Ascochyta gossypii (ascochyta blight of cotton)

CottonSmall, round, whitish spots (up to 2 mm diameter) first appear on the cotyledons and lower leaves of seedlings. The spots have a deep purplish-brown border; they later enlarge and the centre becomes light brown, papery and bears conidiomata. The centre of the spot may fall out and severe spotting results in defoliation of the lower leaves. Early plantings may be stunted.Conidiomata are produced in abundance in the older parts of lesions or after the leaves and stalks are dead. At maturity, sticky masses of conidia issue in streams (cirri or spore tendrils) from the opening at the apex of the conidioma. 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 become pale, liver-coloured and covered with tiny black dots (conidiomata). Cankers may girdle the stem and kill the distal parts.Flowers are not attacked, but mature lint can be destroyed. Lint may show a grey discoloration with conidiomata in half-opened bolls. Plants may die with severe infection (Holliday and Punithalingam, 1970).TobaccoA leaf spot, similar to that observed on cotton, occurs on infected tobacco plants. The darker peripheral area may have concentric zonations. Leaf lesions may develop more frequently on the midrib, causing it to fracture. Lenticular, dark spots may be found on the stem (Holliday and Punithalingam, 1970).

Cultural Control and Sanitary Methods

Planting cotton in rotation with other crops is an effective practice for controlling the disease. Cultivation practices which destroy crop residues are also recommended (Smith, 1950).

Host-Plant Resistance

No host-plant resistance has been developed against A. gossypii.

Severe losses from Ascochyta blight of cotton were reported in the southern states of the USA and in Tanzania, Africa, in the 1940s and 1950s (Higgins, 1940; Wallace, 1948; Morey and Miller, 1949; Smith, 1950; Smith, 1953). Almost 100% of cotton plants in fields in northern Alabama in which cotton had followed cotton were infected by A. gossypii (Smith, 1950). The disease has declined in importance since the 1950s and subsequent research into the disease has been minimal. Estimated seedling losses of 0.12% were reported for the disease in the USA in 1972 (Cotton Disease Council, 1972).