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Distribution and Disease Progress of Phytophthora Root Rot of Fraser Fir Seedlings. C. M. Kenerley, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650, Present address: Department of Plant Pathology and Microbiology, Texas Agricultural Experiment Station, College Station 77843; R. I. Bruck, Associate professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 77:520-526. Accepted for publication 10 June 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-520.

The movement of Phytophthora cinnamomi from infested into noninfested portions of five disease focus plots was monitored by quantifying inoculum density of soil samples (nine sampling periods) and recording Fraser fir seedling mortality (18 assessment dates). Comparison of overlapping inoculum density contour maps with regard to changes in inoculum density, new area infested, and shift in area of highest inoculum density revealed four patterns of propagule distribution. Inoculum density maxima were recorded 26 August and 8 September. Greatest increases in total mortality of seedlings also occurred during this period of the growing season. Final cumulative mortality was not correlated with initial inoculum density or seedling density. Seedling mortality was recorded in areas of each plot previously noninfested, but area of propagule detection was more extensive than that of seedling mortality. Plots of calculated relative rates of disease increase vs. the cumulative seedling mortality illustrated fluctuations in disease progression, indicating a lack of fit with widely acceptable models.