Bacterial blight (Xanthomonas axonopodis pv. malvacearum)
The disease attacks seedlings, leaves, stems and bolls. Seedling attack, usually called seedling blight, results in small, round, water-soaked spots on the cotyledons as they emerge from the seed coat. The spots furnish inoculum (disease source) for the developing true leaves. Leaf spots are translucent, water-soaked, angular and bordered by veins. Affected leaves become browned and blackened. Spread of the bacteria along leaf veins is commonly called vein blight. Affected veins appear black. Younger leaves are more susceptible to vein blight than mature, old leaves.
In older plants black lesions may develop on the stems. These lesions girdle the stem and may cause breaking of the stem when windy. This symptom is known as black arm. Infected bolls have round, shiny lesions, which later become sunken, browned and finally blackened. Infected bolls become deformed and open prematurely. Fibre in diseased bolls is usually stained.
The disease is carried over season to season on infected crop debris and diseased seeds. Spread of the disease is through use of infected seeds and in the field by wind-blown rain water.
- Plant resistant varieties (e.g. Albar 51; Albar G501; BPA; BP 52; SATU; S 2950)
- Use disease-free seeds
- Practise crop rotation of at least 3 years with cereals or legumes
- Practise good field sanitation