Black Rot Review: How to Manage this Disease on Brassica Crops

From Clemson Plant Pathologist Tony Keinath.

Black rot is the most common disease caused by bacteria on brassica crops. It was widespread in the Lowcountry in fall 2022, so it’s time for a review of this serious disease.

Recognizing Black Rot

Most often, black rot starts at the edges of the older leaves on brassicas. Yellow to tan, triangular lesions form and enlarge. The leaf veins inside the lesion turn black. This is a reliable diagnostic symptom to tell if the plant has black rot.

Crops and Resistant Cultivars

Black rot affects all six major crops in the species Brassica oleracea, commonly called head and stem brassicas, including cabbage, broccoli, cauliflower, and Brussels sprout. This species also includes two leafy brassicas, collard and kale. Black rot is less common on mustard and turnip, probably because these crops are different species.

Black rot causes the most loss in collard and kale because it affects the marketable, edible portion of the crop. Black rot damages cauliflower wrapper leaves, which are needed to make quality heads and are sold attached to the head. Black rot also affects the sprouts on Brussels sprouts.

Although black rot affects cabbage and broccoli, it usually doesn’t get into the heads of current hybrid cabbage cultivars. Most green cabbage and one red cabbage recommended in the Southeastern Vegetable Crop Handbook are partially resistant to black rot, along with 3 broccoli cultivars. “Partially resistant” means the symptoms are on the wrapper leaves of cabbage and leaves of broccoli, and the bacteria don’t get into the stem.

Entire plants of collard, kale, and other crops can collapse if black rot bacteria enter the stem. This is called systemic black rot because the bacteria spread through the entire plant. Systemic black is a big problem. Brassica stem tissue takes longer to decompose than leaf tissue, so there is a greater risk of the black rot bacteria surviving in crop debris for more than 1 year.

Seed as a Source

Brassica seed has long been known to be a major source of black rot bacteria and a way to start a new outbreak in a greenhouse or field. Some seed companies report on their websites that they test seeds for pathogens, including black rot, while other seed companies do not say anything about seed testing for pathogens. To avoid black rot, buy seed from companies that test their seed before it is sold.

Management in the Greenhouse

• Keep leaves dry by running a fan after overhead watering.
• Separate flats as much as possible to improve air circulation.
• Separate flats planted with different seed lots and cultivars to reduce the chances of bacteria splashing from contaminated seedlings from one seed lot onto healthy seedlings from other seed lots.
• If you are making sequential seedings from the same seed lot, don’t treat the seedlings, e. g. spray copper, in the first seeding so you can tell if any diseased seedlings appear. The threshold is 1 diseased seedling per 1,000.
• Discard the entire flat of seedlings if any symptomatic seedlings appear.

Management in the Field

• Rotate fields out of head and stem brassicas for 1 year, i. e. plant in 2033 and 2025 with a non-brassica crop in between.
• Don’t plant a brassica cover crop, like oilseed radish and mustard, before or after a brassica crop. While these brassicas are much less likely to be infected, the black rot bacteria could survive on their leaves. Plus, they are hosts for other brassica pathogens.
• Locate successive plantings in separated fields to prevent spread of pathogens, including black rot, from older plantings to younger plantings.
• Preventative applications of copper fungicides will reduce black rot symptoms on cabbage and likely on other crops. Research showed a yield increase in only 1 of 3 tests.
• Research in New York showed that brassica weeds are not a major source of black rot bacteria in the field.
• Disk crop debris as soon as possible after harvest. This is particularly important for cabbage wrapper leaves, which may be heavily infested with black rot bacteria.