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Detection, Isolation, and Pathogenicity of Phytophthora megasperma from Soils and Estimation of Inoculum Levels. G. C. Marks, Postdoctoral Fellow, University of Wisconsin, Madison 53706, Present address of senior author: Senior Forest Pathologist, Forests Commission of Victoria, Melbourne, Australia; J. E. Mitchell, Professor of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 60:1687-1690. Accepted for publication 29 June 1970. DOI: 10.1094/Phyto-60-1687.

Phytophthora megasperma was isolated and identified using 3-day-old alfalfa seedlings floated over flooded soil as bait. The seedlings were injured prior to use by pinching the hypocotyl with a pair of fine tweezers. The fungus sporulated vigorously on the seedling 72-96 hr after immersion, and could be identified directly. Soil isolates were as virulent to vernal alfalfa as an isolate from a tap root lesion. They caused rapid and extensive rot in the unthickened roots and lesions on the tap roots, followed by foliar discoloration, wilting, reduced top growth, small leaves, and frequently plant death. The baiting method was used to study the distribution of the pathogen in alfalfa fields. The highest population density of the fungus occurred in a poorly drained heavy silt loam with a history of severe alfalfa root rot. The fungus could not be detected on contiguous well-drained hill slopes and crests.