Few things are more Southern than collard greens (a selection of Brassica oleracea Viridis group), and in North Carolina we are fortunate to have a climate where we can produce two crops of our favorite Brassicas each year. Some other Brassicas you may be growing are kale, broccoli, and cabbage. They love our cool falls and winters, and are not going to wilt under our mild frosts before they’re ready to be harvested. That being said, they can still get diseases and it is important to keep an eye out for any discoloration or spotting on the leaves. One such disease that can devastate even the best stand of greens in no time is the bacterium Black Rot, Xanthomonas campestris pv. campestris (Xcc). Let’s talk about what to look for and how to prevent an outbreak.

Symptoms

Cole crops have beautiful, leathery leaves in most cases. It is one of the reasons horticulturists have selected blues, reds, purples and greens for ornamental purposes in winter annual beds. Black rot symptoms start at the edges of leaves as V-shaped yellow patches that slowly radiate closer to the base of the leaf. The way Xcc gets into the leaf is through the stomata, the openings in the leaves that allow for gas exchange, and this is usually when the plants are just seedlings. Unfortunately, when a plant is mature, it may be too late. These symptoms can sometimes be mistaken for drought stress or fertility issues.

There can be some black spotting within this yellow patch, and it is easy to confirm if the bacteria is present if you have a simple microscope. Just take a piece from the potentially infected leaf and float it in water for 3-5 minutes. Look at it under the microscope, and if you see a little stream of clear liquid coming from the infection area you most likely have black rot. You can also send a sample off to the Plant Disease and Insect Clinic at NC State. Please contact the Cooperative Extension office for help submitting a sample.

What To Do If You See Disease

Consistent scouting, especially when the weather turns wet and warm, is critical. Cole crops are more susceptible to infection when it is warm and overhead irrigation is used (or it rains!). Remove diseased material and plants as soon as symptoms appear and dispose of or destroy them in an area far away from production fields. Studies have shown that there is little holdover of bacteria in cruciferous weeds in fields, which is good for North Carolina growers, since we have weeds that are in the same family as our favorite Brassicas. Prevention is the best first step. There has been some efficacy with treating with products containing the active ingredient acibenzolar-S-methyl (Actigard) are rated as fairly effective against black rot, if used preventatively. Additionally, copper-based products tank mixed with mancozeb can be used in the field or greenhouse, according to NC State Pathology.

Prevention

The main source of Xcc is infected seed and seedlings, so making sure that you are getting your seeds and plants from a certified, disease-free source is critical. A hot-water treatment upon seed arrival if you cannot verify the cleanliness of the seed source can reduce some potential infection. Make sure to remove and destroy any diseased material in seedling trays as soon as you see it. Try to reduce mechanical injuries to plants as you interact with them, and sterilize tools, hands and equipment between areas. Select resistant or tolerant cultivars for fields with a history of black rot, planting resistant varieties can reduce severe symptoms of disease. It is best to maintain a 3-4 year crop rotation with Brassica crops to reduce disease spread and persistence. Finally, always sanitize boots and equipment when moving between fields that have a history of black rot. It is easy to bring the disease on shared equipment.

Other Diseases to Look Out For

There are several other diseases cool crops are susceptible to, including Alternaria Leaf Spot, Bacterial leaf spot, Clubroot, downy mildew, botrytis crown rot, and powdery mildew. Fusarium and Verticillium wilts can cause similar symptoms to black rot on a quick glance, but they have a distinctive yellowing on one side of the affected plant or leaf and lack any blackening in the veins like black rot. NCSU Plant Pathology’s Brassica portal is an excellent resource for learning more about other diseases our favorite cool crops can get, so we can make sure we have tasty greens throughout the year!

References:

NC State Black Rot Overview

NC State Diseases in Brassicas

Overview of Black Rot

University of Massachusetts Disease Summary

Study in New York (state)

Potential Phage Therapy Treatment Research

Amanda Wilkins is the Horticulture Agent for North Carolina Cooperative Extension in Lee County.