Diversity and Risk Assessment of <i>Phytophthora</i> spp. Found on Plant Roots in Native Plant Nurseries and Interstate Shipping Nurseries in California

For many nurseries, it is difficult to diagnose Phytophthora-infected plants in time to prevent disease incursion and spread, especially if the infection occurs on roots and causes no or limited symptoms on aerial plant parts. Rapid, accurate, and specific diagnostic methods can facilitate the identification and removal of infected plants. This study was conducted to investigate whether specific nursery types within California are more likely to have different Phytophthora species and which hosts are more likely to test positive for a Phytophthora species using immunological and molecular diagnostic tools. From fall 2016 to spring 2019, we collected 1,180 root samples from 15 nurseries, each in California, that fell into one of three distinct plant-sourcing categories. Plant roots were tested with Phytophthora-specific Agdia ImmunoStrips, and positives samples were identified to the species-level by quantitative PCR (qPCR) and sequencing. Of the 1,180 samples collected, 262 (22%) were found positive for infection by a Phytophthora sp., and 152 of these samples (58%) could be diagnosed to the species level. Hosts in the genera Ceanothus (Rhamnaceae), Arctostaphylos (Ericaceae), and Salvia (Lamiaceae) tested positive for Phytophthora spp. most often. A total of 18 Phytophthora species were detected from the root samples over the course of this study. Despite its high rate of false positives (30%), the ImmunoStrip Phytophthora assay is useful as a preliminary screening tool for in-field diagnosis of root infection. qPCR testing of positive ImmunoStrip tests allows for more accurate detection and facilitates species assignment. [Formula: see text] Copyright © 2025 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .