Nursery Plants as a Pathway for Plant Pathogen Invasion Precautions are needed to protect our restoration investments

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Summer 2016 Ecesis, Volume 26, Issue 2

Over the past several years, numerous species of the pathogen Phytophthora (pronounced Fie-TOF-ther-uh) — notorious agricultural, horticultural, and forest plant pathogens — have been detected in California native plant nurseries and restoration sites. For example, the sudden oak death pathogen (P. ramorum), introduced to California on ornamental nursery stock, has killed millions of tanoak (Notholithocarpus densiflorus) and coast live oak (Quercus agrifolia) along the California coast over the past two decades. P. infestans, cause of the Irish potato famine, continues to hinder potato and tomato production. 

Of particular concern is Phytophthora tentaculata — a species that had never been found in the US prior to 2012 — which has been detected in eight California native plant nurseries and on outplanted nursery stock in four restoration areas. To date, P. tentaculata has been identified on several species of California native plants, with sticky monkey flower (Diplacus aurantiacus) observed as the most susceptible (Table 1). P. tentaculata’s complete host range is not yet completely known.

With the broad range of plants susceptible to Phytophthora and other plant pathogens, there is the potential in restoration activities to inadvertently introduce Phytophthora-infected nursery stock into sensitive habitats, setting up a direct pathway for pathogen introduction and spread, and destroying the ecological values that restoration is trying to enhance. However, not all Phytophthora species cause plant diseases — some are adapted to live in water and pose low risk to plants (i.e., P. gonapodyides). Determining the risk of particular Phytophthora species is difficult, so there is much uncertainty as to the threat from P. tentaculata and the more than 20 other Phytophthora species that have been identified over the past 2 years on plants from California native plant nurseries. Phytophthoras are not unique to native plant nurseries; they are also frequently found in ornamental nurseries, agricultural fields, and wildlands.

The impacts of these pathogens in the native environment are uncertain, but the potential ramifications are wide-ranging and may be serious, including widespread plant mortality — as we have seen with P. ramorum — and regulatory quarantine measures affecting the entire state. Several water departments and land management agencies have taken a precautionary approach towards detections of infested plants outplanted in their restoration sites or on nursery stock being grown for their use. In 2015-16, managers suspended plantings, cancelled orders, or invested millions of dollars in solarization and other treatments to clean up contaminated sites. But reduced planting is not an ideal long-term solution to Phytophthora prevention since many of the benefits of restoration are lost when planting is avoided. Native plant nursery stock can be safely utilized by adopting a systems approach — by looking at the entire restoration process from design, seed collection, nursery propagation, through outplanting — to determine how the pathogens are being introduced to new areas, and then improve sanitation. Phytophthoras cannot be totally eliminated but they can be managed so the potential environmental harm is much lower than the real benefits of plantings. 

Efforts are underway to prevent pathogen introduction and spread by implementing Best Management Practices (BMPs) for native plant nurseries and restoration projects. There are simple, but effective practices that native plant nurseries and restoration practitioners can implement to minimize the risk of introducing infected plants into their habitat restoration projects. Utilizing clean pots and tools for planting is paramount. Elevating plants off the ground and installing footbaths into sensitive areas, such as propagation zones, are a few examples of BMPs directed at safeguarding product for field planting. Sanitation is the cornerstone of an effective systems approach and must be the focus in each nursery production step. 

The Phytophthoras in Native Habitats Work Group is bringing all aspects of the problem together to coordinate a comprehensive, unified program of management, monitoring, research, education, and policy to minimize the spread of Phytophthora pathogens. For more information see www.calphytos.org. General information on forest Phytophthora species may be found at http://forestphytophthoras.org and best management practices and other information on P. ramorum is at www.suddenoakdeath.org. The Work Group needs your ideas and observations to prevent unintentional pathogen introductions into high-value habitats. To get involved, contact Janice Alexander, UC Cooperative Extension, Marin County at jalexander@ucanr.edu. — by Susan J. Frankel (USDA Forest Service, Pacific Southwest Research Station), Kathy Kosta (California Department of Food and Agriculture), and Karen Suslow (National Ornamentals Research Site at Dominican University)