole-size, lesions, each with a distinct margin between the healthy and infected tissue. The lesions develop into spots, each with a light center and black ‘webbing’ pattern (Figure 2). Lesions may coalesce. Symptoms can be similar to those of downy mildew, light leaf spot, black leg, black leaf spot, and ringspot. Symptoms caused by M. capsellae are different from those caused by the closely related ringspot fungus, M. brassicicola, as white leaf spots lack the concentric rings of small, black fruiting bodies characteristic of ringspot, usually have a darker margin than ringspot lesions, and typically remain smaller than ringspot lesions.
M. capsellae is spread by asexual spores that are splash-dispersed from lesions on which the spores form. Development and dissemination of spores requires rain or irrigation. The sexual spores of the fungus are aerially dispersed and can spread further than the splash-dispersed asexual spores during windy conditions. As with light leaf spot, cool (50 to 60°F) and wet conditions are ideal for white leaf spot. In most years, white leaf spot does not progress once stem elongation has started on brassica plants that ‘bolt’ (switch from vegetative to reproductive growth) in the spring or early summer, when conditions usually turn warmer and drier in this region.
Management and Prevention
Cultural practices are important in managing light leaf spot and white leaf spot, and are the most effective methods available for organic farming systems. When possible, new fields should not be grown in close proximity—both in time and space—to fields previously infected with either pathogen. Control brassica weeds to reduce the amount of host plants on which the pathogen can survive and spread. Reduce the amount of inoculum that can infect new brassica crops by removing crop debris, brassica weeds and volunteers. Neither of these fungi are soilborne, i.e., they can only survive in soil on infested plant residues. Incorporating brassica residues into the soil results in rapid breakdown of the residues by soil microflora. The light leaf spot pathogen can be seedborne and seed transmitted (see photo, below), so it is important to purchase brassica seed from reputable sources who understand the importance of good seed quality. If a brassica crop becomes infected and it is necessary to irrigate the crop, use less frequent, deeper irrigations to reduce the amount of time the canopy remains wet and, therefore, favorable for infection by these fungi.
For more information on brassica light and white leaf spots, visit the PNW Disease Management Handbook at pnw.handbooks.org. Effective management of any plant disease is dependent on accurate diagnosis, and there are a number of brassica diseases and other problems that can be confused with light and white leaf spots. For assistance with diagnosis of suspect brassica plant samples, submit plant samples to the WSU Plant & Insect Diagnostic Lab in Puyallup for those who reside in western Washington (email@example.com, 253-445-4582), or the WSU Plant Clinic in Pullman for those who reside east of the Cascade Mountains (Rachel.firstname.lastname@example.org, 509-335-3292).
Brassica Seed Pathology Research
The Pacific Northwest is a primary area of brassica seed production for the U.S. and many other countries. Our research on light and white leaf spot is aimed at assessing the seedborne potential of these pathogens, understanding more about the introduction of these fungi into the Pacific Northwest, and the potential risk of these pathogens to brassica seed production in this region. Often cited as being seedborne, but with little published data on this aspect of these fungi, research is needed to assess the potential role of seedborne inoculum in outbreaks of these two leaf spots.
The specific objectives of our research are to:
1. Produce brassica seed lots infected with the light leaf spot
and white leaf spot fungi by inoculating developing pods on plants of Brassica spp. to facilitate natural mode(s) of seed infection.
Preliminary results: We produced cabbage and mustard seed lots infested with P. brassicae. We were not able to produce a seed lot infested with M. capsellae, however.
2. Use these infested seed lots to develop a seed health assay that enables detection and quantification of P. brassicae and M. capsellae in seed.
Preliminary result: P. brassicae was detected on 12.5 ± 2.3% of the mustard seed lot and 0.4 ± 0.5% of the cabbage seed lot we generated. M. capsellae was not detected on seed.
3. Assess rate(s) of seed transmission (the rate at which the infested seed causes infection of seedlings that develop after planting the infested seed) from infested seed lots.
Preliminary result: The light leaf spot fungus developed on 1.6% of the mustard seedlings, which represented a 20% seed transmission rate from the 12.5% infested seeds in that lot. P. brassicae was only detected on 1 of 1,000 seedlings grown from the cabbage seed lot, representing a 25% seed transmission rate from the 0.4% infested seed.
4. Assess organic and conventional seed treatments to eradicate the fungi from seed or reduce the incidence of seed infestation.
5. Develop educational materials and training opportunities on the detection and management of these two diseases.
6. Survey Pacific Northwest areas where light leaf spot and white leaf spot occur to further our understanding of the spread, survival, population diversity, epidemiology and management of these fungi. With the very recent identification of both of these pathogens in Western Washington, this research has increased in importance to brassica stakeholders in this region.
Thank you to Mike Derie, Barbara Holmes, and Dr. Cynthia Ocamb for their assistance with this project. Funding for this project comes from a Seed Matters/Clif Bar Foundation Fellowship, the USDA Western Sustainable Agriculture Research and Education Student Fellowship GW16-055, and the Washington State University Department of Plant Pathology.
Shannon Carmody is a graduate student in Plant Pathology at WSU under the direction of Lindsey du Toit, Professor of Plant Pathology, Department of Plant Pathology, Washington State University Mount Vernon Northwestern Washington Research & Extension Center, Mount Vernon, WA. email@example.com, firstname.lastname@example.org.
Tags: Brassica, fungal disease, light leaf spot, Pest Management, white leaf spot