Black rot is caused by (Xanthomonas campestris PV.) bacteria and is distributed worldwide. The bacteria affects Crucifer crops like broccoli, cabbage, cauliflower, Chinese cabbage, collards, kales, Radish, and turnips.
This is a seed-borne bacterial disease, spread through soil and Infected debris. Crowded plants provide conditions that are ideal for bacterial spread to nearby plants.
Symptoms
Symptoms manifest as wilting at leaf margins.
Wilted tissue becomes chlorotic and progresses to form the characteristic V-shaped lesion associated with this disease.
Within chlorotic tissue, leaf veins turn black, giving the disease its name – black rot.
At advanced stages, affected tissue becomes brown and necrotic. Black leaf veins may extend from the affected leaf into the main stalk where the darkened vascular system may be visible.
As the disease progresses into the vascular system, lesions/spots resulting from systemic invasion may appear along leaf midribs and between leaf veins.
Systemically infected plants may be stunted and develop more severe symptoms on one side of the plant.
In affected cabbage, heads are smaller and outer leaves may deteriorate with time. The disease can progress on cabbage during storage, making the heads unmarketable.
In later stages, affected heads turn black and soft
The rotten heads give a characteristic offensive odour.
Under cool conditions, symptoms may be confused with those caused by Peppery leaf spot or Xanthomonas leaf spot.
Read: How to farm cabbages successfully from planting to harvesting
What are the conditions for disease development?
The black rot organism can survive in crop residue for up to two years.
The bacterium can also infect cruciferous weeds, such as peppergrass (Lepidium virginicum), wild radish (Raphanus raphanistrum), black mustard (Brassica nigra), wart cress (Coronopus Didymus), wild turnip (Brassica campestris) and others.
These weeds, as well as nearby crucifer crops(like broccoli, cabbage, cauliflower, Chinese cabbage, collards, kales, Radish, and turnips), can serve as reservoirs for the bacterium, which may subsequently spread to healthy crops. Though hydathode infection is most common, stomatal entry may occur when plants are subjected to heavy rains or irrigation.
The organism can also enter through natural wounds in the root system during periods of soil saturation. With warm temperatures of 26-30°C (81-86°F), symptoms may appear in 10 to 12 days.
However, under cool conditions, an infected plant may not show symptoms. The spread of the disease in the field generally occurs by wind-blown Black Rot rain, irrigation water, cultivation, insects or animals. The bacterium can be seed-borne, which may result in seedling infection.
Secondary infection from black rot-infected seedlings may occur in nurseries or seedbeds and the disease generally spreads rapidly during transplant/growing operations.
Read Also: The simple secrets of kales farming
How do I control and prevent bacterial rot?
Use high-quality certified seed free of bacteria ( X. campestris pv. Campestris). These are among the best cabbages varieties.
Implement a three-year rotation to noncruciferous crops like bell peppers, squash, eggplant, tomato, cucumber, green beans, carrots, celery, peas and asparagus.
Seedbeds should be geographically isolated from commercial crucifer crops.
Do not mow or clip transplants.
Plant crops in well-drained soils and use irrigation practices that minimize leaf wetness.
Keep fields free of cruciferous weeds. Field sanitation (burn crop residues)
Disinfect seedbeds and equipment with steam or germicidal sprays before use. Control insects to minimize the spread of the pathogen.
Use of copper-based fungicide such as Fungurun (should be sprayed at the early stage of disease infestation)
Read about Pepper bacterial diseases and find out how to identify, control and prevent them.
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