Effects of Oxygen, Carbon Dioxide, and Ethylene on Growth, Sclerotial Production, Germination, and Infection bySclerotinia minor

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Abstract
At O2 concentrations ranging 4-21% (normal air) with CO2 nearly constant at 0.03%, in vitro differences in radial growth or sclerotial production by S. minor were not significant, but at O2 concentrations below 4% both parameters were greatly reduced. Sclerotial production was more sensitive to O2 concentrations below 4% than was radial growth. At concentrations of CO2 .gtoreq. 8% with O2 kept at about 21%, both mycelial growth and sclerotial formation were reduced, but sclerotial formation was more sensitive to high levels of CO2 than was radial growth. Germination of sclerotia increased with increasing O2 concentrations between 1% and 8% with CO2 kept nearly constant at 0.03%, but there were no significant differences in germination at O2 contents ranging from 8-21%. With O2 nearly constant at 21%, there was a significant reduction in sclerotial germination at about 20% CO2, but not at lower concentrations of CO2. In the various O2-CO2 gas mixtures, sclerotial germination was similar to that obtained with similar levels of CO2 when levels alone were increased. No germination occurred in 19% CO2 and O2. Results if in vitro tests with different combinations of concentrations of O2 and CO2 on infection of lettuce tissue by sclerotia, indicated that O2 concentrations of 7.8% and above were more favorable for infection than lower concentrations, and CO2 concentrations .gtoreq. 12.3% were less favorable than lower concentrations. Percent infection of lettuce tissues by sclerotia was less at the respective O2 and CO2 levels than was infection by mycelial disks. Restriction of infection to lettuce tissues at or near the soil surface is not due to impaired aeration at the greater soil depths.