Symptoms of black rot have been recently discovered on a number of cabbage seedlings at a garden center. It is important for growers to know what the symptoms of this disease are and the recommended control methods.
Black rot is caused by the bacterium Xanthomonas campestris. The disease is easily recognized on most crucifers by the presence of yellow, v-shaped or u-shaped areas extending inward from the margin of the leaf (Figures 1 and 2). As the disease progresses, the yellow lesions turn brown and the tissue dies. This vein discoloration progresses toward the base of the leaf as the bacteria spreads through the leaf veins into the stem.
The bacterium spreads through a garden with the help of splashing rain or overhead irrigation. Free moisture in the form of rain or dew is required for the infection to occur. Black rot develops best under warm, wet conditions.
All crucifer crops are susceptible to black rot however, radishes and kale are less susceptible than the others. Plants outside of the crucifer family are not susceptible to this disease. Infected plants may die prematurely and plant quality will be reduced if the disease progresses in the field. Soft rot may also develop after black rot, further reducing quality and storage life.
The best way to control this disease is by purchasing disease-free transplants. When purchasing plants, closely examine the leaves and look for the v-shaped or u-shaped yellow lesions on the older leaves. If a grower has already purchased infected seedlings, they should remove the symptomatic leaves. This may help limit the spread of the pathogen in the field.