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Effects of Infection by Pythium spp. on Root System Morphology of Alfalfa Seedlings. R. P. Larkin, Department of Plant Pathology, University of Missouri, Columbia 65211, Current address: USDA-ARS, Biocontrol of Plant Diseases Laboratory, Beltsville Agricultural Research Center, Beltsville, MD 20705; J. T. English, and J. D. Mihail. Department of Plant Pathology, University of Missouri, Columbia 65211. Phytopathology 85:430-435. Accepted for publication 13 December 1994. Copyright 1995 The American Phytopathological Society. DOI: 10.1094/Phyto-85-430.

Root system morphology of alfalfa seedlings infected by various Pythium spp. was evaluated in greenhouse experiments using morphometric and topological assessment methodologies. Infection of alfalfa seedlings by Pythium spp., particularly P. ultimum and P. irregulare, resulted in reductions in root system growth and changes in root system architecture. Reductions in root system morphological parameters, including total root system length and total numbers of root orders, as well as the topological parameters root system magnitude, altitude, and total exterior path length, indicated overall smaller root system size and complexity in infected plants compared with noninfected plants. Root system branching structure also was affected, with infected root systems developing with a lower degree of branching than noninfected root systems, as represented by fewer orders of branching and a more monopodial type of branching pattern. Although isolates of P. ultimum, P. irregulare, and P. sylvaticum all caused substantial pre- and postemergence damping-off, P. sylvaticum caused no measurable change in root system morphology. In contrast, P. ultimum caused severe reductions in root system size and branching complexity, and P. irregulare, which caused less-severe damping-off, also caused substantial changes in root system morphology. Isolates of P. dissotocum and P. torulosum had no effect on root system growth or morphology. Metalaxyl applied to the soil was effective in reducing the effects of these pathogens, whereas metalaxyl seed treatment was less effective. Use of architectural analysis methods enabled the quantitative assessment of the impact of root pathogens on root system branching structure for the first time.

Additional keywords: Medicago sativa, root topology.