Phytophthora ramorum (Pram) is a pathogen of regulatory concern in the USA, and accurate diagnostics is a key component in the response to potential pathogen outbreaks. Although the molecular diagnostic protocols used in regulatory programs have been evaluated using regulatory samples, to date, no direct comparison of these methods has been analyzed within a nursery setting. A block of 300 camellia plants within a California nursery known to be infested with Pram was simultaneously assayed for visual symptoms, growth medium pH, and moss presence as well as culture isolation and molecular analysis prior to plant destruction. Disease symptoms such as foliar lesions and leaf drop were recorded for each plant prior to foliar and growth medium sampling. All diagnostic assays were highly correlated with one another and disease symptoms, with nested PCR having the best correlation with symptoms, followed by Real-Time PCR then culture. No correlation with disease or diagnostic assays was observed with moss presence or medium pH. Analysis of results allowed diagnostic sensitivity and specificity of the assays to be determined and the performance of each method for diagnosis of Phytophthora spp. or Phytophthora ramorum in camellia tissues and associated potting medium could be compared. Accepted for publication 12 July 2006. Published 16 October 2006.

Data from nursery surveys conducted in 2003, 2004, and 2005 for ramorum blight and dieback caused by Phytophthora ramorum were analyzed to determine which plant cultivars were likely to be infected with P. ramorum. Susceptible plant host genera, species, and cultivars were identified using culture and nested-polymerase chain reaction analyses, and disease incidence was calculated. For the genus, species, or cultivar analyses at least 20 samples were utilized in this study. The results indicated that the genus Camellia had the highest disease incidence, followed by Rhododendron and Viburnum. Camellia cvs. Setsugekka, Jean May, April Remembered, Winter's Fire, Kramer's Supreme, Bonanza, Magnoliaeflora, Apple Blossom, Silver Waves, Yuletide, and Nuccio's Gem showed the highest risk of P. ramorum infection. Four Rhododendron cvs. Purple Splender, Cunningham's White, Unique, and Nova Zembla were also grouped into the high risk category. All other Rhododendron and Viburnum plant species were at medium or low risk of infection by the pathogen. This study indicated that there is a large variation in disease incidence among genera and specific cultivars within a genus. A series of controlled, replicated, and well designed experiments involving a large sample size of plant cultivars is essential in order to identify the underlying reasons for the observed variation in infection risks of the surveyed data. Accepted for publication 25 January 2006. Published 15 March 2006.

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