Take Care to Defend Cruciferous Crops From Black Rot

Black rot in cabbage

This photo shows symptoms of black rot in cabbage. Photo courtesy of Cornell University

Disease Specs
Crop loss because of black rot is not uncommon in Florida. The disease is caused by the bacterium Xanthomonas campestris pv. campestris. Cabbage, broccoli, cauliflower, kale, collards, radish, and other members of the cabbage family are susceptible.

Identification
In the field, the disease is easily recognized by the presence of large yellow, V-shaped areas extending inward from the margin of outer leaves and by black veins in the infected area. In extreme cases, heading may be prevented.

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Although the distribution may be uniform, symptoms often are in clusters and more severe in wet or shaded areas. Diseased plants may appear in rows as a result of spread during cultural operations.

Survival and Spread
The causal bacterium overwinters on and in seed. The organism survives in crop debris and on cruciferous weeds including mustard, pepperweed, and others.

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Splashing or flowing water, shipping and handling of infected plants, equipment, insects, and animals will spread the bacteria, which is seedborne and thus disseminated worldwide. As few as three infected seeds in 10,000 can cause black rot epidemics in a field.

Plants may be affected with black rot at any stage of growth. The bacteria most frequently invade the host plant through hydathodes (pores) at the leaf margins, particularly when plants have been subjected to heavy rains or irrigation resulting in a small, wilted, V-shaped infected area that extends inward from the leaf edge toward the midrib.

The pathogen also can enter the plant through mechanical wounds.

The optimum temperature for organism growth is between 77°F and 86°F. Free moisture in the form of dew, fog, or rain is required for infection and disease development. Under optimum conditions, symptoms may appear on plants seven to 14 days after infection. Symptoms develop more slowly at lower temperatures.

Management Methods
Black rot can be controlled by utilizing an integrated control program based on sanitation. There are no commercially acceptable varieties resistant to the disease at this time. Spraying with copper fungicides may help reduce spread but can cause spotting on foliage.

Failure to implement an integrated control program can result in severe losses because of the many ways in which the black rot bacteria can be introduced or spread into a field.

The following measures will help reduce losses from black rot:
• Crop rotation
• Use of certified, disease-free seed
• Use of hot water treated seed
• Make sure your transplant grower carries out all protective measures, including inspections
• Plow down fields immediately after crop harvest to incorporate debris and speed decomposition
• Reduce spread by waiting for plants to dry before working a field.

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