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Oxygen and Carbon Dioxide Concentration Effects on the Growth and Reproduction of Aphanomyces euteiches and Certain Other Soil-Borne Plant Pathogens. D. J. Mitchell, Postdoctoral Fellow, University of Wisconsin, Madison 53706, Present address of senior author: Department of Plant Pathology, University of Florida, Gainesville 32601; J. E. Mitchell, Professor of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 63:1053-1059. Accepted for publication 20 February 1973. DOI: 10.1094/Phyto-63-1053.

The growth of Aphanomyces euteiches was compared with that of Fusarium solani f. sp. pisi, Phytophthora megasperma, Pythium irregulare, Rhizoctonia solani, and Sclerotium rolfsii under atmospheres containing 1, 5, or 20% O₂ plus 1, 5, or 15% CO₂. F. solani f. sp. pisi and S. rolfsii were most tolerant to low O₂ levels and grew well at 1% O₂; A. euteiches and R. solani were intermediate and maintained 50% or more of their growth in air at 1% O₂; the growth of P. megasperma and P. irregulare at 1% O₂ was reduced to less than 20% of that in air. S. rolfsii grew faster in an atmosphere containing 5% O₂ than in air at all levels of CO₂. F. solani f. sp. pisi was the most tolerant and R. solani was the most sensitive to high CO₂ levels. The growth of P. megasperma at 5% O₂ and of P. irregulare at 1 and 5% O₂ was stimulated by the addition of 5% CO₂. Oospore production by A. euteiches, P. megasperma, and P. irregulare under 5% O₂ in the absence of nutrients was 65, 55, and 26%, respectively, of that in the air control, and a few oospores were formed even when the O₂ concentration was reduced to 1%. Oospore production by A. euteiches was stimulated by 5% CO₂ at all levels of O₂. Oospore production by P. irregulare was stimulated by 5% CO₂ only at 5% O₂, but was consistently inhibited in P. megasperma by any increase in CO₂ above that in air. Inhibition of oospore formation by 15% CO₂ was virtually complete at 1 and 5% O₂ with all three organisms. Zoospore formation by A. euteiches was maximum in air and decreased with decreasing O₂ or increasing CO₂ levels. When the pH was controlled, a few zoospores were formed under all atmospheres tested. The response of pea roots under comparable conditions is discussed with respect to the incidence of pea root rot.