Causal Agent:
Xanthomonas campestris pv. campestris

Distribution:
Worldwide

Symptoms:
Symptoms consist of a localized wilting at the leaf margin followed by a yellowing, though occasionally lesions on leaves may start at wounds created by insects. The yellow tissue turns to yellow-brown and the lesion progresses to a V-shaped area with the base of the V toward the leaf center. Affected areas later turn brown and die. Within the yellow tissue, leaf veins become black giving the disease its name - black rot. At advanced stages the black discoloration may extend from the affected leaf to the main stalk where the darkened vascular system may be evident up or down the stem. One way to identify the disease is to pull off a severely affected leaf and examine the resulting leaf scar at the stem. A black rot infected plant will have characteristic black vascular spots at the leaf scar. The advance of vascular symptoms may proceed into upper leaves where chlorotic lesions resulting from systemic invasion may appear anywhere on the leaf. Plants affected in this way may be stunted and show more severe symptoms on one side. In cabbage, heads are smaller and lower leaves may fall off. The disease can progress on cabbage during storage making the heads unmarketable. Soft rot frequently follows black rot and can reduce stems and leaves of an affected plant to a watery, pungent pulp. Under cool conditions as with winter grown cauliflower, symptoms may be confused with those caused by Pseudomonas syringae pv. maculicola (peppery leaf spot) ox Xanthomonas campestris pv. armoraciae (Xanthomonas leaf spot).

Conditions for Disease Development:
The black rot organism can survive in decomposing crop residue for up to two years. The bacterium can also infect cruciferous weeds, such as pepper grass (Lepidium virginicum), wild radish (Raphanus raphanistrum), black mustard (Brassica nigra), wart cress (Coronopus didymus), wild turnip (Brassica campestris) and others. These weeds, as well as nearby crucifer crops can serve as

reservoirs for the bacterium which can be spread to healthy crops. The bacterium can be seedborne, which may result in seedling infection. Secondary infection from black rot infected seedlings may occur in a nursery or seed bed and spread rapidly during the transplant/growing operation. This secondary spread is especially important when: (1) transplants are grown under conditions of high temperatures and fertility, (2) rotary mowers are used to clip beds, (3) crates of transplants are soaked in water before planting, (4) indicator plants, which may be infected, are used in the seed bed, (5) poorly drained seed beds are flooded during rainstorms, and (6) the same seedbeds are used year after year. The black rot bacterium can enter the plant through wounds or natural openings in the leaves. Though hydathode infection is most common, stomatal entry may occur when plants are subjected to heavy rains or irrigation. The organism can also enter through natural wounds in the root system. This is common during periods of soil saturation. With temperatures of 27-30°C (81-86°F) symptoms may appear in 10 to 12 days. Under cool conditions an infected plant may not show black rot symptoms. If temperatures become warm, the disease "appears suddenly" in an apparently healthy crop. The disease is especially favored when day temperatures are warm and night temperatures are cool. These conditions result in water droplets being formed at the hydathodes providing an excellent avenue for entry of the black rot bacterium. Spread of the disease in the field can occur by wind blown rain or irrigation, cultivation, insects, animals, or movement of contaminated water.

Control:
Use high quality seed free of the black rot organism. Practice a three year rotation with non-cruciferous crops to avoid infection from crop debris. Locate seed beds as far as possible from crucifer crops. Keep fields clean of cruciferous weeds. Seed beds and equipment should be disinfected with steam or germicidal sprays before being used. Transplants should not be mowed or clipped. Crates or transplant containers should be new or sterilized before use. A good pest control program is important to prevent insect spread of the black rot organism. For pest control, boom type sprayers are preferable to air blast sprayers.