VIEW ARTICLE

Etiology

Prevalence and Virulence of Pythium species Associated with Root Rot of Corn in Poorly Drained Soil. Balakrishna Rao, Former Graduate Research Associate, Department of Plant Pathology, Ohio Agricultural Research, Wooster, OH 44691, Present address of senior author: Davey Tree Expert Company, 117 Water Street, Kent, OH 44240; A. F. Schmitthenner(2), R. Caldwell(3), and C. W. Ellett(4). (2)(3)(4)Professor, Assistant Professor, and Professor, respectively, Department of Plant Pathology, Ohio Agricultural Research and Development Center, and The Ohio State University, Wooster, OH 44691. Phytopathology 68:1557-1563. Accepted for publication 30 May 1978. Copyright © 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1557.

Root rot of corn in the field first appeared as small yellowish brown lesions on primary and secondary roots of seedlings at the four-leaf stage. Lesions enlarged and coalesced forming necrotic, black roots. The same sequence occurred as new roots developed. Pythium graminicola was the predominant fungus isolated in June, early July, and again in mid-September. Occasionally Pythium torulosum and P. dissotocum were obtained. Helminthosporium pedicillatum was found infrequently over the entire growing season. Fusarium oxysporum, F. moniliforme, F. roseum, and Pyrenochaeta terrestris were the predominant fungi isolated late in the season. A similar isolation pattern occurred in a greenhouse test with field soil. In steamed soil infested with millet seed inoculum, P. graminicola produced dark brown root rot; P. terrestris, red root rot; H. pedicillatum, infrequent black lesions; F. roseum, scattered red lesions; and F. moniliforme and F. oxysporum scattered necrotic lesions in 70, 30, 10, 12.5, 5, and 7.5% of the root mass, respectively. In water culture pathogenicity tests, P. graminicola produced progressive lesions and P. dissotochum and P. torulosum nonprogressive lesions. Pythium graminicola appears to be the primary incitant of corn root rot early in the season but its prevalence could not account for the severe root rot observed in August and September.