Symptoms and Signs
Black rot symptoms vary depending on the environmental conditions. Early in disease development, typical symptoms consist of angular or V-shaped chlorotic lesions along the leaf edges. With time these lesions will dry up and turn tan or brown. Black veins are often seen within these tan lesions, though they may not always develop. Severely infected leaves may wither and drop off the plant. If systemic infection has taken place, the vascular tissues in petioles and main stems can also turn black. If temperatures are cool, however, symptoms may not be expressed. Atypical symptoms, such as small, brown specks, may also occur and mimic other bacterial diseases.
Comments on the Disease
The most severe losses from black rot have occurred in cabbage and cauliflower; crucifers grown for seed production crops can also be severely damaged. Black rot development is favored by warm, humid conditions. Splashing water from rain or sprinklers spreads the pathogen from plant to plant. Xanthomonas campestris pv. campestris is introduced into greenhouse or field situations primarily on seeds. The bacterium can also survive in soil if infected plant residues have not decomposed. Many cruciferous weeds are important reservoirs of the pathogen.
Management
Because the pathogen may survive in infected plant debris, do not plant a crucifer crop more often than every 2 years in any infested field. When possible, implement crop rotations using nonhosts. Some resistant cabbage cultivars are available.
Many cruciferous weeds host bacteria and must be controlled to prevent continued contamination. Remove weed and volunteer crucifers from production areas. Deep plowing can speed decomposition of infected plant debris, but care must be taken to bury all debris. Avoid sprinkler irrigation wherever possible, and do not plant infested fields during winter and spring when heavy rainfall occurs.
The bacterium can be carried on or in the seed. Plant seed that is free from the pathogen. Seed should be assayed to determine cleanliness and can be hot water treated to reduce infestation; however, hot water or other treatments are not 100% effective and may reduce germination. Transplants produced for field planting should likewise be disease-free. Clipping or mowing transplants before planting may result in widespread contamination of transplants.
Organically Acceptable Methods
Crop rotation, the use of resistant cultivars, weed management in the field and surrounding areas, and the use of pathogen-free seed and transplants are acceptable management strategies in an organically certified crop.