Cotton crops can be attacked by various disease-causing pathogens, hampering the commercial value of this cash crop. Among the pests are nematodes, tiny worms which cause serious damage to the crops. Other pests that cause damage are pink bollworm, boll weevil, Egyptian bollworm, red bollworm, cotton strainers, white flies and cotton aphids. Diseases due to fungus, virus and bacteria also affect the standing crop severely. Disease management can be done by methods like using disease-resistant varieties, crop rotation and cultivating high quality hybrids.
Seedling diseases are caused due to the presence of fungus in the seeds or soil, causing serious damage to the roots of the seedlings. This leads to stunted growth of the plant. Rhizoctania solani, Pythium spp. and Thielaviopsis basicola are some of the fungi that cause seed or root rotting. Seedling diseases are controlled by using fungicides on the seeds and in the soil.
Bacterial blight, also referred to as angular blight, occurs mostly in warm, humid and wet climatic conditions. It causes lesions and yellowing on the leaves and stunted growth of the plant. Advanced stages may cause loss of foliage and lesions on the bolls. Bacterial blight can be managed by seed treatment, rotation of crops and by cultivating resistant varieties.
Boll rot occurs mostly during wet and humid conditions in late summer and fall. Some fungi attack the bolls directly, while in others, they enter through punctured portions of the plant. Brown or black spots appear on the bolls. In extremes cases, the bolls just drop off. This disease can be managed by seed treatment, avoiding excessive application of nitrogen fertilizers, defoliation, insect control and using resistant varieties.
Two types of wilt diseases, Verticillium and Fusarium wilt, are also two major fungal diseases. The disease-causing fungi remain in the soil for lengthy periods, even without the cotton plants. Cool, wet conditions and excessive nitrogen levels are favorable to these fungi. Verticillium wilt, caused by Verticillium dahlia fungus, is the more damaging of the two. Symptoms include yellowing of leaves and the wilting of leaves and bolls. The stem is also affected with brown discoloration. Rotating the crops, avoiding excessive nitrogen application, and using resistant varieties and bedding are effective ways to manage these diseases.
Lesions are formed on the cotton leaves due to the influence of several disease-causing pathogens. Some of these diseases are Ascochyta blight, Cercospora leaf spot and Alternaria leaf spot. Ascochyta blight, also known as wet-weather blight, a fungal disease, occurs as circular brown spots on leaves and cotyledons. Cercospora leaf spots appear as white or light brown spots on the leaves due to potassium deficiency during drought. Ascochyta blight is visible as circular dull brown spots that ultimately produce holes in the leaves. These diseases can be managed by rotation of crops and by applying sufficient potassium.