Black rot, Xanthomonas campestris. This bacterial disease appears on the leaves as yellow, wedge-shaped areas with darkened veins. These leaves are usually wilted. There are black streaks visible in the stem when it is split open. The head finally rots.
Black rot is a wet-weather disease spread in the field by splashing rain or insect feeding.
The bacteria live over in the soil, in plant refuse, or in seed from diseased plants. The only control at present is to use clean hot-water treated seed, crop rotation, and removal of plant refuse the previous season. Insect Problems Cabbage aphid, Brevicoryne brassicae . This whitish, powdery aphid occurs in dense clusters on the leaves where it feeds on plant sap. The leaves of infested plants crinkle and curl, and growth is retarded.