Black rot on cabbage — symptoms and treatment

Severity: high

What is Black rot

Black rot of cabbage is caused by the bacterium Xanthomonas campestris pv. campestris. It is the most serious disease of brassicas — a reputation earned by its seed-borne start, fast spread in the right weather, and the near-impossibility of curing an infected plant. The bacteria colonise the leaf veins and move through the vascular system, working inward from the leaf edges. In severe cases they reach the head and rot it, turning a nearly finished crop into a loss.

Symptoms

Black rot writes its signature at the leaf margin. Yellow lesions form at the edge of the leaf and spread inward in a V shape, broad at the margin and tapering to a point where they follow a vein into the leaf. Trace those veins and you find the second, decisive symptom: they turn black, standing out as a dark network through and beyond the yellowed wedge. The affected tissue then dries out, and badly hit leaves drop from the plant. Where the infection runs down the veins into the head, the head itself rots.

• Early: yellow, V-shaped lesions opening from the leaf margins.
• Diagnostic: veins turning black within and beyond the lesion.
• Advanced: leaves drying and dropping; head rot in severe cases.

The margin-inward V and the blackened veins separate black rot from fungal leaf spots, which tend to be round, scattered and unrelated to the vein pattern.

Causes and conditions

Infected seed is the classic starting point — the bacteria ride inside the seed, so a crop can be doomed before it germinates, and infected transplants bring the same problem ready-grown. From there they spread plant to plant in splashing water: rain, overhead irrigation, and the wet foliage a gardener or tool drags through while working the row. They enter through the natural pores along the leaf margins and through wounds from insects, hail or transplanting. Warm, wet weather drives rapid spread; it needs free water on the leaf to move and infect, so it does much less in dry conditions.

Treatment

Hot water seed treatment — cultural

Timing: pre-planting. Treat seed in 50°C water for 25-30 minutes to kill the seed-borne bacteria before they ever reach the field. Use certified disease-free transplants. Rotate for 2-3 years so that infected debris in the soil has time to break down before brassicas return.

Copper hydroxide — chemical

Timing: preventive. Apply a copper-based bactericide preventively. Copper has limited efficacy once infection is established: it forms a protective film on the leaf surface and cannot reach bacteria already inside the vascular system. It is a shield for healthy tissue, not a cure. Pre-harvest interval: 14 days.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Use certified disease-free seed and transplants — this single step blocks the most common entry route.
• Hot water treat any seed whose origin you are not sure of.
• Rotate for 2-3 years away from brassicas, and control brassica weeds, which host the bacteria between crops.
• Water at the base rather than over the leaves, and never work or harvest the crop while the foliage is wet.
• Space plants for airflow, and destroy infected plants and crop debris rather than composting them.

Frequently asked questions

Will copper spray cure my infected cabbages? No. Copper has limited efficacy once infection is established — it protects healthy tissue but cannot reach bacteria already inside the plant. If plants show blackened veins, remove them and protect the rest of the planting.

Is black rot contagious to other plants? Yes, to other brassicas — broccoli, cauliflower, kale, turnips and brassica weeds. It does not affect unrelated crops. Splashing water and moving through a wet crop are the main ways it travels between plants.

Can I eat a cabbage with black rot? Discard heads showing rot. A plant with only edge lesions on the outer leaves may still produce a usable head — strip the affected leaves and inspect it — but respect the pre-harvest interval above if the crop has been sprayed.