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Occurrence and Control of Soybean Blight
Soybean damping-off, commonly known as "plant death," "collapse," or "black root disease," is a serious issue affecting soybean crops. In years with severe outbreaks, the rate of dead plants in infected fields can range from 5% to 10%, while in highly affected areas, this rate can exceed 30%. In extreme cases, entire blocks of plants may die, leading to widespread crop loss.
The symptoms of the disease are most evident during the seedling stage. Affected plants show reddish-brown, slightly sunken lesions on the main roots and at 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. In severe cases, the plants appear stunted, grow slowly, and the lower stems turn auburn with cracked and ulcerated bark.
The disease tends to be more severe in continuous cropping systems, where bacterial populations in the soil build up over time. Poor seed quality also contributes to higher infection rates, as seeds with mold are often heavily infected. The blight pathogen can be transmitted through seeds, which reduces germination potential and weakens the plant’s resistance. Early sowing increases the risk of disease onset, as seedlings remain vulnerable for longer periods. Leaving diseased crop residues in the field without proper decomposition can further spread the infection. Additionally, heavy pest infestations, poor soil conditions, and nutrient deficiencies can worsen the problem.
To manage the disease, it is recommended to use resistant soybean varieties. Seed treatment is also effective—applying 0.3% of 40% methyl-rhodamine emulsifiable concentrate or 50% thiram wettable powder to the seeds before planting. Crop rotation, especially with grass crops, for at least three years, can significantly reduce disease pressure. Planting in well-drained, high-lying areas is beneficial. Raised bed cultivation or deep furrow planting with sorghum can help manage moisture levels. Avoiding excessive surface humidity and ensuring timely drainage after rain is crucial. Watering should be done based on soil moisture and temperature to prevent waterlogging.
At the early stage of disease development, several fungicides can be applied. These 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. Spraying should be done every 10 days, with 2–3 applications required for effective control. Make sure to spray thoroughly and evenly to cover all parts of the plants.