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Simulation of the Spread of Phytophthora cinnamomi Causing a Root Rot of Fraser Fir in Nursery Beds. K. M. Reynolds, Former graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616, Present address: Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691; H. J. Gold(2), R. I. Bruck(3), D. M. Benson(4), and C. Lee Campbell(5). (2)Professor and director, Biomathematics Program, Department of Statistics, North Carolina State University; (3)(4)(5)Associate professor, professor, and associate professor, respectively, Department of Plant Pathology, Raleigh. Phytopathology 76:1190-1201. Accepted for publication 30 December 1985. Copyright 1986 The American Phytopathological Society. DOI: 10.1094/Phyto-76-1190.

A model was developed to simulate the spread of Phytophthora cinnamomi in Fraser fir in nursery beds. The major system submodels were seedling growth and pathogen spread. The submodels for seedling growth described root and shoot development. Regression models described seedling height as a function of degree-days and shoot dry weight as a function of seedling height and degree-days. Regression models also related shoot dry weight to seedling root attributes. The submodel for pathogen spread described root colonization as a function of soil temperature, interseedling infection across root contacts or small interroot distances, and pathogen spread when soil moisture is suitable for zoospore movement and infectivity. Results of validation studies demonstrated good agreement between the model and portions of field data. Discrepancies were most pronounced in late summer. Alterations in system parameters associated with unusually dry soil conditions in late summer significantly improved agreement between the model and field data. Model response was insensitive both to changes in soil saturation period and to the functional relationship between inoculum efficiency and interroot distance. Parameters whose variation caused weak or inconsistent effects are rhizosphere width in flooded or nonflooded soil and colonization rate of the pathogen along the roots. In contrast, variations in interseedling distance, time of season, probability of interseedling infection by hyphal growth, and chlamydospore production elicited strong and consistent model responses.