Michelle Grabowski, UMN Extension Educator
A bacterial disease of crucifers (cabbage, broccoli, cauliflower, radishes, Brussels sprouts and more) called black rot caused severe damage to many gardeners last year and has been found again in Minnesota this summer. Black rot is caused by the bacteria Xanthomonas campestris pv. campestris. This pathogen often enters a garden on contaminated seed. It can also survive on infected weeds like wild mustard and pepper weed or in plant debris from previous years infected crop.
Black rot can be recognized by the v shaped lesions that form on infected leaves. The tip of the V points towards the mid rib of the leaf. The center of the v is often dead brown tissue which is surrounded by a yellow halo. As the disease progresses, leaf veins turn black within the lesion. The infection can move into the plants vascular system and result in wilt and soft rot.
Black rot enters the leaf through natural openings on the leaf and through wounds. The sticky bacteria are easily spread on a gardener's hands or tools, on splashing water or by insects. The bacterial pathogen thrives in warm wet weather and gardeners may see plants wilt rapidly under these conditions.
The best management of black rot is to prevent introduction of the bacteria into the garden in the first place. Look for seed and transplants that are certified disease free. Inspect transplants for signs of disease and reject any with v shaped leaf spots or wilted yellowing lower leaves. If disease shows up in one or two plants out of many, remove the infected plants completely. If infection is light (a few leaf spots) remove the infected leaves completely. Wash hands and tools with soap and water after touching infected plants.
Once disease is established in the garden, its spread can be slowed by several management practices. Avoid watering when dew is present and in early evening. Bacteria can spread in the dew and will thrive in moisture left on leaves after sunset. Rather water when the sun is bright and will dry leaves quickly. Avoid splashing water on leaves as much as possible. Do not work in plants when they are wet from rain, dew, or irrigation. Bacteria easily spread on hands and tools during this time. Copper sprays can be applied to slow the spread of the disease although they will not cure plants. Always completely read the label before applying a pesticide and follow all instructions.
At the end of the season, till under any plant residue as soon as the crop is harvested. Avoid planting any crucifer in that location for another two years.