Identification and Control Methods of Cotton Stem Blight

Symptoms The disease occurs more severely in the Northeast cotton region, the Yellow River basin and the Yangtze River basin, and the cotton regions along the rivers and coast. Cotton can be affected throughout the entire growth period, with the seedling stage and bud stage suffering the most severe damage. When cotyledons and true leaves are infected, small round spots initially appear with purplish-red edges and grayish-white centers, after which the lesions expand or merge into irregular-shaped lesions. Some lesions develop concentric rings in the center, scattered with small black granules, which are the pathogen's pycnidia. The diseased parts often break apart and fall off. Under high humidity, water-soaked lesions appear on young tender leaves, then spread rapidly as if scalded by boiling water, wilting and turning black, and in severe cases drying up and falling off, turning into bare stems and dying. When petioles and stems are infected, the lesion centers are light brown, with purplish-red surroundings, slightly sunken, and scattered with small black dots on the surface; in severe cases the stems and branches wither, break, or die. Lesions on cotton bolls are similar to those on stems. The center is darker in color, black. Under high humidity the lesions spread rapidly, causing the cotton bolls to become hard, unopened locks, with the bolls incompletely opening or not opening at all.

Pathogen Cotton Ascochyta blight fungus, belonging to the Deuteromycotina fungi. The pycnidia are initially embedded beneath the epidermal tissue of the cotton plant, and after maturing are exposed on the epidermis. The pycnidia are spherical, yellowish-brown, with a slightly protruding circular ostiole at the top, with relatively thin, fragile walls, measuring 82.8-210 x 75-189 (μm), containing many conidia inside. The conidia are oval, unicellular or bicellular, colorless, measuring 4.5-7.3 x 3.5-3.8 (μm). The optimum temperature for pathogen growth is 25°C.

Transmission Routes and Disease Conditions The pathogen overwinters as mycelium or conidia inside and outside cotton seeds or with diseased residues in soil or manure, infecting cotton seedlings the following spring, and producing pycnidia in the diseased parts, releasing conidia that spread through wind and rain and aphids, causing secondary infection. When temperatures stabilize above 20°C during the emergence stage and squaring stage, with continuous overcast and rainy weather lasting 3-4 days, the disease may occur or become epidemic within 3-5 days. In production, low temperatures and abundant rainfall during this period often trigger major outbreaks of the disease. In cotton fields with severe cotton aphid damage, the disease occurs severely. Continuous cropping and extensively managed cotton fields also suffer severe disease occurrence.

Control Methods (1) For seed treatment methods, see Cotton Black Root Rot. (2) Strengthen cultivation management, implement rational crop rotation, carry out intensive farming, raise seedlings and transplant, apply fertilizer reasonably, promote robust cotton plants, and improve disease resistance. (3) Control insects to prevent disease. To control this disease in combination with aphid control, when it is predicted that the disease will become epidemic 1-3 days after rain and aphid numbers are large, spray 1:1:200 Bordeaux mixture or 70% mancozeb wettable powder 500 times solution, 50% benomyl wettable powder 1500 times solution, and add 50% dimethoate emulsifiable concentrate or 80% dichlorvos emulsifiable concentrate 1000 times solution to the above solutions to simultaneously control cotton aphids.