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The Relationship of Infection by Pythium spp. to Root System Morphology of Alfalfa Seedlings in the Field. Robert P. Larkin, Department of Plant Pathology, University of Missouri, Columbia 65211. James T. English, and Jeanne D. Mihail, Department of Plant Pathology, University of Missouri, Columbia 65211. Plant Dis 80:281. Accepted for publication 28 November 1995. Copyright 1996 The American Phytopathological Society. DOI: 10.1094/PD-80-0281.

Alfalfa root system morphology was evaluated in relation to root infection by Pythium spp. in developing seedlings in the field. Metalaxyl treatments (soil drench, seed treatment, or nontreated) were used to produce differing levels of root infection in the field. Experiments were conducted at an upland site on a research farm in Missouri in two consecutive years. Quantitative assessments of root system morphology were made using morphometric and topological classification techniques. A metalaxyl soil drench treatment reduced root colonization by Pythium spp. per unit root length compared with either the metalaxyl-treated seed treatment or untreated control in 1991 through the first 4 weeks after emergence. Root colonization by Pythium spp. was low in 1992 and no differences among treatments could be detected. In the 1991 field test, the metalaxyl soil treatment resulted in increased root system growth and complexity (greater length, more branches, faster growth rates) compared with the seed and control treatments over the first 4 weeks of seedling development. However, no effect of root infection on root system branching structure could be detected in either year. These results suggested that, under favorable environmental conditions, sublethal root infections of alfalfa seedlings by Pythium spp. can cause reductions in root system size and complexity under natural field conditions