No 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.

No abstract

No abstract

Widespread Phytophthora infections have been discovered in nursery stock used in California restoration plantings. In response, nursery Phytophthora best management practices (NPBMPs) designed to exclude Phytophthora from nursery plants were developed to address the need for clean planting stock in restoration projects. A pilot program to implement the systematic use of the NPBMPs, Accreditation to Improve Restoration (AIR), was developed and started in 2018. As of 2020, 13 northern California restoration nurseries have been evaluated and five have met all the program requirements. In 564 tests conducted over four years with a sensitive leachate baiting protocol, no Phytophthora was detected from over 20,000 nursery plants produced in compliance with the NPBMPs. In comparison, Phytophthora was detected in 25% of tests conducted on partially-compliant stock, and in 71% of tests from nurseries following few or no NPBMPs. The AIR pilot program has demonstrated that container stock free of detectable Phytophthora can be reliably produced by adhering to an integrated program of clean nursery production practices. To obtain Phytophthora-free plants for habitat restoration, informed clients were willing to pay increased costs required to produce NPBMP-compliant nursery stock.

Phytophthora pathogens are damaging native wildland vegetation including plants in restoration areas and botanic gardens. The infestations threaten some plants already designated as endangered and degrade high-value habitats. Pathogens are being introduced primarily via container plant nursery stock and, once established, they can spread to adjacent areas where plant species not previously exposed to pathogens may become infected. We review epidemics in California – caused by the sudden oak death pathogen Phytophthoraramorum Werres, De Cock & Man in ‘t Veld and the first USA detections of P. tentaculata Kröber & Marwitz, which occurred in native plant nurseries and restoration areas – as examples to illustrate these threats to conservation plantings.