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Angular Leaf Spot of Cotton |
Symptoms: The disease appears on different parts of cotton plant, both in seedling and mature plant stage. The disease first appears on leaves, which appear water soaked, turn black and dry up often leaving the young seedling green with a black tip. Most of such plants die. In less severely affected plants, points on leaves and stem become water soaked and enlarge into angular reddish spots about 1 mm in diameter. The spots often coalesce and the leaf gradually yellows and drops. Lesions on young stems sometimes cause girdling and referred to as black arm. In older stems the spots may be callused off and leave open cankers. Yellowish bacterial exudate is common on lesions, in moist weather. On bolls, water soaked lesions appear, which coalesce to form irregular, large, brown, sunken areas. Bolls infected when young, may drop prematurely. Older bolls when infected may become distorted and the lint may be discoloured.
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Causal organism: Xanthomonas axonopodis pv malvacearum
Disease Cycle: The bacteria enter the mature seed through the basal end of the chalaza. They over winter in this manner and as contaminants on the surface of the seeds or in the lint attacked to it. Volunteer seedlings are the chief source of primary inoculum when cotton is planted after cotton. Wind blown soil, rain and irregular water are the means of dissemination. Insects have little importance.
Epidimiology: High humidity and moderate temperature (28 °C) favours the development of the disease. Primary infection is favoured by 30 °C and secondary infection is better at 35 °C. presence of moisture is very important for the first 48 hours. Dry and hot weather retards disease development. |
Control:
- Use of healthy seed from healthy plants.
- Delinting seeds with concentrated sulphuric acid then floating the delinted seeds in water and removal of the floating seeds.
- Disinfections of seeds with 1000 ppm streptomycin sulphate solution overnight.
- Destruction of diseased plant debris, and
- Killing of volunteer seedlings.
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