Bacterial Blight (bacterium - Xanthomonas campestris pv. malvacearum): Also known as angular leaf spot, vein blight, black arm and boll rot, depending on the portion of the plant infected. This organism affects all above ground parts of the cotton plant during any stage of its growth. Angular spots first appear on the leaves as water-soaked areas; the spots later turn dark brown to black and are covered with a glazed film. Often the surrounding tissue becomes yellow, giving a halo effect. Leaf spots are limited in size by leaf veins, which result in an angular shape. Infected leaves shed from seedlings and older plants. Occasionally, a black, water-soaked area occurs along a large vein in a leaf. Spots on bolls appear as round, water-soaked areas, but later turn dark brown or black. Spotted bolls may fail to open and lint may be discolored with a yellow stain. Before boll rot is evident, dark, irregularly shaped spots can be found on bracts surrounding the lower portion of the boll. Black spots or cankers may occur on the stems or branches (black arm) causing girdling and death of some branches. Overall, leaf spots and defoliation are the most noticeable symptoms and they are most likely to occur during or following rainy periods. Control measures depend largely on eliminating sources of infection and growing resistant or tolerant varieties. Since the bacterium overwinters in crop residue, plowing under the stubble immediately after harvest and practicing crop rotation will help reduce inoculum in the field. Acid delinting of cottonseed has eliminated carry-over. Most varieties have good resistance to bacterial blight.
