Crop rotation a must for control of black rot disease
Published: Saturday, December 8, 2012 at 6:01 a.m.
Last Modified: Thursday, December 6, 2012 at 12:12 p.m.
In early November, the vegetable plots at the UF/IFAS Marion County Demonstration Gardens were filled with healthy plants — zucchini, sweet potatoes, tomatoes, bell peppers, green beans, onions, beets, lettuce and cole crops (kohlrabi, cabbage, broccoli, collards and kale). About mid-November we noticed V-shaped lesions on the cole crops, with the symptoms most severe on the broccoli. From afar it looked as if the edges of the broccoli plants were burned.
The problem was a disease called black rot, caused by the bacterium Xanthomonas campestris pv. campestris. This particular bacterium only affects cole crops, and is considered to be the most destructive disease in the cabbage family. The bacterium usually is introduced into gardens by infected seeds or transplants (seedlings). It can spread rapidly between plants by rain splash or irrigation water to open wounds, mechanical injury from insect damage, stomata and openings at the margins of the leaves called hydathodes.
Infected plants can take up to 40 days to show visible symptoms, which is about the time we began to notice the symptoms showing up in the plots. The optimum temperature range for bacterium growth is 80-86 degrees. The V-shaped necrotic lesions with blackened veins are a typical symptom of this disease when the bacteria enter through the hydathodes. The bacterium moves through the vascular system of the plant, disrupting water flow and resulting in yellowing of leaves, wilting and even head rot. The bacteria will survive in the soil until infected crop debris is completely decomposed.
An integrated management approach is necessary to control this disease.
■ Purchase seeds or transplants from a reputable source.
■ Use disease-tolerant or resistant varieties.
■ Avoid working in the garden when plants are wet.
■ Remove crucifer (cabbage family) weeds, for example, shepherd's purse and Virginia pepperweed from inside or the perimeter of the garden.
■ Crop rotation is highly recommended. Rogue and burn infected plants or bury deeply. Do not add the diseased plants to the compost pile. Do not plant cole crops in the beds where infected plants were growing for at least two years.
■ Chemical control is difficult, and multiple applications of a copper fungicide in the early stages might help to save the crop. As time progressed, we noticed the disease symptoms showing up in the cabbage, kale and kohlrabi. This turned out to be a teachable moment for us to show Master Gardener Trainees that plants in the same family tend to be affected by the same disease; hence, crop rotation is necessary to starve the pathogen (disease-causing organism). Below is a list of vegetables and their relatives. Keep this list handy when designing your vegetable garden plan each season.
■ Brassicaceae (Cruciferae): broccoli, cabbage, kale, collard, kohlrabi, cauliflower, mustard, turnip, radish, Chinese cabbage
■ Cucurbitaceae: cucumber, melon, pumpkin, squash
■ Chenopodiaceae: beet, spinach, Swiss chard
■ Umbelliferae: carrot, parsley, celery
■ Fabaceae (Leguminosae): various peas and beans, peanuts
■ Solanaceae: tomato, eggplant, white potato, peppers, tomatillo
■ Liliaceae: onion, garlic, chives
For more information on black rot disease, visit: https://edis.ifas.ufl.edu/pdffiles/VH/VH01300.pdf and http://vegetablemdonline.ppath.cornell.edu/factsheets/Crucifers_BR.htm.
Norma Samuel is the Urban Horticulture Agent for the UF/IFAS Marion County Extension Service. Contact her at firstname.lastname@example.org.
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