Organic management of black rot in cole crops
- Black rot is a major disease of cole crops.
- Once black rot is present, it is challenging to get rid of.
- To prevent black rot on your farm:
- Start with clean seed.
- Maintain a four-year crop rotation.
- Manage weeds.
- Maintain overall hygiene.
- Sprays are available that may prevent black rot from spreading further.
Note: The information on this page is meant for commercial growers only. See Growing healthy vegetables for management options specific to home gardens.
Black rot bacteria (Xanthamonas campestris pv campestris) enter the plant through pores at the leaf margins (hydrathodes). The bacteria can also enter the plant through wounds from hail damage, mechanical injury or insect feeding.
Symptoms begin as yellowing at leaf edges, which turns into characteristic V-shaped lesions. As symptoms progress, plants can develop blackened vascular tissue in severe cases.
Control black rot symptoms
If you see black rot symptoms on your farm, there are a few ways to control the spread of the disease:
- If symptoms are limited to a few plants, remove infected plants as soon as symptoms appear. Be sure to remove the entire plant, not just the leaves.
- If you see symptoms in only one field, make sure you maintain clean and hygienic conditions. After working in a field infected with black rot, clean shoes and tools before entering another field .
- Sprays are available that can prevent spread. These products cannot get rid of the bacteria but they can prevent the disease from spreading.
- Copper products can be effective if applied at the right time, and some are approved for organic systems.
- Probiotic sprays, plant defence-enhancing sprays or sprays containing antagonistic bacteria such as Bacillus spp. may be successful in controlling black rot. Such sprays activate a plant's defense system and can cause the plant to put less energy into growth and yield.
- Black rot spreads in warm (75° to 95°F), humid conditions. Apply sprays when conditions are right rather than taking a calendar spraying approach.
- If you are using chemicals, make sure to consult your organic certifier and wear personal protective equipment.
CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.
When treating fruits or vegetables, make sure the plant you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.
Prevent black rot
If you do not have black rot on your farm, it is important to take steps to prevent it.
Start with clean seed
In warm, humid weather, a single infected seed in as many as 10,000 could lead to a black rot outbreak.
Different companies may have very different approaches to screening for and treating black rot. This information is not always disclosed on their websites.
- Before buying cole crop seeds (broccoli, cauliflower, Brussels sprouts, mustard greens, etc.) and even brassicas in cover crop mixtures such as tillage radish, call your seed company and make sure they can certify that their seed is disease free.
- You also can treat your own seed. The most common approach is hot water treatment. See the University of Massachusetts Extension’s overview chart of water temperatures and length of treatment for various plant species and diseases.
- If you are buying transplants, make sure that your supplier used disease-free seed or treated their seed.
- Plant resistant varieties when possible. There are a number of resistant cabbage varieties on the market.
Maintain a four year rotation
If you have had black rot in your field, a full four year rotation is important to prevent the disease in the future. This includes cover crops. Tillage radish should also be considered as part of the brassica rotation.
- While some sources say that only a 2-year rotation is necessary, waiting three to four years is recommended in Minnesota.
- Make sure to completely till under any diseased tissue, and remove any severely diseased tissue throughout the season.
Weeds can carry disease. There are plenty of weeds in Minnesota in the Brassicaceae family, such as shepherd's purse, black mustard and field mustard, that serve as hosts for black rot.
The black rot bacteria can move from field margins into your crop if you have weeds from the brassica family near your fields.
Maintain hygienic conditions
Even if you do not see any black rot symptoms, keep your tools and equipment clean.
- Use new or sterilized trays and soil for seeding transplants. Clean your boots and machinery as often as possible.
- Irrigation water and rain may disperse black rot bacteria over short distances. Water your plants in the morning after the dew has evaporated from plant leaves.
- Plan for wider spacing between plants if you have had problems with black rot in the past.
- Prevent water pooling around irrigation equipment.
- Consider water movement when selecting a site. Avoid fields with poor drainage and fields that may receive runoff from fields where cole crops were recently grown.
Reviewed in 2019