A Comparative Analysis of Detection Techniques Used in US Regulatory Programs to Determine Presence of Phytophthora ramorum in Camellia japonica 'Nucio's Gem' in an Infested Nursery in Southern California

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.