Several cabbage samples submitted to the Plant Disease Clinic have been diagnosed with black rot. Black rot is caused by the bacterium, Xanthomonas campestris pv. campestris and can attack all types of crucifers (cabbage, cauliflower, broccoli, turnip, etc).
Plants can be affected at any growth stage. The initial symptoms of infection are the presence of small, yellow-brown, V-shaped areas at the leaf margins. As the lesions enlarge, the nearby veins turn black and affected areas dry out and turn black or brown. A cross- section of the stem will show a distinct ring of discolored or decayed tissue. Root systems on infected plants are usually less extensive than on healthy plants.
The disease cycle of the black rot pathogen starts in infected seed or crop residue. The bacteria are spread to healthy plants by splashing water, mechanical spread or by insects. Black rot is favored by wet weather and temperatures between 80-86o F. Using disease-free seed and transplants are essential for black rot control. Plant in areas where air circulation and soil drainage are good. Fall tillage will hasten the decomposition of infested crop residue and decrease pathogen populations. Rotating fields out of crucifers for three years is advisable for disease control and soil management. Spraying with copper bactericides can be effective if conditions are not favorable for pathogen spread and pathogen populations are already low. Late season infections of black rot can be tolerable if precautions are taken to keep infection levels small.
This article originally appeared in the July 28, 1995 issue, p. 115.