Other Phytophthora Diagnosis & Management

Read on for the following resources: Sampling for Phytophthora species; Best Management Practices for Nurseries; and Guidelines for Restoration and Fieldwork.

Sampling in Habitat Restoration Plantings

Swiecki, Tedmund J.; Bernhardt, Elizabeth A.; Bourret, Tyler B.; Frankel, Susan J. 2025. Sampling to detect soilborne Phytophthora infestations in California habitat restoration plantings: a technical guide. Gen. Tech. Rep. PSW-GTR-279. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 63 p. doi.org/10.2737/psw-gtr-279. 15MB. Download PDF

This illustrated guide provides background information and guidance to help resource management professionals and land managers identify habitat restoration sites that have been affected by introduced root-rotting Phytophthora species. Restoration sites have become persistently infested with soilborne Phytophthora species by the planting of nursery stock with Phytophthora root rot. These infestations can spread beyond planted material into adjoining native vegetation, resulting in expanding areas of plant decline and mortality. Root-rotting Phytophthora species decay fine roots and may cause basal stem cankers. This damage induces shoot symptoms related to acute or chronic water stress in infected plants. Because many other agents and environmental conditions can induce similar or identical shoot symptoms, diagnosis of Phytophthora root rot requires sampling and testing to detect Phytophthora in the root systems of affected plants. We provide guidance for sampling plants to detect soilborne Phytophthora by baiting of root/soil samples. Topics include strategies to optimize detection and minimize false negative results; details of sample collection, including timing, plant selection, collection and handling; and phytosanitary practices to prevent spread of contamination. We describe specific methods for baiting samples with green (unripe) pears to detect Phytophthora. Identification of Phytophthora infestations in restoration areas can inform management to prevent further pathogen spread within and beyond infested sites.
Leachate baiting can be used in nurseries to detect Phytophthora in container plants. For more information see “Testing Procedures for BMPs for Producing Clean Nursery Stock” from Phytosphere Research. A video describing and demonstrating this testing procedure is available at this link.

Best Management Practices for Nurseries

Guidelines to Minimize Phytophthora Pathogens in Restoration Nurseries (AKA “Nursery BMPs”) – The Phytophthoras in Native Habitats Work Group complied these guidelines to help design and maintain a nursery system that excludes Phytophthora and other plant pathogens to the best extent possible. These are intended for professional nursery growers that supply plants to wildland restoration projects. (Latest draft updated March 2021)
A sampling and diagnostic guide is under development; until a more formal publication is available, Phytosphere Research’s “leachate baiting” protocol may be useful to test for Phytophthora species in nursery container stock. Also see the updated How to construct a zoospore collection vessel (ZCV) – version 2 posted by Phytosphere.
Frequently Asked Questions about the BMPs (updated August 2023)
Buying healthy plants: What to look for at a nursery – guidelines created for the California Native Plant Society but useful to anyone wanting to work with plant growers on producing and purchasing healthy nursery stock. (Latest draft updated December 2017)
Understanding results from the CDFA lab – a handout for nurseries
Affordable Soil Heat Treating System: An example from The Watershed Nursery (video)
Nursery Container Cleaning Instructions: An example from The Watershed Nursery (video)

Guidelines for Restoration and Fieldwork

These draft guidelines and best management practices (BMPs) aim to prevent and manage Phytophthora species during restoration activities and other field work, including trailwork and construction projects. This guidance is targeted for use in rare plant and other high-value habitats but may be applied to all areas where sustaining natural resources is a priority.

