Cotton
Bacterial Blight of Cotton
Xanthomonas citri subsp. malvacearum
Bacteria
In a Nutshell
- Waxy, water-soaked spots with red to brown border on leaves, stems and bolls.
- Spots become brown.
- Black cankers on stem and branches.
- Premature defoliation.
Can also be found in
Cotton
Symptoms
Bacterial blight starts out as angular, waxy and water-soaked leaf spot with a red to brown border on leaves, stems and bolls. The angular appearance is due to the restriction of the lesions by fine veins on the cotton leaf. In some cases, the spots on the leaf blade may spread along the major leaf veins. As disease progresses, these lesions gradually turn into brown, necrotic areas. The infection of stems results in black cankers that grow around the vascular tissues and girdle them, causing the portions above the canker to die and premature defoliation of the plant. A white waxy crust containing the bacterium may form on old leaf spots or cankers. Bolls may become infected causing boll rot, rotted seeds and discolored lint. Infected bolls have round, rather than angular, lesions that initially may appear water-soaked. As the infection proceeds, bolls lesions will be sunken and dark brown or black.
Recommendations
Organic Control
Apply talc-based powder formulations containing the bacteria Pseudomonas fluorescens and Bacillus subtilis against the bacteria X. malvacearum. Use extracts of Azadirachta indica (neem extract). Apply growth regulators that prevent unrestrained growth to avoid infection with bacterial blight.
Chemical Control
Always consider an integrated approach with preventive measures and biological treatments if available. Seed treatment with authorized antibiotics and seed dressing with copper oxychloride are very effective against the bacteria causing cotton bacterial blight.
What caused it?
Cotton bacterial blight is caused by Xanthomonas citri subsp. malvacearum, a bacterium that survives in infested crop debris or seeds. It is one of the most devastating diseases of cotton. Significant rainfall events and high humidity, combined with warm temperatures, favor the development of the disease. The bacteria enter leaf tissues through the natural openings in the leaves (stomata) or mechanical wounds. This explains why the disease is most severe following storms that produce heavy rains or hail. Since the infections may be seed-borne, the delinting of seeds through an acid treatment has been instrumental in minimizing the spread of bacterial blight through contaminated seed. Seedlings growing from volunteer plants may also be a source of primary infection by bacterial blight.
Preventive Measures
- Plant high-quality, disease-free seeds or seeds delinted through an acid treatment.
- Using blight-resistant varieties is the more efficient method to avoid the disease.
- Scout fields, identify infected plants and remove them as well as some neighboring plants.
- Keep the canopy as open as possible to reduce humidity and promote drying of the foliage.
- Do not cultivate or move equipment through fields when foliage is wet.
- Do not irrigate with overhead irrigation systems.
- Harvest infested fields as soon as possible to avoid more losses.
- Remove plants as soon as possible from fields and burn them.
- Incorporate infected residue deep into the soil to favor decomposition.
- Plan a crop rotation with non-susceptible plants.
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