Brassica black rot, also known as Cabbage or Crucifer black rot, is a bacterial disease of brassica crops (such as cabbage, Brussels sprouts, kale) caused by Xanthomonas campestris pv. campestris. It causes V-shaped yellow lesions that move from the outer edges of the leaves inwards, with nearby veins turning black and thickening. Foliar symptoms can appear similar to those of plant stressors including drought, overwatering, or over-fertilization. Once established in the leaves, this disease can cause black discoloration inside the stem, which will become visible when cut. As the disease progresses, the roots may also turn black.
Symptoms
Black rot symptoms may not develop for more than a month after cruciferous vegetables start to grow. Initial symptoms are irregular, dull, yellow blotches that appear on the edges of leaves. As the disease progresses, these blotches expand into V-shaped areas with the wide part of the “V” at the edge of the leaf and the point of the “V” toward the attachment point of the leaf to the plant. The V-shaped areas are initially yellow, but eventually become brown and necrotic (i.e., dead) in the center with a yellow border or halo.
Veins in affected areas are brown or black, forming a net-like pattern (often most visible when leaves are held up to the light). Later, interior stem tissue (specifically the water-conducting tissue) will also turn brown or black. At this point, affected plants tend to show symptoms of wilting. Black rot can also predispose vegetables to other rot diseases such as bacterial soft rot (see University of Wisconsin Garden Facts XHT1224 “Bacterial Soft Rot”).
Causative agent
Black rot of crucifers is caused by Xanthomonas campestris pv. campestris (Xcc). This bacterium is most often introduced into a production field or garden on or in seeds and transplants of susceptible vegetables. By some estimates, a single infected seed in 10,000 can lead to a severe outbreak of the disease if environmental conditions are favorable. Favorable conditions include warm temperatures (approximately 80°F) and high humidity. Once introduced into a field/garden, Xcc can survive in residues from susceptible vegetables or on weed hosts. Xcc can subsequently enter susceptible plants through roots, through natural openings in leaves or through wounds made by tools, rough handling, or insect feeding. Cruciferous plants grown near infected plants and healthy plants handled with the same tools as diseased plants are at highest risk of becoming infected.
Infection
Most often, primary infection occurs when infected seed or transplants enter a field. Some brassica weeds, like mustards and radishes, can also carry and spread the disease. Once established in a field, the disease spreads via splashing water, wind, equipment, workers, and some insects. The bacteria can survive in and on infested soil-bound plant debris that can survive for up to two years. Favorable conditions include high moisture, hot temperatures, and poor airflow.
There are no curative treatments available to combat black rot once the disease has occurred. However, when disease severity is low, copper-containing fungicides that are labeled for use on cruciferous vegetables may help limit additional disease development even though this disease is caused by a bacterium. Fungicides containing peroxyacetic acid and/or hydrogen peroxide (such as SaniDate) can help reduce viable inoculum when on the outside of plants. Reducing inoculum may minimize new infections. At harvest, vegetables with low levels of black rot may be salvageable. Remove symptomatic leaves (or other plant parts) and store the remaining parts of the vegetables in a cool, but not overly wet environment.
Disease Cycle
The black rot disease cycle begins with primary infection, which can occur as early as the seedling stage. The bacteria can survive in or on brassica seeds, infecting seedlings as they grow. These infections may not present with distinct symptoms, or may mimic other forms of stress. If seedlings are grown in a greenhouse, infected flats or soil can be a source of inoculum. Transplants that have been clipped via mowing can carry disease as well. Once a plant is infected, the bacteria can be spread from one plant to another by splashing water, wind, or contact with equipment and some insects. During favorable conditions, the bacteria is able to enter a plant through the hydathodes (water pores on the outer edge of the leaf) or wounds, which can be caused by insects or machinery. Heavy black rot infection can often be followed by secondary soft-rot infections. At the end of a season, the bacteria can survive on seeds, or in soil-bound plant debris.
Prevention
Prevent introduction of Xcc into your field/garden by using certified disease-free crucifer seeds and transplants. If certified disease-free seed is not available, use hot water seed treatments to eliminate Xcc. Treat seeds of Brussels sprouts, collards, and cabbage for 35 minutes in water that is 122°F. Treat seeds of broccoli, cauliflower, kale, kohlrabi, rutabaga, and turnips for 20 minutes in water that is 122°F. DO NOT plant cruciferous vegetables in the same area of your farm/garden every year; rotate (i.e., move) these vegetables to different locations, ideally every 3rd year.