“Restoration guidance” covers restoration site preparation and construction; guidelines for planting at field sites; procedures for sanitizing tools, surfaces, and footwear; and clean water specifications. (Also in Spanish)
“Best Management Practices for Preventing Phytophthora Introduction and Spread: Trail Work, Construction, Soil Import” by Tedmund Swiecki and Elizabeth Bernhardt, Phytosphere Research, was written for the Golden Gate National Park Conservancy (January 2018). In 73 pages, it provides an introduction to Phytophthoras, a risk assessment model to determine which BMPs are needed under various conditions, as well as trail work, construction and soil import BMPs for use in sensitive habitat, known infested areas and park lands.
“Guidance for contaminated or sensitive sites” is for restoration sites that has been confirmed to contain an Phytophthora infestation or are an especially sensitive habitat. (Also in Spanish)
Guidelines for general construction in Spanish
“Holding nursery guidance” presents a set of practices to avoid contamination of nursery stock being held for planting at restoration sites.
“Guidance to reduce the risk of Phytophthora and other plant pathogen introductions to mitigation sites” (aka “Guidance for environmental regulators to reduce the risk of Phytophthoras and other plant pathogen introductions to restoration sites”) – this document is intended for professionals involved in regulating mitigation and restoration.
“Start clean and stay clean when visiting native plant habitats” – this two-page handout provides guidance on how to reduce contaminated mud and plant materials from being transported to native plant habitats.
“Come Clean Leave Clean” – This 3:11 minute video video from the California Invasive Plant Council was created in 2023 in collaboration with the Oakland Civicorps and Hip Hop for Change about site hygiene to reduce the spread of weed seeds and pathogens.

Phytophthora resources by species

Phytophthora tentaculata

Phytophthora tentaculata was initially detected in a native plant nursery causing a severe root and crown rot in sticky monkey flower, Diplacus aurantiacuss ubsp. aurantiacus (Scrophulariaceae) in 2012. Since then it has been detected in several nurseries and a few restoration sites where outplanted stock was found to be infected.

CDFA Plant host list for Phytophthora tentaculata, February 2016
USDA Phytophthora tentaculataÂ Pest Alert, February 2015
CDFA Nursery Advisory No. 01-2014Â Phytophthora tentaculata, November 2014

Rooney-Latham, Suzanne; Cheryl Blomquist; Ted Swiecki; Elizabeth Bernhardt. 2015. Phytophthora tentaculata. Forest Phytophthoras 5(1). doi10.5399/osu/fp.5.1.3727. A compendium of information on Phytophthora tentaculata that includes photographs of field symptoms, a table of known hosts and more, geared to a technical audience and containing many photomicrographs. (PDF for download)

Rooney-Latham, S., C. L. Blomquist, T. Swiecki, E. Bernhardt, and S. J. Frankel. 2015. First detection in the US: new plant pathogen, Phytophthora tentaculata, in native plant nurseries and restoration sites in California. Native Plants Journal 16:(1) 23-27. Abstract: Phytophthora tentaculata KrÃ¶ber & Marwitz (Pythiaceae) has been detected in several native plant nurseries in 4 California counties and in restoration sites on orange sticky monkey flower (Diplacus aurantiacus (W. Curtis) Jeps. subsp. aurantiacus [Scrophulariaceae]), toyon (Heteromeles arbutifolia (Lindl.) M. Roem. [Rosaceae]), coffeeberry (Frangula californica (Eschsch.) A. Gray [Rhamnaceae]), and sage (Salvia spp. L. [Lamiaceae]). These findings are the first detections of P. tentaculata in the US and the first ever on these host plants. Phytophthora species are a known problem in horticultural nurseries, but little attention has been placed on native plant or restoration nurseries. The potential for plant pathogens to be outplanted along with native plant nursery stock is very high, posing a threat to neighboring forests. http://npj.uwpress.org/content/16/1/23.abstract.

General Pathology Resources

Threats to oaks and other native plants from root-rotting Phytophthora species (handout by Dr. Tedmund J. Sweicki for the University of California Oak Health Virtual Workshop, April 2020)

Reid, A. (2006), Sampling and testing for plant pathogens. Department of Agriculture and Food, Western Australia, Perth. Bulletin 4683.

Resources for Regulators

Sample text from Santa Clara Valley Water District for General Project Conditions for Phytophthora and Phytophthoras and Nursery Stock. Posted January 2023.