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Effects of Oxygen and Carbon Dioxide Tensions on Sporangium and Oospore Formation by Phytophthora spp.. D. J. Mitchell, Former Research Assistant, Department of Plant Pathology, University of California, Riverside 92502, Present address of senior author: Department of Plant Pathology, University of Wisconsin, Madison 53706; G. A. Zentmyer, Professor, Department of Plant Pathology, University of California, Riverside 92502. Phytopathology 61:807-812. Accepted for publication 12 February 1971. DOI: 10.1094/Phyto-61-807.

The effects of aeration on sporangium production and oospore production by several species of Phytophthora were determined by incubating cultures in continuous-flow (100 ml/min) atmospheres containing 0, 5, 15, or 30% CO2 plus 1, 5, or 20% O2. Sporangium production was examined after growing cultures of P. cactorum, P. capsici, P. cinnamomi, P. citrophthora, P. palmivora, and P. parasitica in a glucose-nitrate-sterol liquid medium for 72 hr, washing the resulting mats with sterile deionized water, and incubating the suspended mats in open petri plates in aerated plexiglass chambers exposed to fluorescent light at 24 C for 24 hr. Oospore production by P. cactorum, P. megasperma var. sojae, and in pairings of the opposite mating types of P. capsici, P. cinnamomi, P. drechsleri, and P. palmivora was evaluated after 12 daysí growth on glucose-nitrate agar with sterol in open petri plates in aerated chambers at 20 C in darkness. Sporangium formation was reduced by decreasing O2 concentration or increasing CO2 levels from those in air. Oospore production was greater at O2 concentrations of 1% and 5% than in air, but decreased with increasing CO2 concentrations when the O2 level was 1, 5, or 20%.