Once your cruciferous vegetables are growing, be sure to fertilize them appropriately. In particular, inadequate nitrogen can predispose plants to black rot. Also, be gentle with cruciferous vegetables to prevent any wounds that might serve as entry points for Xcc. Limiting overhead irrigation as possible can reduce likelihood of bacterial pathogen spread. Avoid working with plants when they are wet to help limit spread of Xcc. If severe black rot develops, promptly remove symptomatic plants as well as all cruciferous plants within a three to five foot radius. Dispose of these plants by burning (where allowed by local ordinance), burying or composting them. If you decide to compost, make sure your compost pile heats to a high enough temperature and that any infested material decomposes for at least one year before it is reincorporated into your garden. For more information on how to properly compost, contact your local county Extension office. Depending upon the scale of production, it may be possible to decontaminate any pots, tools, or other gardening items that have come into contact with Xcc-infected plants or Xcc-infested debris by treating them for at least 30 seconds with 10% bleach or 70% alcohol (preferable for metal tools because of its less corrosive properties). Rubbing alcohol and many spray disinfectants typically contain approximately 70% alcohol.
Cultural Control
Cultural management includes scouting regularly to identify the presence of the disease early, before it has had a chance to spread and cause significant damage. The following practices can help mitigate the risk of this disease:
- Plant resistant varieties when possible
- Plant certified disease-free seed (may include process of hot-water seed treatment
- Rotate away from susceptible brassica crops for ≥ 3 years
- Maintain proper spacing between plants
- Plant in areas with good airflow
- Avoid overhead irrigation
- Avoid working in fields when plants are wet
- Manage host weeds
- Remove and destroy infected plants
- Destroy infested plant debris
- Disinfest tools and equipment
Chemical Control
Properly-timed copper or biopesticide elicitors such as acibenzolar-S-methyl can also be used to manage disease. For Wisconsin-specific fungicide information, refer to the Commercial Vegetable Production in Wisconsin (A3422), a guide available through the UW Extension Learning Store website. Or, for home garden fungicide recommendations, see Home Vegetable Garden Fungicides (D0062), a fact sheet available through the UW Plant Disease Diagnostic Clinic website. Always follow label directions carefully.
Resources
- Commercial Vegetable Production in Wisconsin (A3422) from the UW Extension Learning Store. This guide offers the latest recommendations for disease, insect, and weed management in Wisconsin’s most common commercial vegetable crops. Also included are lime and fertilizer recommendations as well as insect identification information and keys.
- UW Plant Disease Diagnostics Clinic. The University of Wisconsin-Madison/Extension Plant Disease Diagnostics Clinic (PDDC) provides assistance in identifying plant diseases and provides educational information on plant diseases and their control.
References
- Bess Dicklow, M., R. Hazzard, A. Cavanagh, and S. Scheufele. 2022. “Brassicas, Black Rot.” Center for Agriculture, Food, and the Environment: UMass Extension Vegetable Program. September 2022. https://ag.umass.edu/vegetable/fact-sheets/brassicas-black-rot.
- Liu, Zeci, Huiping Wang, Jie Wang, Jian Lv, Bojie Xie, Shilei Luo, Shuya Wang, et al. 2022. “Physical, Chemical, and Biological Control of Black Rot of Brassicaceae Vegetables: A Review.” Frontiers in Microbiology 13 (November): 1023826. https://doi.org/10.3389/fmicb.2022.1023826.
- McGrath, Margaret Tuttle. 1994. “Black Rot of Crucifers.” Cornell Vegetable MD Online. November 1994. http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_BR.htm.
- Pape, Andrew. 2021. “Black Rot of Crucifers.” Plant Disease Diagnostics Clinic. May 11, 2021. https://pddc.wisc.edu/2015/07/06/black-rot-of-crucifers/.
- Smart, Christine D., and Holly W. Lange. 2010. “Managing Black Rot of Cabbage and Other Crucifer Crops in Organic Farming Systems.” EOrganic. September 15, 2010. https://eorganic.org/node/4957.
- Vicente, Joana G., and Eric B. Holub. 2013. “Xanthomonas Campestris Pv. Campestris (Cause of Black Rot of Crucifers) in the Genomic Era Is Still a Worldwide Threat to Brassica Crops.” Molecular Plant Pathology 14 (1): 2–18. https://doi.org/10.1111/j.1364-3703.2012.00833.x.
Written by Amanda Gevens, Ariana Abbrescia, Andrew Pape, Russell Groves, and Ben Bradford. Last updated Jul 2024
