Formation and Germination of Chlamydospores of Phytophthora parasitica Under Various Oxygen and Carbon Dioxide Tensions. N. Ioannou, Department of Plant Pathology, University of California, Davis 95616. R. G. Grogan, Department of Plant Pathology, University of California, Davis 95616. Plant Dis. 69:400-403. Accepted for publication 18 October 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-400.

Mycelial mats of Phytophthora parasitica from tomato were incubated in continuous-flow atmospheres containing various concentrations of O₂, CO₂, and N₂, and the numbers of sporangia and chlamydospores produced after 2 wk were determined. Total sporulation (sporangia plus chlamydospores) was maximal at O₂ concentrations from 21 to 1% and at all CO₂ concentrations from 0.03 or less to 6.5%. Low CO₂ combined with O₂ concentrations decreasing from 21 to 2.5% favored production of abundant sporangia with few chlamydospores (sporangium to chlamydospore ratio about 85:15). At 1.25% O₂, sporangia and chlamydospores were produced in about equal numbers, but at 0.3% O₂, formation of both spore types was reduced to nearly nil. Increased CO₂ from 1 to 6.5% inhibited formation of sporangia and stimulated chlamydospore formation (sporangium to chlamydospore ratio 5:95). Above 6.5% CO₂, production of chlamydospores decreased, and at 21% CO₂, it was reduced by 30%. At 2–2.5% CO₂, chlamydospore formation was inhibited below 1% O₂. Chlamydospores incubated in atmospheres of various concentrations of O₂ and CO₂ on cornmeal agar (CMA) germinated predominantly by means of mycelium-producing germ tubes. In soil extract (SE) or on SE agar, chlamydospores germinated predominantly by means of sporangia. This type of germination and sporangium production from mycelium was maximum in atmospheres containing 0.03% CO₂ and 2.5–21% O₂. Below 2.5% O₂, chlamydospore germination in SE (sporangium production) was reduced, and at 0.2% O₂, it was nearly nil. Germination on CMA (mycelium-producing germ tubes) was maximal from 0.5 to 21% O₂. At 0.2% O₂, germination still was reduced about 50%. In constant 9–10% O₂ and CO₂ increasing above 0.03%, chlamydospore germination, both on CMA and SE, was maximal at 0.03% CO₂ and decreased with each increment in CO₂ concentration to nearly nil at 28% CO₂. At 0.03% CO₂, 90% of chlamydospores germinated by means of sporangium-producing hyphae, whereas at 2–10% CO₂, 60–100% of the germinating chlamydospores formed mycelium instead of sporangia.