Black rot is a bacterial disease of cole crops and other brassicas, caused by Xanthomonas campestris pv. campestris. One of its most distinctive symptoms is a yellow V-shaped lesion that begins at the edge of a leaf and points inward. As the infection progresses, the veins within affected tissue turn dark brown or black.

The bacteria enter through natural openings or wounds and move into the plant's vascular system. They spread in infected seed and transplants, splashing water, contaminated tools and infected plant debris. Warm, wet conditions favour rapid spread. Severe infections can stunt plants, cause extensive leaf loss and sometimes kill young plants.

How to Fix It

There is no cure once the bacteria have entered the plant's vascular system. Remove severely infected plants and dispose of them rather than composting them. Remove brassica debris after harvest and clean tools that have contacted infected plants.

Start with certified disease-free seed and healthy transplants. Rotate brassicas with unrelated crops for several years, control brassica-family weeds, provide good spacing and avoid overhead watering. Don't work among plants while they're wet, when bacteria can be moved easily from plant to plant.

Biological products containing certain Bacillus species may provide some preventive suppression, but they won't cure infected plants. For the home garden, clean planting material, sanitation, crop rotation and keeping foliage dry are the most dependable controls.