Phytophthora Introductions in Restoration Areas: Responding to Protect California Native Flora from Human-Assisted Pathogen Spread

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.

Diversity of Phytophthora Species Detected in Disturbed and Undisturbed British Soils Using High-Throughput Sequencing Targeting ITS rRNA and COI mtDNA Regions

“…The spread of Phytophthora has been linked to human activities such as plant movement/trade and other pathways such as soil on shoes, machinery, and water [23][24][25][26][27]. The soil environment plays an integral role in the spread and establishment of Phytophthora pathogens, which can persist long term in soil in the form of resilient thick-walled spores.…”

Investigating the Role of Restoration Plantings in Introducing Disease—A Case Study Using Phytophthora

“…Few studies have investigated the frequency and severity of disease introductions resulting from supplementary planting outside of forestry plantation settings. Accidental Phytophthora introductions on planting material used for habitat restoration in three separate locations in California have been documented [21]. While the biosecurity mea-sures introduced at donor nurseries to prevent further introductions required significant changes to existing practices, the measures described to remediate sensitive, inaccessible habitats following disease introduction were significantly more onerous, costly and time-consuming [21].…”

“…Accidental Phytophthora introductions on planting material used for habitat restoration in three separate locations in California have been documented [21]. While the biosecurity mea-sures introduced at donor nurseries to prevent further introductions required significant changes to existing practices, the measures described to remediate sensitive, inaccessible habitats following disease introduction were significantly more onerous, costly and time-consuming [21]. Remediation is also less likely to succeed long-term compared to preventing initial introductions [62], which native plant nurseries in California showed can be achieved by implementing biosecurity protocols to maintain Phytophthora-free stock [19,58].…”

“…A further study of close to 4000 water, soil and root samples collected from 133 British plant nurseries revealed high infestations, with 50% of samples testing positive for Phytophthoras belonging to 63 different taxa [18]. High levels of Phytophthora infestation were also reported from container plant nurseries in Oregon [19] and southern California [20], some of which raise native plant stock to restore ecologically sensitive sites, and indeed disease outbreaks in three such sites in California originated from plants translocated from nurseries [21]. This demonstrates the potential for restoration plantings to act as an introductory pathway for disease.…”

Lignin nanoparticles containing essential oils for controlling Phytophthora cactorum diseases

“…There is a very high risk related to the introduction of Phytophthora species into new habitats via the movement of plants (Frankel et al, 2020; Santini et al, 2013). For instance, P .…”