Spatial Pattern of Phytophthora Root Rot and Dieback of Azalea in Container-Grown Nursery Stock. D. M. Benson, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. C. Lee Campbell, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 69:1049-1054. Accepted for publication 14 June 1985. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-1049.

Spatial pattern of azaleas with visual symptoms of Phytophthora root rot caused by Phytophthora cinnamomi and Phytophthora dieback caused by several Phytophthora spp. was assessed for container-grown azaleas at three nurseries. Phytophthora root rot was found to be random. Indices of dispersion (variance-to-mean ratio and Morisita’s index) and Poisson and negative binomial probability distributions were used to evaluate incidence of disease from quadrat sampling of nursery sections containing 400 plants. Each section had 40 quadrats. Disease incidence ranged from 4 to 58% depending on plant age and cultivar. Phytophthora dieback of azalea, a distinct phase of the Phytophthora syndrome, was aggregated at two nurseries and random at the third. Secondary production and dissemination of inoculum during favorable environmental periods may account for the spatial aggregation of plants with symptoms of Phytophthora dieback. Sampling procedures to estimate disease incidence of Phytophthora diseases should be improved with an understanding of spatial pattern.