| [Image: Picture of cotton leaves with discolored veins] |
| [Image: picture of angular leaf spot on cotton leaves] |
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Symptoms
Bacterial blight can affect plants at any stage, and leaves, stems and bolls of the cotton plant may be infected. Initial symptoms are small, circular, dark green to brown lesions on the cotyledons. On leaves, brown or black angular spots restricted by small veins occur, hence the name “angular leaf spot.”
Over time these spots enlarge and cover large areas of the leaf. Often lesions can also form along the veins of the leaf. In severe infections, the petioles and stems of plants are covered with black cankers, a condition called “black arm.” Infected bolls have circular, sunken, dark brown spots. The infection may extend to the inside of the boll and stain the lint. If dry conditions prevail after seedling infection, these early season infections do not cause significant losses. Dry periods stop the progression of the disease before the bolls are infected. The redevelopment of the disease late in the season after boll set can cause significant losses, because bolls are infected.
Causal agent and disease development
Bacterial blight is caused by the bacterium Xanthomonas campestris pv malvacearum. The bacterium is usually introduced into the field by contaminated seed, but it may survive on crop residues in the soil. During germination the cotyledons are infected and the disease spreads to the leaves and eventually to the stems and bolls. Free water is required for infection, and the disease is spread by splashing or windblown rain and irrigation water. Because of the need for water, symptoms are generally more severe in fields receiving overhead irrigation or frequent rainfall.
Control
Once a serious problem in cotton, bacterial blight has been reduced by acid delinting that removes bacteria from the seed and by not planting seed from infected fields. In addition, the high level of resistance in most varieties has greatly reduced the incidence of the disease.