Cataloging Phytophthora Species of Agriculture, Forests, Horticulture, and Restoration Outplantings in California, U.S.A.: A Sequence-Based Meta-Analysis.

California contains a diverse flora, and knowledge of the pathogens that threaten those plants is essential to managing their long-term health. To better understand threats to California plant health, a meta-analysis of Phytophthora detections within the state was conducted using publicly available sequences as a primary source of data rather than published records. Accessions of internal transcribed spacer (ITS) ribosomal DNA were cataloged from 800 Californian Phytophthora isolates, analyzed, and determined to correspond to 80 taxa, including several phylogenetically distinct provisional species. A number of Phytophthora taxa not previously reported from California were identified, including 20 described species. Pathways of introduction and spread were analyzed by categorizing isolates' origins, grouped by land-use: (i) agriculture, (ii) forests and other natural ecosystems, (iii) horticulture and nurseries, or (iv) restoration outplantings. The pooled Phytophthora metacommunities of the restoration outplantings and horticulture land-use categories were the most similar, whereas the communities pooled from forests and agriculture were least similar. Phytophthora cactorum, P. pini, P. pseudocryptogea, and P. syringae were identified in all four land-use categories, while 13 species were found in three. P. gonapodyides was the most common species by number of ITS accessions and exhibited the greatest diversity of ITS haplotypes. P. cactorum, P. ramorum, and P. nicotianae were associated with the greatest number of host genera. In this analysis, the Phytophthora spp. most prevalent in California differ from those compiled from the scientific literature.

Promise and Pitfalls of Endemic Resistance for Cultural Resources Threatened by Phytophthora ramorum.

Invasive forest pathogens can harm cultural, economic, and ecological resources. Here, we demonstrate the potential of endemic tree pathogen resistance in forest disease management using Phytophthora ramorum, cause of sudden oak death, in the context of management of tanoak (Notholithocarpus densiflorus), an ecologically unique and highly valued tree within Native American communities of northern California and southern Oregon in the United States. We surveyed resistance to P. ramorum on the Hoopa Valley Indian Reservation and Yurok Indian Reservation in a set of study sites with variable management intensities. Variation in resistance was found at all sites with similar mean and variation across stands, and resistance tended to have a random spatial distribution within stands but was not associated with previous stand management (thinning or prescribed fire) or structural characteristics such as tree density, basal area, or pairwise relatedness among study trees. These results did not suggest host, genetic, management, or environment interactions that could be easily leveraged into treatments to increase the prevalence of resistant trees. We applied epidemiological models to assess the potential application of endemic resistance in this system and to examine our assumption that in planta differences in lesion size-our measure of resistance-reflect linkages between mortality and transmission (resistance) versus reduced mortality with no change in transmission (tolerance). This assumption strongly influenced infection dynamics but changes in host populations-our conservation focus-was dependent on community-level variation in transmission. For P. ramorum, slowing mortality rates (whether by resistance or tolerance) conserves host resources when a second source of inoculum is present; these results are likely generalizable to pathogens with a broader host range. However, when the focal host is the sole source of inoculum, increasing tolerant individuals led to the greatest stand-level pathogen accumulation in our model. When seeking to use variation in mortality rates to affect conservation strategies, it is important to understand how these traits are linked with transmission because tolerance will be more useful for management in mixed-host stands that are already invaded, compared with single-host stands with low or no pathogen presence, where resistance will have the greatest conservation benefits.

Standardizing the nomenclature for clonal lineages of the sudden oak death pathogen, Phytophthora ramorum.

Phytophthora ramorum, the causal agent of sudden oak death and ramorum blight, is known to exist as three distinct clonal lineages which can only be distinguished by performing molecular marker-based analyses. However, in the recent literature there exists no consensus on naming of these lineages. Here we propose a system for naming clonal lineages of P. ramorum based on a consensus established by the P. ramorum research community. Clonal lineages are named with a two letter identifier for the continent on which they were first found (e.g., NA = North America; EU = Europe) followed by a number indicating order of appearance. Clonal lineages known to date are designated NA1 (mating type: A2; distribution: North America; environment: forest and nurseries), NA2 (A2; North America; nurseries), and EU1 (predominantly A1, rarely A2; Europe and North America; nurseries and gardens). It is expected that novel lineages or new variants within the existing three clonal lineages could in time emerge.

Economic impacts of non-native forest insects in the continental United States.

Reliable estimates of the impacts and costs of biological invasions are critical to developing credible management, trade and regulatory policies. Worldwide, forests and urban trees provide important ecosystem services as well as economic and social benefits, but are threatened by non-native insects. More than 450 non-native forest insects are established in the United States but estimates of broad-scale economic impacts associated with these species are largely unavailable. We developed a novel modeling approach that maximizes the use of available data, accounts for multiple sources of uncertainty, and provides cost estimates for three major feeding guilds of non-native forest insects. For each guild, we calculated the economic damages for five cost categories and we estimated the probability of future introductions of damaging pests. We found that costs are largely borne by homeowners and municipal governments. Wood- and phloem-boring insects are anticipated to cause the largest economic impacts by annually inducing nearly $1.7 billion in local government expenditures and approximately $830 million in lost residential property values. Given observations of new species, there is a 32% chance that another highly destructive borer species will invade the U.S. in the next 10 years. Our damage estimates provide a crucial but previously missing component of cost-benefit analyses to evaluate policies and management options intended to reduce species introductions. The modeling approach we developed is highly flexible and could be similarly employed to estimate damages in other countries or natural resource sectors.