VIEW ARTICLE

Ecology and Epidemiology

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 (φ_m). 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 φ_m 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 φ_m, were calculated from infection levels. Rhizosphere width (W) did not depend on inoculum density. The model obtained for rhizosphere width as a function of φ_m 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 = φ_m (- mb). The same experimental system was used to study the effect of duration of soil saturation (2, 4, 8, and 16 hr) (P_s) 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 P_s^0.63.