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Relative Prevalence and Virulence of Phytophthora Species Involved in Rhododendron Root Rot. H. A. J. Hoitink, Associate Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691; A. F. Schmitthenner, Professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691. Phytopathology 64:1371-1374. Accepted for publication 14 May 1974. DOI: 10.1094/Phyto-64-1371.

Phytophthora cinnamomi was found to be the most important species involved in the rhododendron root rot syndrome in the United States. P. cactorum and P. citricola cause root rot, but occur more frequently as dieback pathogens. P. cryptogea, P. lateralis, P. megasperma, and P. gonapodyides are involved in the root rot syndrome, but are considered minor root pathogens. P. citrophthora causes stem dieback only. All isolates of P. cinnamomi from ericaceous plants appeared to be equally virulent in our tests; however, some strains of P. cinnamomi isolated from nonericaceous plants were significantly less virulent than others to the susceptible rhododendron hybrid cultivar Purple Splendour. The hybrid cultivar English Roseum was resistant to all eight Phytophthora spp. Roots were rotted by all species, but regenerated from the crown in the top 2- to 4-cm layer of soil.