Xanthomonas campestris pv. campestris
In most cases, the damage on the cabbage leaves appear only in late summer. The main symptoms show as yellow, wedge-shaped patches on the edge of leaves, later moving inwards onto the leaf and downwards into the stem. This symptom distinguishes black rot from Fusarium wilt, where symptoms move upwards from the ground level towards the stem. As the disease progresses, the yellowing part of the leaf enlarges and turns brown as the tissues die. Leaf veins turn black at the final stage of the disease, thereby the common name of the disease. Eventually, the leaf collapses. The pathogen may enter the stem and spread through the vascular system, something that is visible as a ring of black discoloration when cut near the soil surface.
The symptoms are caused by the soil-borne bacterium Xanthomonas campestris, which survives in infected crop debris or seeds for up to 2 years, or on weeds of the Brassica family for longer periods. It affects a large number of vegetables in the cabbage family (including broccoli, cauliflower, turnip, radish, kohlrabi). The bacterium spreads to healthy plants via water splashes and enters the tissues through different paths, among other injuries. Once a plant becomes infected, the disease spreads quickly to other cabbages. If soils or seeds are contaminated, the first symptoms can be observed already in the propagation bed. Environmental conditions that favor the bacteria and the infection process are high humidity and temperature in the range of 25-30 °C. Densely planted crops provide conditions that are ideal for bacterial spread to nearby plants. In those conditions, crop yields can be reduced by as much as 75-90%.
A hot water bath for 30 minutes at 50 °C is the recommended treatment to sterilize the seeding material. This is not 100 percent effective against black rot but reduces the incidence of the disease considerably. The drawback is that it might lower the seeds germination rate.
Always consider an integrated approach with preventive measures together with biological treatments if available. Seed treatments with hot water are very effective to block the contamination of fields. Foliar treatments with copper based fungicides every seven to ten days are also used to slow the spread of the disease. Unfortunately, these treatments can cause black spots to develop of the outer leaves of the cabbage.