Black rot on broccoli — symptoms and treatment

Severity: high

What is Black rot

Black rot is caused by the bacterium Xanthomonas campestris. It is seed-borne, which means it can arrive in a garden hidden inside apparently healthy seed or transplants, and it spreads rapidly in warm, wet weather. The bacteria enter the leaf and travel in the plant's vascular system, so the infection moves inward from the leaf edges toward the stem and eventually into the head. Because it is inside the plant rather than on the surface, it is difficult to stop once established — hence the high severity rating.

Symptoms

The classic sign is a V-shaped lesion opening from the leaf edge and pointing inward, with the wide part of the V at the margin. These lesions start yellow and turn brown as the tissue dies. Look closely at the veins running through and beyond the damaged wedge: they darken to black, a giveaway that the bacteria are moving in the plant's plumbing rather than just sitting on the leaf. As the infection reaches the head, the head discolours and quality collapses. In advanced cases the tissue breaks down into a foul-smelling soft rot — the smell is a reliable field clue that you are dealing with a bacterial problem, not a fungal one.

• Early: V-shaped yellow lesions opening from the leaf margins.
• Progressing: lesions turning brown; veins blackening; leaves dying back.
• Advanced: head discoloration; foul-smelling soft rot.

The V-from-the-margin shape plus blackened veins distinguishes black rot from fungal leaf spots, which are typically round and scattered across the leaf blade.

Causes and conditions

The bacteria arrive most often on contaminated seed or infected transplants — a seed-borne start is the usual explanation for an outbreak in a clean garden. Once a plant is infected, the bacteria are moved to neighbours by splashing rain, overhead irrigation, and by people and tools brushing through a wet crop. They enter through the natural pores at the leaf margins and through any wound, including insect damage and transplant handling. Warm, wet weather is what turns a few infected plants into a lost planting: it is the combination of heat and leaf wetness that lets the disease spread rapidly.

Treatment

Copper hydroxide — chemical

Timing: preventive. Apply a copper-based bactericide preventively — before symptoms appear, because copper protects healthy tissue and cannot reach bacteria already inside the plant. Hot water treat seeds (50°C for 25 min) to eliminate the seed-borne source of infection. Pre-harvest interval: 14 days.

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

Prevention

• Start with certified disease-free seed or transplants — the seed-borne route is the most common way black rot enters a garden.
• Hot water treat seed of uncertain origin before sowing.
• Rotate away from brassicas; do not follow broccoli with cabbage, kale, cauliflower or related crops in the same ground.
• Water at the base rather than overhead, and stay out of the crop while the foliage is wet — walking a wet row spreads the bacteria plant to plant.
• Remove and destroy infected plants and crop debris rather than leaving them in the bed; clean up brassica weeds too.

Frequently asked questions

Can I cut off the affected leaves and save the plant? Usually not. By the time the veins are blackening, the bacteria are already in the plant's vascular system beyond the visible lesion, so removing the leaf leaves the infection behind. Copper protects healthy plants; it does not cure infected ones.

Is it contagious to my other plants? It spreads readily to other brassicas — cabbage, cauliflower, kale, and brassica weeds — but not to unrelated crops. Splashing water and working in the crop while it is wet are the main ways it travels.

Can I eat broccoli after spraying copper? Yes, after respecting the pre-harvest interval given above. Heads showing discoloration or soft rot should be discarded, not eaten.