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Effect of Dissolved Oxygen Concentration on the Relative Susceptibility of Shortleaf and Loblolly Pine Root Tips to Phytophthora cinnamomi. S. W. Fraedrich, Former graduate research assistant, Department of Forestry, Clemson University, Clemson, SC 29634-1003, Present address: Forestry Sciences Laboratory, USDA Forest Service, P. O. Box 70, Olustee, FL 32072; F. H. Tainter, professor, Department of Forestry, Clemson University, Clemson, SC 29634-1003. Phytopathology 79:1114-1118. Accepted for publication 6 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1114.

Exposure of shortleaf and loblolly pine lateral roots to oxygen concentrations of 00.25 mg/L for durations of 624 hr before inoculation increased root susceptibility to infection by Phytophthora cinnamomi as compared with roots maintained continuously under oxygen concentrations of 7 mg/L. Symptom development in lateral roots progressed more rapidly in roots that had been stressed with low oxygen than in roots maintained under high oxygen. Maintenance of pine seedlings at oxygen concentrations of 0.251.0 mg/L for 12 and 24 hr before inoculation had no subsequent effect on root susceptibility to infection or the rate of symptom development in infected roots of either pine species. Results suggest that essentially anaerobic conditions are necessary in the rhizosphere before pine root tips are predisposed to attack by P. cinnamomi. Further, no evidence was found to support the hypothesis that differences in tolerance to low soil oxygen concentrations between loblolly and shortleaf pine are linked to their relative susceptibilities to infection by P. cinnamomi.