Ascochyta blight (Ascochyta gossypii)
The disease is caused by fungus Ascochyta gossypii. It may manifest as seedling blight, a leaf spot, a stem canker, and as a boll spot. The first symptoms are small, round, white, purple-ringed spots on the cotyledons and lower leaves. The spots become somewhat elongated and raised on the upper surface. Later they change to a light brown, the purple ring around the outside disappears, and the diseased tissue often falls out. The upper small leaves, petioles and buds are often infected, and the plant dies. The almost bare stems, with a few small leaves at the tip, are characteristic of the disease at the later growth stages.
Stem infection, which occurs only during consecutive days of cloudy, wet weather, leads to the formation of lesions, which may reach several centimetres in length, with cracks and ragged edges. The centre of these lesions becomes pale, liver-coloured and covered with tiny black dots (bodies of fungal spores, conidiomata). Cankers may encircle the stem and kill the distal parts. Flowers are not attacked, but mature lint can be destroyed. Lint may show a grey discolouration with conidiomata in half-opened bolls.
The disease is favoured by long periods of rain and cool weather. It is also seed-borne.
- Use disease-free seeds.
- Practise crop rotation with crops non-related to cotton (e.g. cereals).
- Practise good field sanitation and cultivation practices which destroy drop residues.