Cotton seedling blight, also known as rot disease or black root disease, is a major root infection that affects young cotton plants. This disease is widespread globally and has long been a historical issue in cotton cultivation. It is commonly found across various regions in China, with the Yellow River Basin being heavily affected, and it is also a significant problem in Xinjiang's cotton-growing areas. In contrast, the incidence is relatively lower in the Yangtze River Basin. Nationally, the disease incidence ranges from 5% to 40%, and in some years, it can exceed 50%, leading to severe seedling death, thinning of rows, and even total crop failure. The pathogen has an extremely wide host range, affecting not only cotton but also over 200 other plant species, including sorghum, millet, corn, potatoes, kenaf, soybeans, peanuts, eggplants, sugar beets, and various forest trees. It particularly causes damage to Solanaceae, Leguminosae, Caryophyllaceae, Moraceae, and cruciferous crops.
Symptoms typically appear during the germination stage, when young roots and shoots are infected, leading to rotting buds and seedlings. After emergence, symptoms first appear at the base of the stems near the soil surface, starting as yellow-brown spots that gradually expand into sunken lesions. Severe cases result in dark brown rot, causing the seedlings to die. Infected seedlings can be easily pulled out from the soil, which is a key diagnostic feature. In some cases, cotyledons may show irregular yellow-brown spots, eventually breaking and falling off. True leaves are generally not infected, but they may become dehydrated. Hyphae often adhere to infected plants and surrounding soil. During rainy seasons, even mature plants can be affected, showing blackish-brown lesions at the stem base. Once the outer layer rots, the inner wood fibers become exposed, and the stem may break off, leading to plant death.
The pathogen responsible for this disease is *Rhizoctonia solani*, a fungus with strong survival capabilities in the soil. It spreads through hyphae and can form sclerotia under unfavorable conditions, allowing it to survive for months or even years. The optimal temperature for growth is between 17°C and 28°C, while the pathogenic range is slightly lower, between 16°C and 25°C. It is highly adaptable and can withstand high temperatures, freezing, and drought. The bacteria thrive in moist soils, particularly in the top 5–10 cm of soil. Under high humidity and heat, its survival time is reduced to about 4–6 months.
The infection cycle begins with the presence of pathogens in the soil, crop residues, and fertilizers. These pathogens remain dormant in mycelium or sclerotia until stimulated by cotton seeds or seedling exudates. They then invade directly or through wounds and natural openings. Symptoms usually appear within 12 hours of infection, and seedlings may die within two to three days. The cotyledon stage is the most vulnerable period for cotton seedlings. If the soil temperature remains around 15°C for an extended period, even under cold or rainy conditions, the disease can become severe. Infected tissues spread hyphae to nearby plants, forming clusters of diseased seedlings. If the weather is wet before harvest, the pathogen can infect the seeds, becoming a source of infection for the next season. Residues left in the field produce sclerotia, ensuring the survival of the pathogen in adverse conditions.
Environmental factors such as climate and soil play a critical role in disease occurrence. In northern and Xinjiang cotton regions, early spring cold and wet conditions increase disease risk. In southern areas, if planting occurs under dry conditions, blight may become the primary disease. Early sowing, due to low ground temperatures, leads to slow emergence and prolonged infection periods, weakening the seedlings' resistance. Excessive nitrogen fertilizer application can worsen the disease, while adding organic matter like straw or green manure helps reduce it by stimulating beneficial microbial activity in the rhizosphere, which suppresses *Rhizoctonia* growth. Continuous cotton cropping increases pathogen buildup, while crop rotation with corn or wheat can significantly reduce disease incidence. Poor drainage, heavy soil, and low-lying fields also contribute to higher disease severity.
To manage cotton seedling blight, integrated control strategies are essential. There are currently no resistant cotton varieties, so prevention is key. Cultural practices such as crop rotation, proper soil preparation, and timely irrigation help create favorable growing conditions and strengthen seedlings. Deep plowing in the fall, winter irrigation, and avoiding early sowing when temperatures are below 15°C at 5 cm depth are important. Field leveling and good drainage are necessary to prevent waterlogging. Seed treatment with fungicides, sterilization of seedbeds, and the use of decomposed manure can further reduce disease risk. Timely removal of diseased seedlings and maintaining proper soil aeration are also crucial. By combining these methods, farmers can effectively manage this persistent and damaging disease.
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