Key Messages from 2019 Plant Pathogen Symposium

Symposium participants enjoyed a hands-on tour through the Presidio Native Plant Nursery, where science-based best practices are in place to prevent the spread of Phytophthora fungus.

Forestry Commission / Mick Biddle

This June, scientists and land managers from as far as Australia and New Zealand gathered at the Presidio’s Golden Gate Club for “Healthy Plants in a World with Phytophthora: the seventh Sudden Oak Death Science and Management Symposium."

This symposium series began as a way for scientists and managers to come together and address the massive devastation that Sudden Oak Death had caused across much of the West Coast, and it maintains that focus. Over the course of the day, we learned all about pathogens in the Phytophthora genus, which includes the disease causing agent behind Sudden Oak Death. Researchers are developing new tools to understand how and why Phytophthora species can damage ecosystems, and it is critical that nursery practitioners and land managers apply that understanding towards stewarding park landscapes. This event was a unique opportunity to share experience and knowledge across disciplines.

Although some disease in an ecosystem is normal, the ornamental plant trade that moves plants and soil across state and national borders are introducing new species that may have the ability to invade and cause more damage. We don't know the long-term outcomes of these species. In the case of Phytophthora ramorum or Sudden Oak Death, the exotic fungus was particularly deadly to California native oaks and tanoaks. Local scientists and land managers are looking for ways to prevent new diseases from becoming established and creating similar devastation.

Many researchers have focused on the source of the problem: where and how are new Phytophthora species introduced into new areas? Native Phytophthora species are present at low levels in undisturbed park wildlands, but the data is showing that planting infected plant stock from nurseries is a major way that new species are introduced into landscapes. Furthermore, there's evidence that Phytophthora strains growing in captive facilities tend to spread faster and be more deadly. Could it be that out-planting natives with the aim of restoring degraded habitat is doing more harm than good?

Fortunately, scientists, land managers, and nursery practitioners are becoming savvy to the problem and are taking steps to make sure that restoration efforts continue to be a force for reviving our irreplaceable park ecosystems. One equation to cut risk of habitat restoration efforts? Ensuring plant stock is clean, and continuing to monitor Phytophthora on the landscape.

Several California native plant nurseries, including those run by the Golden Gate National Parks Conservancy, are following science-based best management practices to stop the spread of Phytophthora. New accreditation programs offer opportunities for practitioners to level up their knowledge, and fungicide alternatives like solar treatment are also gaining traction. Yet it’s taking time for other nurseries to follow suit. Many commercial nurseries are still selling infected plant stock, shifting the responsibility onto consumers to test their purchases before planting them. We have a ways to go towards getting the economy, consumer society, and the law to catch up with the science around Phytophthora.

In the meantime, researchers are building an understanding of how Phytophthoras are moving through California landscapes. Because disturbance and new planting can spread pathogens, knowing which areas may be particularly vulnerable to infection is critical. Certain vegetation types and other ecological factors are associated with higher infection and spread rates. For example, pathogens can spread through waterways and via animal vectors, like grazing cattle or wild pigs. Using these risk factors along with vegetation maps and data from different park lands, researchers hope to identify potential "Phytophthora hotspots" to inform park management and plan strategic habitat restoration.

A third solution thread emerged at the symposium: communication. Across disciplines, across jurisdictions—it doesn’t matter what incredible work we’re doing if we’re not sharing it with those who are out in the field, standing at the front of a boardroom, or stewarding the land across the road.

Our struggle with Phytophthora is a story that tells many stories; it is a poster child for larger, systemic global trade issues. Voices from this symposium echoed that curiosity and collaboration are paving a path forward.

For more information on the Seventh Sudden Oak Death Science and Management Symposium, please visit the University of California Division of Agriculture and Natural Resources website.