Comparative Pathogenicity of Pythium myriotylum and P. irregulare to the Soybean Cultivar Bragg

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Abstract
Greenhouse pre- and postemergence tests on the soybean cultivar ''Bragg'' were performed with an autoclaved Arredondo fine sand (approximately WHC [waterholding capacity] 8%, pH 6.2) noninfested or infested with oospores or mycelial inoculum of P. irregulare and P. myriotylum. Inoculum densities were 25, 50, 100, 250, 500, and 1000 oospores/g of soil, except the highest inoculum density with P. myriotylum was 5000 oospores/g of soil. In preemergence tests with mycelial inoculum, P. myriotylum significantly reduced the mean height and number of plants, whereas P. irregulare significantly reduced the plant height only in 1 test. In preemergence tests with oospores of P. irregulare, a significant plant height reduction occurred at inoculum densities of 500 and 1000 oospores/g of soil. Root dry weight was significantly reduced in postemergence tests at 250 and 500 oospores/g of soil. With P. irregulare at 250 oospores/g of soil, plant height was significantly reduced at 20, 23, 25 and 33.degree. C. Root weight also was reduced at 20, 30, and 35.degree. C. With P. myriotylum at 250 oospores/g of soil, plant height was reduced at 20, 23, 25, and 30.degree. C. Root weight was reduced at 20, 25, 30 and 35.degree. C. Oospores of P. myriotylum in pre- and postemergence tests did not affect root weight nor plant height. P. myriotylum was not recovered from the soil.

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