Cause Xanthomonas campestris pv. campestris is a bacterium that overwinters in plant refuse for up to 2 years and is carried internally and externally on seed. The bacterium infects other crucifer crops and weeds. It may be spread in the field by water (rain, irrigation), insects, equipment, and animals. When spread by water, bacteria enter water pores (hydathodes) at leaf margins or occasionally through insect or other mechanical injuries. From the leaf, the infection proceeds up and down the main stem and then becomes systemic. Disease development may stop in unfavorable weather only to resume when conditions are favorable. Optimum temperatures for development are 80°F to 86°F. Water in the form of dew, rain, or sprinkler irrigation is necessary for spread.
Symptoms Tissue at leaf margins becomes yellow; chlorosis progresses toward the leaf center, creating a V-shaped area with the base of the "V" at the leaf midrib. Veins in these areas become dark brown or black; when the leaf is held up to a strong light source, they show up as a black net. Vascular tissue of the main stem then becomes blackened. As the infection becomes systemic, symptoms (the same as those found from direct infection) may appear anywhere on the plant. Unilateral stunting of some leaves occurs. In infected seed, the first symptoms appear on cotyledons, which rapidly wilt and drop off; usually, however, the infection has reached the stem and proceeds up.
Cultural control
- Use clean, pathogen-free seed.
- Control cruciferous weeds and insect pests.