Bacterial blight [Xanthomonas campestris pv. malvacearum (Smith 1901) Dye.1972], of cotton produces angular spots on leaves and squares, causes defoliation, lesions on sterns and bolls and reduces yield. Downward spread of the disease from the principal veins of the leaf blade or by direct infection of lateral branches results in the black arm symptom. This is the most serious form of bacterial blight and occurs in seasons most favourable to the pathogen or when crops have been severely stressed. Each season bacterial blight is present in varying levels of severity in the majority of crops. The grower.is most aware of this disease by the damage it causes to the boll. This consists of discolouration of the lint, destruction of one or more locules,.bolls failing to open and increased boll drop.