Five detection methods were comparatively tested on putative Phytophthora ramorum field samples from 41 wild plant species. The tested methods included two culture-based assays, a DAS-ELISA-based polyclonal assay, a nested PCR-based assay, and a TaqMan real-time PCR assay. Diagnostic values including sensitivity, specificity, positive predictive value and negative predictive value were calculated for each method. The effects of host species, seasonality and host location were analysed and compared between two laboratories. Significant effects of season, host species and laboratory were detected. It is concluded that a combination of either culturing and molecular diagnosis or of two molecular assays is the most promising approach to diagnose this pathogen. Based on the results of this and other studies, diagnosis should occur as much as possible during wet and warm periods favourable to the pathogen, and proficiency tests should be performed to compare results obtained with molecular approaches in different laboratories. Furthermore, length of time lapsed between sample collection and processing strongly affected the diagnostic sensitivity of culture-based methods, and therefore needs to be taken into account when comparing results from different laboratories.