Abstract
Over the past several years, plantings of California native plant nursery stock in restoration areas have become recognized as a pathway for invasive species introductions, in particular Phytophthora pathogens, including first in the U.S. detections (Phytophthora tentaculata, Phytophthora quercina), new taxa, new hybrid species, and dozens of other soilborne species. Restoration plantings may be conducted in high-value and limited habitats to sustain or re-establish rare plant populations. Once established, Phytophthora pathogens infest the site and are very difficult to eradicate or manage—they degrade the natural resources the plantings were intended to enhance. To respond to unintended Phytophthora introductions, vegetation ecologists took a variety of measures to prevent pathogen introduction and spread, including treating infested areas by solarization, suspending plantings, switching to direct seeding, applying stringent phytosanitation requirements on contracted nursery stock, and building their own nursery for clean plant production. These individual or collective actions, loosely coordinated by the Phytophthoras in Native Habitats Work Group ensued as demands intensified for protection from the inadvertent purchase of infected plants from commercial native plant nurseries. Regulation and management of the dozens of Phytophthora species and scores of plant hosts present a challenge to the state, county, and federal agriculture officials and to the ornamental and restoration nursery industries. To rebuild confidence in the health of restoration nursery stock and prevent further Phytophthora introductions, a voluntary, statewide accreditation pilot project is underway which, upon completion of validation, is planned for statewide implementation.
Highlights
California is home to exceptional botanical diversity
Defined as “the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed” [3], ecological restoration is most commonly conducted in response to human-caused habitat loss or degradation associated with infrastructure construction, invasive species introductions, and climate change
We present our experiences of restoration site Phytophthora introductions and subsequent remediation efforts to draw attention to this pathway for pathogen movement and the benefits of prevention, primarily achieved by the use of clean planting stock
Summary
California is home to exceptional botanical diversity. The California Floristic Province is recognized as a global biodiversity hot spot—an area with more than 1500 endemic plants and less than 30% of its original natural vegetation remaining [1,2]. Further sampling in restoration planting sites and native plant nurseries resulted in the detection of numerous new Phytophthora taxa and dozens of known Phytophthora species [10,11,12]. The risk that Phytophthora could be introduced via nursery-grown plants to GGNRA sites with rare and endangered plants became apparent in 2014 when land managers learned of Phytophthora infestations in SFPUC restoration sites. The BMPs, which are strictly adhered to, include sterilization of reused plant containers, heat treatment of potting soil, drainage improvements in growing areas, sterilization of footwear, tools, and equipment, and additional routine sanitation measures.
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