Black rot: identify, prevent, control

Worst in warm, wet, humid weather, December to April, in subtropical and tropical regions.

Black rot is a bacterial disease of brassicas that turns leaf edges yellow and blackens the veins. It spreads in warm wet weather through splashing water and infected seed, and a serious case can rot whole plants from the leaf margins inward.

How to identify it

• Yellow V-shaped patches spreading inward from the leaf edges
• Blackened veins running through the affected tissue
• A foul smell as the rot advances and turns soft
• Stunted plants with leaves dropping under heavy infection

How to prevent it

• Use clean, certified seed, since the disease is often seed-borne
• Rotate brassicas and avoid replanting them into infected ground
• Water at the base and avoid wetting the foliage
• Space plants well and keep beds weed-free for airflow

Organic control, step by step

• Remove and bin affected plants promptly, never composting them
• Avoid working among wet brassicas, which spreads the bacteria
• Spray a copper-based product as a protectant in wet weather, following label rates
• Clean tools and boots to avoid carrying the bacteria to clean beds
• Rest the bed from brassicas for a couple of seasons
• Clear all crop debris at the end of the season

Plants it attacks