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Zoospore Inoculum Density of Phytophthora cinnamomi and the Infection of Lateral Root Tips of Shortleaf and Loblolly Pine. 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, and A. E. Miller. Professor, and assistant professor, respectively; Department of Forestry, Clemson University, Clemson, SC 29634-1003. Phytopathology 79:1109-1113. Accepted for publication 6 June 1989. Copyright 1989 The American Phytopathological Society. DOI: 10.1094/Phyto-79-1109.

The relationship of spore concentration of Phytophthora cinnamomi to frequency of infection of shortleaf and loblolly pine roots was determined. Pine seedlings grown in sand culture were root-dip inoculated for 3 hr in spore suspensions ranging from 14 to 2,000 spores/ml. Shortleaf pine roots were more susceptible to infection by P. cinnamomi than were roots of loblolly pine. Comparison of fitted regression functions for the two species indicated that the regression lines were not coincident. Differences between the species were evident at spore concentrations of 80280 spores/ml. At spore concentrations outside this range, differences in susceptibility between loblolly and shortleaf pine were undetectable, because either too high or too low a percentage of roots of both species was infected. Spore concentrations required to infect 50% of lateral roots were approximately 100 and 178 spores/ml for shortleaf and loblolly pine, respectively.