Cabbage is the crop most often affected by black rot, however, other crucifers such as broccoli, cauliflower, mustard, kohlrabi or Brussels sprouts may be affected. The first symptom one is likely to notice is a ‘V’ shaped lesion on the margin of the leaf (Figure 1). However, the symptom on Brussels sprouts observed recently are irregular, jagged lesions on leaves (Figure 2). The plants represented in Figures 1 and 2 are different varieties of Brussels sprouts. The differences may be due to differences in susceptibility of the two cultivars or the cultivar in Figure 2 may have been infected at an earlier age than the one in Figure 1. Figure 3 shows two severely affected plants next to a relatively healthy plant.
Black rot is most severe in wet, warm weather. The emergence of this disease during a rather cold spring may mean that the disease started in a greenhouse situation.
The bacterium that causes black rot, Xanthomonas campestris, survives in crop residue. Thus, crop rotations that avoid crucifers should lessen the severity of the disease. Sanitation in the greenhouse should help to lessen the amount of the bacterium that can cause more disease. The causal bacterium may also be transmitted through seeds, therefore, every effort should be made to plant seed that has been tested and found free of the bacterium. Inspect transplants for symptoms before planting. Avoid practices which add to free water on plant surfaces. Products which contain copper as an active ingredient may help to lessen the spread of the disease. However, copper products may also cause lesions on leafs under some circumstances. Although there may be varietal differences in susceptibility, compete levels of resistance are not available in commercial cultivars. More information about general pest management can be found in the Midwest Vegetable Production Guide for Commercial Growers, (mwveguide.org). This article was originally published at veggiediseaseblog.org
Figure 1. Typical symptoms of black rot include ‘V’ shaped lesions on the margins of leaf.
Figure 2. Angular lesions on Brussels sprouts.
Figure 3. Black rot on Brussels sprouts.