Black rot was observed on both ornamental cabbage and kale in the UMass Extension Plant Diagnostic Lab this week. This disease occurs when the bacterium Xanthomonas campestris pv. campestris enters the hydathodes (leaf pores) and begins to colonize the leaves. Characteristic symptoms of black rot include marginal browning of leaves and V-shaped lesions extending inward from the leaf margins. Blackened leaf veins may be observed in affected areas. Affected tissue turns yellow, then brown in the center. Bacteria may also enter stomata or wounds, including those caused by insect feeding. This type of infection produces small, irregular lesions that do not begin at the leaf margins. Lesions may continue to enlarge until a significant percentage of leaf tissue is diseased.
X. campestris pv. campestris can survive for some time in undecomposed plant debris. Cruciferous weeds are also susceptible to black rot, providing another avenue for overwintering and a source of inoculum in spring. Infected seeds and seedlings are also common sources of disease in the greenhouse. Overhead irrigation, insects, and human activity can spread the pathogen from plant to plant. The disease tends to be more severe in warmer weather: infection and symptom development occur most rapidly at 75-95°F (25-30°C). At lower temperatures, the disease can progress slowly without the appearance of symptoms. The bacterium is inactive below 50°F (10°C).
Plant disease-free seed and seedlings. Most major seed companies test seed for X. campestris pv. campestris. Manage weeds in and around the greenhouse, especially those species in the Brassicaceae. Control insect pests. Remove infected plants and plant debris from the greenhouse. Use drip irrigation rather than overhead if possible and space plants adequately to facilitate drying of foliage. Do not work with plants when foliage is wet. Copper-based fungicides may be used to slow the progress of black rot within the crop, but it cannot cure infected plants.