What Is Turnip Black Rot – Learn About Black Rot Of Turnips
By: Amy Grant
Black rot of turnips is a serious disease of not only turnips, but most other crucifer crops as well. What exactly is turnip black rot? Turnips with black rot have a bacterial disease caused by the pathogen Xanthomonas campestris pv. campestris. As mentioned, black rot targets members of the Brassica family – from turnips to cabbage, broccoli, cauliflower, kale, mustard and radish. Because the disease afflicts so many crops, it is important to learn about turnip black rot control.
What is Turnip Black Rot?
The bacteria X. campestris enters leaf pores at the margin and moves down into the vascular system of the leaf. Upon inspection, infected leaves are marked by a notched or “V” shaped lesion at the leaf margin and appear to have black to dark grey fibers running through the leaf tissue. Once the leaves are infected, they rapidly degrade. Infected turnip seedlings collapse and rot soon after infection.
Black rot of turnips was first described in 1893 and has been an ongoing problem for farmers since that time. The pathogen spreads rapidly, infecting seed, emergent seedlings, and transplants. The disease is spread by splashing water, windblown water, and by animals and people moving through the crop. Symptoms on a turnip with black rot will first appear on lower foliage.
The disease is most prevalent in warm, wet weather. It survives in cruciferous weeds like shepherd’s purse, yellow rocket and wild mustard, and in crop debris, surviving for a short time in soil. Black rot of turnips spreads rapidly and may be spreading well before any symptoms can be observed.
Turnip Black Rot Control
To control the spread of black rot in turnips, only plant turnips in areas that have been free from cruciferous debris for over a year. Use disease free seed or resistant varieties if possible. Keep the area around the turnips weed free.
Sanitize garden equipment to prevent the spread of the disease. Use a drip irrigation system or water plants at their roots. Remove and destroy any cruciferous crop debris.
Apply bactericides at the first sign of leaf infection. Repeat the application weekly while weather conditions favor the spread of the disease.
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Black rot, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc), is a significant disease of cabbage and other crucifer crops worldwide. The disease was first described in New York on turnips in 1893, and has been a common problem for growers for over 100 years. The pathogen thrives in warm, wet weather, spreading from plant to plant by splashing water, wind blown water droplets, and by workers or animals moving from infected fields to healthy fields. Xcc can spread rapidly during transplant production in greenhouses or seed beds, and could be spreading long before any symptoms are observed. The bacterium can infest seed, infecting young seedlings as they emerge. The pathogen can also survive in cruciferous weeds, such as yellow rocket, Shepherd’s purse, and wild mustard, as well as in crop debris in the field.
Figure 1. The cabbage above shows typical black rot symptoms, with V-shaped lesions moving into the leaf from the leaf margin. Photo credit: Chris Smart, Cornell University.
Figure 2. Transplants with black rot symptoms are shown above. While these plants are clearly diseased, it is important to remember that bacteria can be invading plants even if no symptoms are observed. Photo credit: Holly Lange, Cornell University.
How should seed be treated?
While hot-water seed treatment can be done effectively on a stovetop, it is much better to use a precision water bath or sous vide machine. The temperature of water for treating seed varies from 118 to 125 F, depending on the crop, and the treatment period likewise varies from 15 to 30 minutes. Pre-heating seed at 100 F is recommended. Equipment for treating seed, including precision water baths, were purchased for several locations in the mid-Atlantic and northeast regions through a project funded by the Northeastern IPM Center. Additionally, extension specialists were trained so that they could assist growers who want to hot-water treat their seed. Contact Meg McGrath to find the nearest location. It is important to use the appropriate treatment protocol for a crop to achieve control of pathogens without damaging the seed. Protocols are listed in Table 1.
Sous vide (right) is a new machine that became available after the workshop training project. It is an immersion circulator precision cooker pod for cooking food in vacuum-sealed bags. Precise temperature control combined with water circulation makes it suitable for hot water seed treatment at a lower cost than scientific water baths. Several units are available on the web for less than $200. A unit used for treating seed is shown right and demonstrated in a video prepared by Amy Ivy (retired Horticulture Educator, Cornell Cooperative Extension Clinton County). Seed needs to placed in a porous container as done with water baths (not a sealed plastic bag used for food cooking) and also kept submerged during treatment.
See also Treatments for Managing Bacterial Pathogens in Vegetable Seed for guidelines for treating seed on a stovetop.
Look for dry, brown, V-shaped areas with black veins at the margin of older leaves this is a characteristic symptom of this disease.
- Use only certified, black rot-free seed the bacterium can survive up to 3 years in seed. Seed should be treated at 50°C for 25 minutes (cabbage and Brussels sprouts), and 20 minutes for cauliflower and broccoli). An alternative is to soak the seed in 0.5% sodium hypochlorite for 30 minutes.
- Nursery practices:
- Decontaminate seedling trays by dipping them in 10% bleach, rinse and dry.
- Raise nursery plants in soil-less potting mix, or pasteurised soil mixes.
- Keep nursery areas at distance from fields where cabbage (and other brassica) crops are grown.
- Inspect seedlings frequently if infections occur, destroy the seedlings and those in a buffer region around.
Select well-drained fields, and plant on raised beds.
- Remove weeds from around the cabbage (and other brassica) fields, especially weeds in the brassica family.
- Ensure there is enough space between plants in the field.
- Remove the remains of the crop immediately after harvest, or plough in the remains the bacterium can survive several months in stems or leaf debris in the soil.
- Practice crop rotation so that there is a 3-4-year gap between susceptible brassicas grown on the same land.
Cabbage, Chinese cabbage, mustard and turnip are susceptible, broccoli and Brussels sprouts less so, and radish is quite resistant.
Chemical control is not recommended for this disease where cabbages (and other brassicas) are grown for household use. Where they are grown for sale, it is recommended that copper fungicides be used as preventative sprays in nurseries. Spraying of plants in the field is not recommended.
AUTHOR Grahame Jackson
Information (and Photo 1) is from Diseases of vegetable crops in Australia (2010). Editors, Denis Persley, Tony Cooke, Susan House. CSIRO Publishing. Photo 2 David B. Langston, University of Georgia, Bugwood.org.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production , implemented by the University of Queensland and the Secretariat of the Pacific Community.
This fact sheet is a part of the app Pacific Pests and Pathogens
The mobile application is available from the Google Play Store and Apple iTunes.
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