VIEW ARTICLE

Ecology and Epidemiology

Development of Phytophthora Root Rot of Alfalfa in the Field and the Association of Rhizobium Nodules with Early Root Infections. F. A. Gray, Research Director, Farm Seed Research Corporation, San Juan Bautista, CA 95045; R. B. Hine, Professor, Department of Plant Pathology, University of Arizona, Tucson 85721. Phytopathology 66:1413-1417. Accepted for publication 3 May 1976. Copyright © 1976 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-66-1413.

Initial root infections of alfalfa, caused by Phytophthora megasperma, occurred 4-8 weeks after planting in naturally infested field soil and were primarily associated with nodules incited by Rhizobium meliloti. In pasteurized (temperature increased to 115 C over a 4-hour period) field soil artificially infested with P. megasperma, seedling death was 24% higher with R. meliloti-treated seed than in nontreated controls. Increases in seedling death were not observed when treated and untreated seed were planted in field soil naturally infested with P. megasperma and R. meliloti. In a 4-month-old alfalfa stand, taproot lesions occurred from 4-25 cm below the soil surface with maximum lesion development at depths of approximately 17 cm. Phytophthora megasperma was recovered at soil depths as great as 56 cm in Mojave clay and 80 cm in Gila silt loam and Grabe clay loam.