Etiology of Rhododendron Dieback Caused by Four Species of Phytophthora. D. M. Benson, Department of Plant Pathology, North Carolina State University, Raleigh 27650. R. K. Jones, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Plant Dis. 64:687-691. Copyright 1980 American Phytopathological Society. DOI: 10.1094/PD-64-687.

Rhododendron dieback was present in all nine nurseries surveyed across North Carolina from 1976 to 1979. Phytophthora spp. isolated from plants with dieback symptoms included P. cactorum, P. citricola, P. heveae, and P. nicotianae var. parasitica. All species caused dieback in the greenhouse when plants with young tissue were sprayed with zoospore suspensions. Lesions on shoots developed as light to dark brown necrotic areas in 2–3 days. Mature tissue on intact plants was not infected by zoospores, but mature tissue was colonized by the fungi entering the plant through young tissue. A wedge-shaped necrosis developed in the leaf blade of mature leaves colonized through petioles from infected stems. In laboratory studies, the abaxial leaf surface was more susceptible than the adaxial surface to infection by zoospores. Infection counts did not differ between detached young leaves and detached mature leaves, although penetration of detached mature leaves was slower. Zoospores of P. heveae penetrated 0, 17, 67, and 100% of the detached young leaves in 6 hr at 16, 20, 25, and 30 C, respectively. Oospores were formed in infected host tissue by all species. Sporangia of P. cactorum, P. heveae, and P. nicotianae var. parasitica were formed in infected tissue after 12 hr when tissue was wet. Cultivar response to dieback varied. Under nursery conditions, Roseum Elegans and Roseum Pink had the least dieback (3.5% infection) and Scintillation the most (78.2% infection). Cultivars of intermediate response were Catawbiense Album, Chionoides White, Nova Zembla, Purple Splendour, and Spring Dawn.

Keyword(s): Rhododendron maximum, shoot blight.