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Effect of Pythium ultimum and Metalaxyl Treatments on Root Length and Mycorrhizal Colonization of Cotton, Onion, and Pepper. U. Afek, Postdoctoral Fellow, Department of Plant Pathology, University of California, Riverside 92521. J. A. Menge, and E. L. V. Johnson. Professor, and Staff Research Associate, Department of Plant Pathology, University of California, Riverside 92521. Plant Dis. 74:117-120. Accepted for publication 13 August 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0117.

Root length and mycorrhizal colonization of cotton, onion, and pepper inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus intraradices were generally greater in fumigated soil than in nonfumigated soil. Five weeks after VAM fungus inoculation, 3942% of roots were colonized in fumigated soil, compared to 2126% in nonfumigated soil. VAM colonization of roots increased to 6471% following treatment with the fungicide metalaxyl. Root lengths and VAM colonization of the three crops were reduced significantly in fumigated soil following infestation with Pythium ultimum and were similar to those in nonfumigated soil. Metalaxyl did not affect root length or VAM colonization in fumigated soil. P. ultimum, Fusarium solani, and Rhizoctonia solani were isolated from the roots of cotton, onion, and pepper grown in nonfumigated soil. The most commonly isolated fungus was P. ultimum.