Black Rot in Cole Crops

Black rot is caused by Xanthomonas campestris pv. campestris, and as a seedborne pathogen, this disease is frequently observed on Pennsylvania farms unless the cole crop seed has undergone a hot water seed treatment process or the grower has selected black rot resistant varieties for their planting.

Black rot symptom expression and severity can vary according to the host, the age of the host (when infected), the strain of the bacteria, and environmental conditions. In most cases, a grower will notice small water-soaked leaf spots that, over time, will become brown with chlorotic or yellow margins. Growers may also notice yellow V-shaped yellow lesions along the leaf edge. As the disease progresses, these leaf lesions will become brown and necrotic in appearance.

Systemic infection by black rot can also be seen in some cole crop plantings. When systemic infection is observed, the vascular tissues in the leaf petioles and plant stems turn black in color. If the air temperatures are high, systemic infection may cause leaf lesions to develop in the center of the leaves. Under cooler conditions, cole crops may be asymptomatic of black rot infection and may not express any visual symptoms of infection.

Growers who detect black rot in their fields should rotate to non-hosts for a 2–3-year period. The bacteria that causes black rot, Xanthomonas campestris pv. campestris cannot survive in the soil and can only survive on crop residues. Once the crop residues decompose in the soil, the threat of black rot infection diminishes.

Growers noticing black rot in their cole crop fields should use fixed copper sprays (1.0 lb. active ingredient/A) to reduce the spread of black rot infection in the field. Copper can be phytotoxic (high rates) to some cabbage varieties and may cause black flecks to form on the wrapper leaves. Overall, crop protectant sprays are not very effective in managing black rot in cole crops.