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Epidemiology of Phytophthora Root Rot of Fraser Fir: Root Colonization and Inoculum Production. K. M. Reynolds, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; 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 27695-7616. Phytopathology 75:1004-1009. Accepted for publication 16 April 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/Phyto-75-1004.

Phytophthora cinnamomi grew equally well on attached primary roots of 3-yr-old Fraser fir seedlings and on excised roots. At optimal temperatures for fungal colonization (27 C), a Beta function fit to the data (R2 = 0.95) predicted a linear colonization rate exceeding 18 mm/day. The effects of time and soil moisture content on chlamydospore production by P. cinnamomi on zoospore-inoculated 3-yr-old Fraser fir primary roots incubated in nonsterile sandy loam were examined. No chlamydospores formed in roots maintained at 5% soil moisture, and very few formed within roots maintained at 10 or 15%. Maximum numbers of spores were recovered from soil 17 days after burial of roots in the 10% soil moisture treatment. Production of chlamydospores occurred primarily at the root surface or in its immediate soil environment since large numbers of chlamydospores were obtained by soil sieving. A linear regression model for chlamydospore production as a function of incubation time in soil and soil moisture content was obtained (R2 = 0.91): C = 0.004(24.11 + 7.47 D - 0.16D2)M in which C = number of spores per millimeter of root, M = percent soil moisture content, and D = days following burial of roots in soil. Sporangium production on zoospore-inoculated 2-yr-old Fraser fir root tips was assessed on Büchner-funnel tensiometers. Sporulation was sparse and confined to the first 5-mm of root tip whether roots were placed directly on tensiometers following inoculation or preincubated in soil. Sporangia only occasionally formed on short roots and this appeared to be due to prior occupation of most short roots by other fungi. Sporangium production on root tips was extremely variable. A model for sporangium production as a function of soil matric potential, time, and preincubation treatment was developed by using stepwise regression (R2 = 0.66): S = Bĝ + B1 T + B2 T2 + 0.000018 M3, in which S = number of sporangia produced, M = soil matric potential (mb), T = incubation time on tensiometer (hr), Bĝ = a constant (- 13.13 for preincubated roots, - 14.13 for nonincubated roots and B1 and B2 are third-order functions of soil matric potential, the functional form depending on whether the roots had received a preincubation.