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Occurrence and Control of Soybean Blight
Soybean damping-off, commonly known as "dead plant," "collapse," or "black root disease," is a serious issue that affects soybean crops during the early growth stages. In years with severe outbreaks, up to 5-10% of plants may die in infected fields, while in highly affected areas, the mortality rate can exceed 30%. In extreme cases, entire blocks of plants may be lost, leading to significant yield losses and even crop failure.
The symptoms of soybean blight are most noticeable during the seedling stage. Affected plants show reddish-brown, slightly sunken lesions on the main roots and the base of the stem near the ground. The outer layer of the root (cortex) becomes cracked and ulcerated. Initially, the pathogen's mycelium is colorless but gradually turns brown. As the disease progresses, the plants appear stunted, grow slowly, and develop an auburn-colored stem with cracked and ulcerated bark.
The occurrence of the disease tends to be more severe in continuous cropping systems, where bacterial populations in the soil accumulate over time. Fields with poor seed quality often experience higher infection rates, as seeds that are moldy or damaged are more likely to carry the pathogen. Seed transmission plays a key role in disease spread, reducing both germination rates and the plant's ability to resist infection. Early sowing can also increase the risk, as it extends the period of vulnerability for young seedlings. Additionally, leaving diseased crop residues in the field without proper decomposition can further promote disease development.
Soil conditions also play a critical role. Heavy infestations of pests, poor soil fertility, and inadequate fertilization contribute to more severe disease outbreaks. Soybeans grown in poorly drained or compacted soils are especially susceptible.
To manage soybean damping-off, several control strategies are recommended. First, planting disease-resistant soybean varieties is essential. Before sowing, seeds should be treated with chemical protectants such as 0.3% 40% methyl-rhodamine emulsifiable concentrate or 50% thiram wettable powder. Crop rotation is another effective measure—rotating with grass crops for at least three years helps reduce pathogen buildup in the soil. Choosing well-drained, high-lying fields and practicing ridge cultivation or deep furrow planting with sorghum can help improve drainage and reduce moisture levels around the roots.
Proper watering is also important. Irrigation should be based on soil moisture and temperature to avoid overwatering. Once symptoms appear, fungicides can be applied. Recommended treatments include 40% triethylphosphonate aluminum wettable powder diluted 200 times, 70% ethyl phosphorus manganese zinc wettable powder diluted 500 times, 58% metalaxyl manganese zinc wettable powder diluted 500 times, and 69% mancozeb manganese zinc wettable powder diluted 1000 times. These should be sprayed every 10 days, with 2–3 applications needed for effective control. Make sure to spray thoroughly and evenly for best results.