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Etiology

Comparative Pathogenicity of Pythium myriotylum and P. irregulare to the Soybean Cultivar Bragg. J. W. Southern, Research Assistant, Plant Pathology Department, University of Florida, Gainesville 32611, Present address of senior author: Department of Plant Pathology, University of Minnesota, St. Paul 55108; N. C. Schenck(2), and D. J. Mitchell(3). (2)(3)Professor, and Assistant Professor, respectively, Plant Pathology Department, University of Florida, Gainesville 32611. Phytopathology 66:1380-1385. Accepted for publication 26 May 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1380.

Greenhouse pre- and postemergence tests on the soybean cultivar Bragg were performed with an autoclaved Arredondo fine sand (approximately WHC 8%, pH 6.2) noninfested or infested with oospores or mycelial inoculum of Pythium irregulare and P. myriotylum. Inoculum densities were 25, 50, 100, 250, 500, and 1,000 oospores/g of soil, except the highest inoculum density with P. myriotylum was 5,000 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 one test. In preemergence tests with oospores of P. irregulare, a significant plant height reduction occurred at inoculum densities of 500 and 1,000 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 per gram of soil, plant height was significantly reduced at 20, 23, 25, and 33 C. Root weight also was reduced at 20, 30, and 35 C. With P. myriotylum at 250 oospores/g of soil, plant height was reduced at 20, 23, 25, and 30 C. Root weight was reduced at 20, 25, 30, and 35 C. Oospores of P. myriotylum in pre-and postemergence tests did not affect root weight nor plant height. Pythium myriotylum was not recovered from the soil.