Black rot is a bacterial disease that affects crucifers (vegetables in the cabbage family). The pathogen, Xanthomonas campestris pv. campestris, is particularly damaging to cabbage and cauliflower, but turnip, rutabaga, collard, and kohlrabi are also susceptible. Broccoli is somewhat resistant, and radish is usually highly resistant. Wet conditions favor pathogen growth and spread
Symptoms: The first symptoms are usually yellow V-shaped areas at the leaf margins. As the diseased area of the leaf expands, the area turns a mottled brown, and the leaf veins in the affected areas 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 viewed in cross section. Once the disease is present, it can easily spread to nearby plants, especially if the plants are crowded.
Control: There are several cultural practices that can help to control black rot.
- Remove infected plants.
- Remove any debris (the bacteria can persist and overwinter in diseased debris).
- Do not plant crucifers in the same site for at least 2 years.
- Plant black rot tolerant varieties if possible.
- Crucifer plants should be located where air and soil drainage is good.
- Avoid overhead irrigation.
- Good sanitation practices involving quick removal of old debris and the use of clean equipment are important.
- Although weekly sprays of copper products can slow the spread of an epidemics, the cultural practices described above are likely to be more effective.
This article originally appeared in the June 24, 1994 issue, p. 98.
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