Phytopythium helicoides, which belongs to the algae (Chromista), Oomycota, Pythiales, Pythiaceae and Phytophthora, is a quarantine pathogen that causes brown rot of fruits, stem rot and root rot, along withother symptoms that can damage severaltreespecies in urban landscaping. Therefore, disease management requires rapid and accurate diagnosis. The present study used recombinase polymerase amplification (RPA) in conjunction with the CRISPR/Cas12a system to identifyP. helicoides. The test exhibited high specificity and sensitivity and could detect10 pg.µL-1 of P. helicoides genomic DNA at 37 ℃ within 20 minutes. The test results were visible by excitation of fluorophores by blue light. This groundbreaking testis able to detect P.helicoides in artificially inoculated Rhododendron leaves. The RPA-CRISPR/Cas12a detection assay developed in this study is characterizedby its sensitivity, efficiency, and convenience. Early detection and control of P. helicoides is crucial for the protection of urban green coverspecies.