Response of Selected Nursery Crop Plants to Inoculation with Isolates of Phytophthora ramorum and Other Phytophthora Species

Many nursery crops are susceptible to root and foliage diseases caused by numerous species of Phytophthora . Phytophthora ramorum causes sudden oak death of trees and ramorum leaf blight and shoot dieback on numerous nursery plants, including rhododendron ( Rhododendron spp.), viburnum ( Viburnum spp.), pieris ( Pieris spp.), and camellia ( Camellia spp.) in Europe, the United States, and British Columbia, Canada. We sought to evaluate relative susceptibility of a selection of ornamental nursery crops by inoculating detached leaves with several species of Phytophthora known to infect rhododendrons, and to compare the relative virulence on those species to isolates of P. ramorum . The results indicated that many plants were susceptible under these experimental conditions, while others were not. On a given host, symptoms caused by all species of Phytophthora were identical except for differences in pathogen virulence. Plant species were identical except for differences in pathogen virulence. Plant species within genera or cultivars within species varied in susceptibility to isolates of P. ramorum and other species of Phytophthora . Phytophthora ramorum , P. citricola , P. citrophthora , and P. nicotianae were the most virulent pathogens on most of the host plants inoculated. Some plants were susceptible to several species of Phytophthora , while others were susceptible only to P. ramorum . Inoculation of detached leaves of `Nova Zembla' rhododendron, lilac ( Syringa vulgaris ), or doublefile viburnum ( Viburnum plicatum var. tomentosum ) under controlled conditions with different species of Phytophthora or isolates of P. ramorum (both mating types) indicated significant relative differences in species or isolate virulence.