Learn How to Protect Brassica Crops from Black Rot

By Scout Nelson

Black rot is an uncommon but highly damaging bacterial disease that affects brassica crops. Although it does not appear frequently in South Dakota, extended periods of warm temperatures and high humidity can create ideal conditions for rapid disease development and serious crop losses.

Information provided by South Dakota State University Extension highlights that black rot is caused by the bacterium Xanthomonas campestris pv. campestris (Xcc). The disease can infect many brassica crops, including cabbage, broccoli, cauliflower, kale, Brussels sprouts, bok choy, turnips, radishes, mustard greens, and other related plants. The bacteria may also survive on certain weeds and crop residue.

Black rot can develop at any growth stage but is often seen in young transplants during spring or later in the season after several days of hot, humid weather. The bacteria enter plants through natural openings on leaf edges or through wounds caused by insects, hail, or mechanical damage. Water from rain, irrigation, dew, or splashing helps spread the disease.

One of the most common symptoms is yellow, V-shaped lesions that begin along leaf margins and move toward the center of the leaf. As the disease progresses, veins within affected areas turn dark brown or black. Large sections of leaves may eventually die, and severe infections can cause stem rot, head rot, and complete plant collapse.

Seed-borne infections are especially dangerous because symptoms can appear shortly after germination. Young seedlings often wilt and die within two weeks.

Managing black rot requires prevention and early action. Farmers are encouraged to use certified clean seed, purchase healthy transplants from trusted suppliers, and consider resistant crop varieties when available. Removing infected plants, cleaning equipment, avoiding overhead irrigation, and reducing plant stress can also help limit disease spread.

Crop rotation is another important tool. Since the bacteria can survive in crop residue for several years, fields with a history of black rot should avoid brassica crops for two to three years.

Preventive copper-based products may provide limited protection before disease develops, especially during periods of warm and humid weather.

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