Cotton seedling blight, commonly referred to as rot disease or black root disease, is a significant root infection affecting young cotton plants. This disease is widespread across the globe and has long been a historical problem in cotton cultivation. In China, it is prevalent in many regions, with the Yellow River Basin being heavily affected, as well as the Xinjiang cotton-growing area. The Yangtze River Basin experiences a lower incidence, but overall, the disease affects between 5% to 40% of cotton fields nationwide. In years of resurgence, some areas may see over 50% infection rates, leading to severe plant death, sparse stands, and even total crop loss in extreme cases.
The pathogen responsible for this disease, *Rhizoctonia solani*, has an extremely broad host range, infecting over 200 plant species, including sorghum, millet, corn, potatoes, kenaf, soybeans, peanuts, eggplants, sugar beets, and various forest trees. It particularly affects plants from the Solanaceae, Leguminosae, Caryophyllaceae, Moraceae, and cruciferous families.
Symptoms typically appear early in the germination stage. Before emergence, young roots and shoots are often infected, leading to rotting buds and seedlings. Once the plants emerge, the first signs appear at the base of the stem—yellow-brown spots that gradually expand into sunken lesions. In severe cases, the entire stem becomes dark brown and rots, causing the plant to die. Infected seedlings can be easily pulled from the soil, a key diagnostic feature. In some cases, cotyledons may also show irregular yellow-brown spots, eventually leading to leaf drop. True leaves are usually not infected, but dehydration can cause them to wilt.
*Rhizoctonia solani* thrives in the soil and can survive for extended periods, forming sclerotia under unfavorable conditions. These structures allow the fungus to persist for months or even years. The optimal temperature for growth is 17–28°C, though the pathogenic range is slightly lower, around 16–25°C. The pathogen is highly adaptable, surviving in high temperatures, freezing, drought, and acidic or alkaline soils. It prefers moist conditions, with the highest concentration found in the top 5–10 cm of soil.
The infection cycle begins with the presence of mycelium or sclerotia in the soil or crop residues. When cotton seeds germinate, the pathogens become active and invade through natural openings or wounds. Symptoms appear within a day or two, and seedlings may die within a few days. Young seedlings, especially those in the cotyledon stage, are most vulnerable. If soil temperatures remain around 15°C after planting, even in cold or rainy weather, the disease can develop rapidly.
Environmental factors such as climate, soil type, and farming practices significantly influence the occurrence of blight. Early sowing in cold conditions increases the risk of infection due to prolonged exposure and weakened plant resistance. Overuse of nitrogen fertilizers can worsen the problem, while adding organic matter like straw or green manure helps suppress the pathogen by stimulating beneficial microbial activity in the rhizosphere.
Continuous cotton cropping leads to pathogen buildup in the soil, increasing disease pressure. Crop rotation with corn, wheat, or other non-host crops can help reduce the pathogen population. Poorly drained, heavy soils and low-lying fields are more prone to infection. Proper field preparation, including deep plowing, timely irrigation, and maintaining good drainage, is essential for prevention.
Control strategies should focus on prevention, combining cultural and chemical methods. Soil sterilization, proper seed treatment, and the use of resistant varieties (where available) are important. Timely seedling care, such as weeding, soil aeration, and avoiding premature removal of mulch, can improve plant health. Applying phosphorus and potassium fertilizers, managing pests, and removing diseased plants promptly are also critical steps in reducing the impact of this devastating disease.
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