I have observed the disease black rot of cabbage and broccoli this year. I have discussed the basics of this disease in April (Issue 701). Here, I would like to share some photos and observations. For a brief background, black rot is a bacterial disease that becomes systemic-that is, moves within the plant. The disease may overwinter in crop residue or may be brought in on seed/transplants.
The first set of photos is of an outbreak that occurred on first crop cabbage in early July. Figure 1 shows typical early symptoms on cabbage. If such symptoms occur when the heads are well formed, it is unlikely that any yield loss will result. Figure 2, on the other hand, shows severe symptoms on a plant that may render the head unmarketable.
Figure 1. Initial symptoms of black rot on cabbage often include V-shaped necrotic and chlorotic lesions on the leaf margin. Such lesions will probably not be important to marketability if the heads are close to maturity.
Figure 2. Severe symptoms of black rot on cabbage threaten to affect maturity and marketability of cabbage.
The next set of photos show symptoms of black rot on broccoli on a second crop of broccoli in September. The symptoms range from minor to severe. While I have not observed black rot to affect the heads of broccoli directly, it is possible for severe symptoms to affect yield and marketability of broccoli. Initial symptoms are similar to those on cabbage-lesions including necrotic and chlorotic V-shaped lesions (Figure 3). More severe lesions may include the entire leaf (Figure 4). Affected lower leaves such as in Figure 4 may not be important, however the bacterium is systemic and may move to other parts of the plant.
Figure 3. Initial symptoms of black rot of broccoli are similar to those of cabbage and include marginal V-shaped lesions.
Figure 4. The older broccoli leaf in the photo has severe symptoms of black rot.