Cabbage black rot is a disease that normally affects cabbages grown in late summer. The disease is caused by a bacterium called Xanthomonas campestris pv. Farmers who grow cabbages during raining seasons do face this challenge as 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.
Black rot, also called Xanthomonas campestris, is the most common cabbage disease. Preventing it is a very crucial step a cabbage farmer should take because affected cabbages can not be treated. While others spray copper-based fungicides, research and experience has proved that it is all a sheer waste of time and resources as cabbage black rot can not be treated.
This leafy disease can be identified on the leaf as a "V" shaped yellow colour, moving from the edge inwards and downwards into the stem. This symptom distinguishes Black rot from Fusarium wilt where symptoms move upwards from the soil level towards the stem.
Cabbage black rot poses a big problem in areas of high humidity, reducing crop yields by as much as 75-90%. The bacteria that cause Black Rot can survive and spread via wild hosts, soil, water droplets or infected seed.
As the disease can not be cured, we will only talk about how to prevent it and what to do once you have spotted affected plants in your garden or field.
1. Remove infected plants.
Since water, wind and animals or human beings moving around within the field can all transport the bacteria from infected plants to healthy ones, removing infected plants and affected leaves is strongly recommended, as this helps containing the disease.
2. Remove lower leaves.
Prune off all leaves close to the ground and leaves that show signs of the disease. You should use clean tools regularly dipped in bleach to avoid unintentionally transmitting healthy plants with the dangerous diseases.
3. Increase in-row and inter-row plant spacing.
Increase the gap between plants to allow a better movement of air and a slightly dryer microclimate. This helps to reduce the spreading of the disease from 1 plant to another. This will also bring the crop to harvest earlier, which will help to further reduce the danger of black rot.
4. Grow resistant varieties.
There are varieties that are resistant to black rot, mosaic tobacco virus cabbage and other diseases. Such varieties, usually hybrid varieties, are strongly recommended as they help you have a better yield, thanks to their genetic disease fighting mechanisms. Consult your agronomist or seed house for further assistance.
5. Use clean, non-contaminated seeds
Ask your seed company to hot water treat your crucifer seeds to kill the black rot bacteria. You can also do it yourself. Hot water treatment can be used to destroy the bacteria that may be infesting your seed. For cabbage and Brussels sprouts, soak seed for 25 minutes in 122°F water; for Chinese cabbage, broccoli, cauliflower, collard, kale, kohlrabi, rutabaga or turnip, soak for 20 minutes in 122°F water. Mustards, watercress and radish are more susceptible to heat damage, and should be soaked for 15 minutes in 122°F water.
6. Practise Crop rotation and good farming techniques.
You could grow carrots or strawberries after harvesting your cabbages. You are strongly advised to practice crop rotation, and not repeat the same crop or any vegetable crop in a field that has recently been used to grow cabbages. In addition, you can also use a strong chemical such as Citrox P which is sprayed on the whole field soon after harvesting. You apply Citrox P daily for 7 days at a rate of 15ml per 200 litres. Citrox P will, within 14 days, kill all diseases, bacteria and fungi that could have build up in your field.