Introduction About cotton Cotton diasesis

Verticillium Wilt (fungus –Verticillium albo-atrum): Young plants infected with Verticillium wilt show yellow leaves and stunting, and often die. Following the seedling stage, older plants exhibit a chlorotic mottling on the leaf margins and between the major veins. (Figure 4) Plants attacked during later stages of growth display a mottling on the lower leaves first, later progressing toward the top of the plant as the season progresses. Often a single branch shows symptoms in the early stages of disease. Yellow progresses inward, followed by brown, and the leaf finally dies. Severely affected plants shed all their leaves and most of their young bolls. These plants may survive throughout the growing season and send out young sprouts or shoots from the base of the plant. Verticillium wilt is difficult to distinguish from Fusarium wilt. Leaf symptoms are similar and the internal tissues of the stems are discolored in both diseases. The only reliable way to distinguish the wilts is through a laboratory isolation of the fungus. The fungus causing Verticillium wilt can survive in the soil as small, dark resting bodies called sclerotia. The sclerotia can withstand adverse environmental conditions. Susceptible plants growing in Verticillium infested soil may not be severely attacked if environmental conditions are not suitable for fungal growth. The disease is more prevalent during periods of cool, wet weather. The fungus is not transmitted in seed, but can be introduced in the field by infested gin trash and burrs. The disease is more common on heavier soils than on light sandy soils. Control measures include avoiding excess nitrogen, crop rotation, shallow cultivation to avoid root pruning, avoiding excess irrigation, and planting resistant varieties.
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.