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WSU Puyallup Ornamental Plant Pathology

Phytophthora (fy-toff-thor-uh)


Phytophthora is a group of microscopic organisms that generally harm plants (plant pathogens). More than 150 species have been collected and formally described, but between 200-500 species are estimated to exist. It is not known how many species are present in Washington, although nearly thirty species have been recovered from ecosystems in Oregon and Alaska.

Phytophthora sp. sporangia

The photo on the left shows empty sporangia growing on the edge of a Phytophthora colony in water.  Sporangia are structures that produce and release zoospores, the motile swimming spores.

You can imagine the  production of these microscopic structures and spores on the surface of a leaf during a rain event. As rain drips through the canopy or blows in the wind, Phytophthora spores can be spread to new hosts.

What are Phytophthora?

Phytophthora is a Greek term that generally translates as ‘plant destroyer’. The name is fitting for a few species that are truly plant destroyers. For example, Phytophthora ramorum has killed thousands of tanoak trees in California, P. cinnamomi can infect more than 5000 plant species, and P. infestans displaced many Irish communities during the Irish Potato Famine in the mid 1800s.

  • Phytophthora are microbes that harm plants (plant pathogens).
  • They are considered “water molds” and they are more closely related to brown algae than fungi.
  • Phytophthora are often spread via the movement of infected plants and contaminated soil.
  • Phytophthora species cause economically important plant diseases such as potato late blight and sudden oak death.
  • Phytophthora diseases on plants are identified by symptoms, microscopic examination and molecular testing.
  • Read more about Phytophthora and other Oomycetes in streams here.

Invasive Phytophthora species

Phytophthora species can become invasive after they have been introduced to new environments and become established on new hosts. Sudden Oak Death is an example of the capacity of a Phytophthora species to invade ecosystems in the western US.

Although federal and state agencies actively try to protect Washington’s industries and ecosystems through monitoring and management (e.g. regulations), the emergence of another invasive Phytophthora species is a major threat.

This is a particular concern to western Washington given the pressure from major trading hubs such as our ports.

Spread of Phytophthora

The spread of Phytophthora species throughout the globe has had detrimental consequences on economies and ecosystems. This result has inspired agencies such as USDA APHIS to implement strict regulations on material and industries linked to the spread. For example, APHIS confirmed the accidental shipment of Phytophthora ramorum on plants from Nurseries in Washington State and Canada in mid 2019. This event is a wake up call that Washington’s industries need additional support and capacity to manage Phytophthora diseases.

Monitoring for Phytophthora

Monitoring Washington’s environments and industries is critical to detect the invasion of a Phytophthora species early. The earlier an invasive species is detected after it is introduced to a new area, the more effective we can control and prevent its impacts.

Students and other volunteers can help by conducting early detection surveys as part of our Volunteer Stream Monitoring Program.

Stream monitoring programs are effective approaches to detect Phytophthora species in watersheds. This approach has been used widely to detect the spread of P. ramorum and focus control efforts, ultimately reducing the threats of this species to our industries and ecosystems.

Stream monitoring for Phytophthora

Management of Phytophthora

Much of our research is dedicated to helping Washington’s industries and land managers control Phytophthora diseases.

YouTube Videos about Phytophthora