Tactical Diagnostic Strategies at Native California Ornamental Nurseries for Preventing Phytophthora Havocs in Wildlands.

Native plant restoration efforts in California are continuously challenged by Phytophthora-induced diseases, particularly through nursery introductions, resulting in significant losses and distribution of pathogens into restoration areas. Effective diagnostic strategies can help manage Phytophthora, yet determining the most suitable method, considering specificity, sampling time, resource availability, and expertise, remains uncertain. This study compares the effectiveness of five diagnostic methods: irrigation leachate baiting, ImmunoStrip® for Phytophthora, culture-based isolations, recombinase polymerase amplification (RPA, targeting genus-specific trnM-trnP-trnM locus), and quantitative polymerase chain reaction (qPCR, targeting genus-specific atp9-nad9 locus) on artificially inoculated and on naturally infested ornamental plants. For artificially inoculated plant testing, two native California ornamentals, Frangula californica and Heteromeles arbutifolia, were inoculated with three Phytophthora species: P. cinnamomi, P. cactorum, and P. cryptogea, and sampled three times, every six weeks post-inoculation. The sampled plants, along with the non-inoculated controls, were subjected to the five detection methods for Phytophthora detection. Among the detection assays, RPA, qPCR, and irrigation leachate baiting had the highest Phytophthora detection rates, regardless of the host, sampling time, or pathogen species (P < 0.01). For naturally infested plants, molecular assays consistently detected Phytophthora from three horticultural and two native nurseries (one accredited through Accreditation to Improve Restoration (AIR) program and other in process), with a subset of samples independently validated by another laboratory. The AIR-certified nursery showed no Phytophthora and the lowest detection probability across methods. These results indicate that implementing strategic detection approaches, combined with best management practices, can improve detection and prevent Phytophthora spread to wildlands.