Ludhiana, June 21: Leaf spots on cotton due to different fungal and bacterial pathogens have gained significance with the cultivation of Bt cotton, say PAU plant pathologists Dr HS Rewal and Dr Chander Mohan.

According to them, last year, high incidence of pathogen-alternaria blight was reported in many fields in Bathinda, Mansa and Faridkot districts. Because of the attack of different fungal and bacterial pathogens, the cotton leaves develop pale green leaf spots with irregular margins. These spots enlarge, turn brown and are recognized easily due to the presence of concentric zones. Severe infection causes shedding of leaves specially where soil is light in texture.

The disease is more serious on plants having low vigour, or those which are raised in fields having low potash. According to the scientists, another type of leaf spot is also gaining importance. The pathogen attacks both leaves as well as the bolls. Circular to semi-circular brown coloured spots with broad violet margins appear on leaves, bracts as well as bolls. Later, small dark dot like structures are formed inside these spots. The fungus is seed borne and also survives on dead leaves. The disease is favoured by high humidity and rains. Dense growth in Bt cotton is favourable for the development of this pathogens.

As per these scientists, bacterial leaf blight, also known as angular leaf spot, is characterized by black arm and boll rot, depending upon the portion of the plant affected. The disease appears on leaves in the form of minute, water soaked, angular spots which later on turn brown. The disease extends up to the veins and veinlets, which also turn black.

Spots on bolls appear as round water soaked areas which later turn dark brown or black, and are slightly depressed. Spotted bolls may fail to open and lint may be discoloured with a yellow stain. Before boll rot is evident, dark irregularly shaped spots are observed on bracts surrounding the lower portion of bolls. Under severe disease conditions, defoliation takes place. The bacterium survives in seed as well as plant debris.

The scientists have highlighted that seed treatment before sowing is the best remedy against the leaf spot.