What it is
Black rot is one of the most destructive diseases of cabbage and other brassicas worldwide, caused by the bacterium Xanthomonas campestris pv. campestris. It typically enters at the leaf margins through water pores, producing yellow, V-shaped lesions that point inward, with blackened veins. The infection can become systemic, blackening the vascular tissue and spreading into the head, causing serious storage losses. It is seedborne and spreads long distances on seed.
Symptoms to look for
- Yellow, V-shaped lesions starting at the leaf margins and pointing inward
- Blackened veins within and around the lesions (a diagnostic sign)
- Lesions turn brown and necrotic in the centre with a yellow border
- Blackening of internal stem/vascular tissue when the stem is cut
- Wilting, systemic spread into the head, and rot in storage; heads develop black spots
When it spreads
Warm, humid conditions with rain, dew, or overhead irrigation that splash the bacteria. It enters through leaf-margin water pores (hydathodes) or wounds from insects, hail, or handling. The bacterium survives on and in seed, in crop debris, and on cruciferous weeds, and can spread rapidly during transplant production before symptoms appear.
Treatment
Field practices
Use certified pathogen-free seed and hot-water-treated seed, grow transplants away from production fields, plant resistant cultivars where available, rotate away from crucifers for at least two to three years, control cruciferous weeds, avoid overhead irrigation, disinfect tools, and deep-plough debris after harvest.
Organic options
There is no cure once plants are infected. Remove and destroy infected plants and debris and do not compost them. Copper sprays give only partial protection and must be preventive; they cannot stop systemic infection. Avoid overhead irrigation and working plants when wet. Prevention through clean seed is far more effective than any spray.
Active ingredients
- copper hydroxide
- copper oxychloride
Chemical options
No chemical cures black rot. Fixed-copper bactericides can slow leaf-surface spread if applied preventively but do not control systemic infection. Money and effort are far better spent on clean seed, hot-water seed treatment, resistant varieties, and sanitation than on sprays.
Active ingredients
- copper hydroxide
Safety
Managed mainly by clean seed and sanitation, not spraying, so chemical exposure is low. If copper is used, wear gloves, a mask, and eye protection and follow the label pre-harvest interval. Wash hands and disinfect tools after handling infected plants to avoid spreading the bacteria, and do not save seed from infected crops.
Preventing it next season
The foundation is clean seed: use certified pathogen-free or hot-water-treated seed and inspect transplants carefully. Add resistant varieties, crop rotation, weed control, dry foliage (no overhead irrigation), tool sanitation, and debris removal. Because it is seedborne and spreads before symptoms show, prevention is everything.