Pathogen Poses Threat to Habitat Restoration and Open Space Projects in California

By Isaac Swanson, Tanner Harris, and Kari Dupler

Phytophthora is a water-borne fungus transmitted through contact with infected soil, water, or leaves. Some Phytophthora species are relatively innocuous, but others–like Phytophthora ramorum, the cause of Sudden Oak Death–can be deadly to woody shrubs and trees and even to some grass and marsh species native to California. Phytophthora damages and sometimes kills plants by attacking plant tissues.

Figure 1: Tree die-off in a mixed conifer-tanoak forest infected by Phytophthora ramorum. Image: USDA Forest Service, n.d.

Sudden Oak Death has killed millions of oak trees in California in recently decades (Figure 1). Even when plants survive a Sudden Oak Death infection, effects from the infection can cause root and crown dieback, leaf discoloration, and stunted growth.  Over 50 species of Phytophthora have been found in nurseries in California, and many of these species have similar effects as P. ramorum.

Why is Phytophthora of concern for habitat restoration and open space projects? To start, plants infected by a Phytophthora tend to be much less vigorous than uninfected plants, so nursery plants free of Phytophthora will likely grow larger and more robust when planted in Phytophthora-free areas than infected plants will. Second, we would not want to introduce deadly pathogens into clean wildlands or restoration sites. Doing so would further degrade the habitat potential of the wildland or restoration site while wasting money invested in the original planting effort and subsequent efforts to clean up the site. Third, some recent restoration efforts unintentionally introduced Phytophthora into extremely rare plant communities, further imperiling already-endangered habitats. We want to ensure that we do no harm in our quest to do good. Moreover, it is expensive and nearly impossible to get rid of Phytophthora once a site is infected.

How does Phytophthora spread and what areas are at the most risk?

Figure 3: Distribution of Phytophthora ramorum in California. Image: Food and Agriculture Organization of the United Nations, 2016

Phytophthora presents the greatest risk to coastal areas from Monterey to Southern Oregon (Figure 3) due to its temperate, moist climate. Nursery stock is the primary pathway for the introduction of Phytophthora into wildlands and restoration sites. Recent studies have found that over sixty percent of nursery plants in California are compromised by aggressive strains of Phytophthora. Phytophthora quickly spreads among nursery plants when plant containers are not properly sanitized before reuse, when potting soil is not heat-treated or sterilized before use, when nurseries irrigate plants with water sources compromised by Phytophthora, and when container plants sit on soil infected with Phytophthora. The larger the plant container, the higher the risk that the plant will carry an aggressive strain of Phytophthora due to the length of time spent in the nursery and the risk of coming into contact with Phytophthora during the plant container upsizing process. Any restoration project using container plants–especially woody native plants from nurseries that do not follow clean nursery practices–is at high risk for infection of Phytophthora.

Projects that occur near Phytophthora-infected soil, water bodies like creeks and ponds, or host species such as California bay trees, are at particularly high risk as these are primary vectors for the spread of Phytophthora (a list of Phytophthora host species can be found here. Phytophthora can be transmitted via soil on vehicle tires and footwear, via irrigation with water from a Phytophthora-contaminated pond, or via infected leaves dispersed by wind or water.

Beyond restoration sites, other areas at high risk for Phytophthora contamination include forested areas, chaparral, areas near confirmed occurrences of Sudden Oak Death or other Phytophthora-related diseases, areas next to roads or trails, sites along Phytophthora-contaminated creeks or lakes, and sites with large numbers of California bay trees or other host species.

What can be done to prevent the spread of Phytophthora?

Strategies for preventing the spread of Phytophthora include the following:

  • Source plants from nurseries that use clean, sanitary nursery practices designed to prevent the transmission of Phytophthora. Such nursery practices should include routinely disinfecting shoes, potting soil, plastic containers, and equipment; growing plants on raised benches several feet off of the ground; and eliminating any areas with standing water. A detailed summary of best management practices for Phytophthora management can be found here.
  • Specify small plant container sizes, where possible, instead of large containers to reduce the amount of time plants remain in the nursery.
  • Grow container plants in an on-site nursery located away from bay trees, chaparral, forest, and sources of standing water or runoff.
  • Test container plants for Phytophthora before planting them in the field.
  • Install container plants in late summer or fall when the ground is drier.
  • Consider growing plants from locally-sourced seed instead of using container plants in sensitive plant communities or in habitats that support endangered plant species.
  • Remove dirt and sanitize vehicles, footwear, and tools (including wood chippers) with sanitizing agents like bleach, alcohol, quaternary ammonium compounds, or peroxides before entering project sites.
  • Import only dry straw wattles and gravel. Wet fiber rolls and gravel can host
  • Verify that water used for irrigation comes from a treated water source and is not stored in ponds.
  • Test for Phytophthora on high risk-sites before and after restoration to catch any Phytophthora-infected areas early on. It is important to test pre-restoration so that expensive plants don’t get installed in an area that already has
  • Be especially careful moving soil, water, and container plants during the rainy season to avoid contaminating plants, equipment, and other materials.
  • Avoid using imported wood chips from unknown or unreliable sources unless they are from a source known to be free of Phytophthora.

Figure 4: Sourcing plant material from nurseries that elevate container plants off the ground is a proven method for reducing the transmission of Phytophthora. Image: The Watershed Nursery, 2018

What to do in the event of a Phytophthora outbreak

If an aggressive Phytophthora is discovered on a project site, various techniques can mitigate the risk of the pathogen spreading, including:

  • In hot, sunny areas, moist heat treatment of contaminated soil via solarization can heat soil to a temperature sufficient to kill Phytophthora (Figure 5).
  • Fungicides, biological control agents like Trichoderma asperellum, and film-forming polymers (anti-transpirants) can be sprayed on infected plants to kill Phytophthora. Click here for further information.
  • Remove existing California bay trees or other host species.
  • Controlled burns can be effective in managing

Figure 5: Heating soil via solarization can kill Phytophthora in hot climates. Image: Plant Health International, 2018

Phytophthora containment is all about risk management. Phytophthora presents a serious risk to restoration sites and open space projects, particularly in coastal California, but following best management practices established by nursery and restoration specialists can minimize the risk of introducing aggressive strains of Phytophthora to project sites.




  • California Oak Mortality Task Force. (2019). Best Management Practices. [online] Available at: [Accessed 11 Jan. 2019].
  • California Oak Mortality Task Force. (2016). Guidelines to Minimize Phytophthora Contamination in Restoration Projects. [online] Available at: content/uploads/2016/04/Restoration_guidance_FINAL-111716.pdf [Accessed 11 Jan. 2019].
  • California Oak Mortality Task Force. (2019). Other Phytophthora species in California’s Native Habitats. [online] Available at: native-habitats/ [Accessed 11 Jan. 2019].
  • Peterson, E., Larson, E. and Parke, J. (2018). Management of foliar infection of rhododendron by Phytophthora ramorum with film-forming polymers and surfactants. [online] Available at: [Accessed 11 Jan. 2019].
  • Swiecki, T. and Bernhardt, E. (2013). A Reference Manual for Managing Sudden Oak Death in California. [online] Available at: [Accessed 11 Jan. 2019].