Black Rot

Black rot may sound like the perfect name for your next Halloween character, but this bacterial disease can wreak havoc on plants in the cabbage family (Brassicaceae).
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Popular brassicas, or cole crops, include broccoli, cauliflower, kohlrabi, horseradish, Napa and Chinese cabbages, collards, turnips, rutabagas, Brussels sprouts, watercress, kale, radishes, bok choy, mustard. Rapeseed (canola) is also a member of this family. And all of these plants are susceptible to black rot, a close cousin to bacterial spot.

Symptoms of black rot on cabbage leaf (David B. Langston, USDA FS) CC BY 3.0

Black rot symptoms 

The early symptoms of black rot (Xanthomonas campestris pv. campestris) don’t look like anything serious. You will see some chlorosis (yellowing) along leaf edges (margins) and some V-shaped lesions pointing toward the center of the plant. Then, those lesions may dry up and fall away, lulling you into thinking the problem has resolved itself. Instead, black rot bacteria have gained a foothold in the phloem and xylem of your cabbage. Wilting and dieback occur as they populate and block these vascular tissues. ​

Black rot lifecycle (Gkwan) Public Domain

If you cut an infected stem longitudinally, you will see blackening of the vascular tissue. [Just be sure to disinfect your cutting tool afterward so that you don’t spread the disease to other plants.]

Managing black rot in the garden

Since this disease is most likely to occur in warm, humid weather, planting your cole crops after any chance of Indian summer has passed can help prevent it. The moisture left behind from overhead watering can also create the perfect conditions for black rot to take hold, so water your cabbages at ground level. Resistant varieties are available, so shop for those if you have already had problems with black rot. Also, it’s a good idea to use crop rotation with cabbages and cauliflower in particular, as these two crops are the most likely to be affected. Since this bacteria can survive on cruciferous weeds, try keeping your cabbage patch weed free.