Black rot is a bacterial disease that affects crucifers (vegetables in the cabbage family). The bacterial pathogen, Xanthomonas campestris pv. campestris, is particularly damaging to cabbage and cauliflower, but turnip, rutabaga, collard, kohlrabi, and Chinese cabbage are also susceptible. Broccoli is somewhat more resistant, and radish is usually highly resistant.
The first symptoms are usually yellow V-shaped lesions at the leaf margins. As the diseased area of the leaf expands and turns brown, the leaf veins in the affected area may appear black. Eventually, the leaf collapses. The pathogen may enter the stem and spread to all parts of the plant through the vascular system. Infected stems show a ring of black discoloration when cut near the soil surface. Black rot infection and spread is favored by wet conditions and temperatures in the range of 80-86 F. Crowded plants provide conditions that are ideal for bacterial spread to nearby plants.
To control black rot in the home garden:
This article originally appeared in the August 13, 1999 issue, p. 109.