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Epidemiology of Phytophthora Root Rot of Fraser fir: Estimates of Rhizosphere Width and Inoculum Efficiency. K. M. Reynolds, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695; D. M. Benson(2), and R. I. Bruck(3). (2)(3)Professor, and assistant professor, respectively, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Phytopathology 75:1010-1014. Accepted for publication 16 April 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1010.

Fraser fir seeds were germinated for 12 days in a nonsterile, sandy-loam soil infested with different chlamydospore densities of Phytophthora cinnamomi in Büchner funnel tensiometers at - 100 mb matric potential (φm). After this time, tensiometers were either set to 0 mb for 16 hr and then reset to - 100 mb; set directly to - 10, - 25, - 50 mb; or left at - 100 mb φm for 5 days. The percentage of infected seedling radicles was determined by plating onto a selective agar medium. Seedling rhizosphere width, at the different inoculum densities and φm, were calculated from infection levels. Rhizosphere width (W) did not depend on inoculum density. The model obtained for rhizosphere width as a function of φm accounted for 82% of the variation (P = 0.0001) and is given by: W = 4.01(1 - M)-0.72 in which W = rhizosphere width (mm) and M = φm (- mb). The same experimental system was used to study the effect of duration of soil saturation (2, 4, 8, and 16 hr) (Ps) on inoculum efficiency (E) by using Ferriss' model. The model developed, which accounted for 63% of the variation (P = 0.0004), was E = 0.04 Ps0.63.