Occurrence and Control Methods of Common Cotton Boll Diseases

Cotton boll disease is a composite disease caused by multiple pathogens, often manifesting as boll rot, which affects both cotton yield and cotton quality. Losses caused by boll rot reach more than 10% of total production. Common boll diseases in cotton fields in Henan Province include boll blight, anthracnose, red rot, pink rot, black boll, and others. They mostly occur in rainy years in mid to late August, and are especially severe during consecutive rainy days with high temperature and high humidity.

I. Symptoms of Boll Diseases

Boll blight: This disease mostly occurs on mature bolls in the lower part of the cotton plant, starting from the base, suture, or tip of the boll. It begins as a dark bluish-green water-soaked area, gradually spreading to the entire boll, turning bluish-brown, and finally becoming black, oily, irregular lesions with gradually lighter edges, and the boundary between diseased and healthy areas is not obvious. If environmental conditions are suitable, within 3-5 days of onset, the entire boll surface turns oily bluish-brown or black, after which a thin layer of downy mold grows on the boll surface, and the cotton boll becomes black and rotten or becomes a hardened, unopened boll segment. In late-onset cases, only the boll shell turns black, with less effect on the lint.

Anthracnose: In the early stage, dark red small spots mostly appear near the boll tip, gradually expanding into brown round spots with purplish-red edges, slightly raised, and sunken in the middle. When the climate is humid, red-brown sticky material is produced in the center of the lesions. In severe cases, the diseased boll stops growing, rots, or becomes a hardened unopened boll segment. In mild cases, the boll opens early, and the lint is brown.

Red rot: The lesions mostly occur from the base, tip, or suture of the cotton boll. In the early stage, they appear as dark green water-soaked areas, then spread to the entire boll, causing black rot, with a pink or pinkish mold layer growing on the surface.

Pink rot: Its symptoms are similar to red rot, except that the mold layer of pink rot is thicker, appearing as a loose pink fluffy material, and when humid, the mold layer turns pinkish-white; whereas the mold layer of red rot is thinner and denser.

Black boll: The affected cotton boll first turns light brown, and the whole boll becomes soft. In the later stage, raised small dots grow on the boll surface, initially white, later developing into black. On severely affected cotton bolls, the boll shell becomes hard, covered with a layer of gray-black mold, becoming a sooty black boll, often not falling off, remaining hard on the fruit branch, with the internal fibers becoming gray-black hard masses that cannot split open.

II. Control Methods

Agricultural control: ① Implement intercropping and relay intercropping. Adopt various forms of intercropping and relay intercropping such as cotton-grain, cotton-potato, and cotton-radish to increase ventilation and light penetration in cotton fields, reduce field humidity, create ecological conditions favorable for cotton growth but unfavorable for disease occurrence, and also increase income. ② Reasonable fertilization. Avoid excessive and too-late application of nitrogen fertilizer, and increase the application of phosphorus and potassium fertilizers. The suitable ratio of nitrogen, phosphorus, and potassium is 1:0.4:0.8. Apply more farmyard manure. ③ Reasonable irrigation and drainage. Never use flood irrigation; fine-flow furrow irrigation is preferable. Drain promptly after rain to reduce field humidity. ④ Careful pruning. Prune in time, remove old leaves, and pull out empty plants to increase ventilation and light penetration in cotton fields. Take the pruned branches and leaves outside the field and bury them. ⑤ Pick and collect diseased bolls. Promptly pick and collect cotton bolls that have already developed disease. Diseased cotton bolls are all over 40 days old, and the internal fibers have already formed. If not picked, the disease will continue to develop, causing rot and rapid spread; picking them can still recover a certain yield. The picked diseased bolls can first be dipped in 1% ethephon solution and then dried in the sun.

Chemical control: Generally, cotton bolls begin to develop disease 30 days after flowering. When some green bolls are present in the lower part of the cotton field, promptly spray 80% mancozeb, 58% metalaxyl-mancozeb, metalaxyl-thiram, or oxadixyl and other chemicals to protect the cotton bolls. Spraying at the early stage of disease can also control the development of the disease. Adding 27% high-film to the spray can form a film on the surface of the cotton boll, prevent pathogen invasion, and also prevent rain erosion, providing better protection.