Christine D. Smart, Cornell University
Holly W. Lange, Cornell University
Black rot, caused by the bacterium Xanthomonas campestris pv. campestris (Xcc), is a significant disease of cabbage and other crucifer crops worldwide. The disease was first described in New York on turnips in 1893, and has been a common problem for growers for over 100 years. The pathogen thrives in warm, wet weather, spreading from plant to plant by splashing water, wind blown water droplets, and by workers or animals moving from infected fields to healthy fields. Xcc can spread rapidly during transplant production in greenhouses or seed beds, and could be spreading long before any symptoms are observed. The bacterium can infest seed, infecting young seedlings as they emerge. The pathogen can also survive in cruciferous weeds, such as yellow rocket, Shepherd’s purse, and wild mustard, as well as in crop debris in the field.
Figure 1. The cabbage above shows typical black rot symptoms, with V-shaped lesions moving into the leaf from the leaf margin. Photo credit: Chris Smart, Cornell University.
Figure 2. Transplants with black rot symptoms are shown above. While these plants are clearly diseased, it is important to remember that bacteria can be invading plants even if no symptoms are observed. Photo credit: Holly Lange, Cornell University.
All crucifer crops are susceptible to black rot, including cabbage, broccoli, cauliflower, Brussels sprouts, Chinese cabbage, kale, radish, turnip, mustard, rutabaga, watercress, and arugula.
Figure 3. The cabbage field on the left has been destroyed by the black rot pathogen. Portions of the field on the right have been overtaken by cruciferous weeds which can serve as a source of inoculum. Photo credit: (left) Chris Smart, Cornell University; (right) Julie Kikkert, Cornell University .
Symptoms of black rot generally begin with yellowing at the leaf margin, which expands into the characteristic "V"-shaped lesion. The bacterium commonly enters the plant through the hydathode, or water pore, on the margin of the leaf; however, damage to leaves due to insect feeding, hail, or mechanical injury can also enable pathogen entry. The bacterial infection becomes systemic, meaning that the bacterium can enter the veins of the plant and spread into the cabbage head, which can lead to serious losses in storage. Blackening of the vascular tissue is typical in severe infections.
Figure 4. Hydathodes (or pores) on the margin of this cabbage leaf (left) exude plant sap or guttation droplets early in the morning. These hydathodes are the most common entry method for Xanthomonas campestris pv. campstris (which causes black rot). The leaf on the right is showing symptoms of black rot, with the lesion starting at the location of insect damage. Photo credits: Holly Lange, Cornell University.
Figure 5. Internal vein blackening caused by the black rot pathogen. This head would rot completely during storage. Photo credit: Holly Lange, Cornell University.
Prevention is the best line of defense and is especially important in organic production. There are three preventative measures that can reduce the risk of a black rot outbreak:
Anything that can be done to reduce leaf wetness and water splash will help reduce disease spread. This includes watering plants in the morning so that leaves dry prior to sunset, maintaining your irrigation system to reduce the likelihood of ponding, increasing spacing between plants, and orienting rows with prevailing winds to maximize air flow and drying.
Figure 6. Cabbage and cauliflower plants at this production facility are watered early in the morning so leaves will dry quickly. Photo credit: Chris Smart, Cornell University.
As with most bacterial pathogens, managment can be very difficult when the weather is conducive to disease. Once a plant is infected, there is no rescue treatment since the infection is systemic. Copper-based products are effective in reducing spread from infected to healthy plants.
NOTE: Before applying ANY pest control product, be sure to read and understand the safety precautions and application restrictions, and make sure that the brand name product is listed in your Organic System Plan and approved by your certifier. For more information see the eOrganic article Can I Use this Product for Disease Management on my Organic Farm?
Although black rot can be severe, following the prevention strategies described above will reduce the risk of this disease. Although the pathogen can survive on farms, we know that this is not the most common source of inoculum on farms that use a minimum three year rotation; instead, the pathogen is most commonly brought onto farms on seed or plants. In New York, new strains of the pathogen enter the state each year. Thus, planting only clean seed and disease-free transplants are the most important management practices in regions with cold winters. In locations with mild winter temperatures, the risk of maintaining the pathogen on farms is greater.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.