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Rhizoctonia solani can cause seed decay, preemergence damping-off of soybean seedlings, root rot on plants of all ages and aerial blight. On seedlings and older soybean plants, localized reddish-brown lesions develop near the soil line. The red color of the lesions is a good diagnostic guide, but it is best observed immediately after plants are removed from the soil because the color fades as plants are exposed to the air. Lesions are usually confined to the cortical layer (outer layer) of the hypocotyl or stem. If the lesions remain localized, the stem usually remains firm and plants will survive although they may be stunted. If lesions girdle the entire stem, plants may be extremely stunted or less vigorous than healthy plants, resulting in uneven stands. The foliage of infected plants may have an off-color or yellow cast, and the root systems may be poorly developed with decayed lateral roots.
Crusting, hard pan layers, herbicide injury, deep planting, poor seed quality, hail damage, insect damage, mechanical injuries, poor fertility or other factors that delay germination and emergence favor the development of Rhizoctonia root rot. Aerial blight appears particularly in the southern United States during prolonged periods of high humidity and high temperatures. Leaves, petioles and buds may exhibit disease symptoms beginning on the lower or middle parts of the plant and moving upward. Lesions are reddish brown, then turn brown or black; leaves show brown necrotic areas and may drop prematurely. A brownish web of the fungus may spread across infected leaves, stems and entire plants. To reduce losses plant good-quality seed with a good germination rate under good seedbed conditions, minimize or avoid stresses that delay germination and emergence and use an appropriate fungicide seed treatment. Wide row spacings or lower plant populations which help increase air movement in the canopy may help reduce aerial blight. |
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