Epidemiology of Phytophthora Root Rot of Fraser fir: Estimates of Rhizosphere Width and Inoculum Efficiency

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Abstract
Fraser fir seeds were germinated for 12 days in a nonsterile, sandy-loam soil infested with different chlamydospore densities of Phytophthora cinnamomi in Buchner funnel tensiometers at -100 mb matric potential (.psi.m). After this time, tensiometers were either set to 0 mb for 16 hr and then rest to -100 mb; set directly to -10, -25, -50 mb; or left at -100 mb .psi.m for 5 days. The percentage of infected seedling radicles was determiend by plating onto a selective agar medium. Seedling rhizosphere width, at the different inoculum densities and .psi.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 .psi.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 = .psi.m (-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.