USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Forest Disease Resistance, and Pathogen Evolution & Biogeography

The Institute of Forest Genetics disease resistance program is devoted to understanding genetic interactions between tree hosts and fungal pathogens, using both native and exotic pathogens as models for study. The disease resistance program is directed by Bro Kinloch, who works in collaboration with Det Vogler. Dr. Vogler also studies the evolution, systematics, biogeography, and landscape-level interactions of native and exotic pathogens (primarily rust fungi) with their plant hosts. In the same rust pathosystem, these can involve phylogenetically disparate hosts (e.g., white pine species and gooseberries or currants). The resistance program works in close collaboration with Region 5 and Region 6 Genetic Resources program.

The white pine/blister rust pathosystem

A disease native to Asia, white pine blister rust was introduced separately into both eastern and western North America early in the 20th century. In both cases, the vector was seedlings of native eastern white pine imported from European nurseries, where they had become infected. Blister rust had first appeared in Europe in the mid-nineteenth century, and within three decades had spread across the continent wherever the popular eastern white pine had been planted. The life cyle of blister rust includes an alternate host.

In western North America, the rust spread in serial jumps from a nursery near Vancouver, British Columbia, south through the Cascades and Sierra Nevada, and east to the northern Rocky Mountains. All native white pine are susceptible to damage. Economic impact has been most acute on the two large commercial species, western white pine and sugar pine, but ecological damage has been equally severe on high elevation species such as whitebark and limber pines in many places. More recently, an outbreak was recorded in the Sacramento Mountains of New Mexico on southwestern white pine. Infection is not always uniform; it often forms a mosaic pattern over the landscape, where patches of intense infection give way to areas of lighter or no infection.

Mechanisms and inheritance of resistance
A multi-year survey of western white pine species has identified major resistance genes to the white pine blister rust fungus (Cronartium ribicola) in sugar pine, western white pine, southwestern white pine, and limber pine. Virulence to at least two of these genes (in sugar pine and western white pine) was found to be highly specific, in a classic gene-for-gene relationship.

Fungal virulence
Studies of the population genetic structure of the blister rust pathogen (Cronartium ribicola) revealed that it was (a) highly outcrossing, (b) depauperate in genetic variation, and (c) had a metapopulation structure. Virulence in the pathogen is inherited cytoplasmically. Distribution and frequency of two virulence genes were mapped in the western U.S.

Deployment strategies for durable resistance
Field performance after 25 years of genetically highly-selected families of western US white pines is being evaluated. Strong mechanisms other than major gene resistance exist. mechanisms and efficacy of partial resistance (i.e., not conditioned by major genes, and not vulnerable to specific virulence) were described in sugar pine and western white pine, and their role in breeding program is being evaluated.

Evolution and biogeography of fungal pathogens within forested ecosystems
We are using molecular genetic, population genetic, mycological, and plant ecological approaches to understand the associations of fungal pathogens with their hosts and the roles of such pathogens within forested ecosystems. Information on how genes, organisms, and environments interact provides critical information for those who manage such ecosystems for sustainability and commodity production. Research priorities include the pine stem rust fungi (Cronartium and Peridermium species) and pitch canker disease of Monterey pine and associated coastal Californian Pinus species.

Biogeographic studies on pine stem rust fungi and their hosts

This research involves extensive field data and sample collection documenting distribution of pine stem rust fungi in the western U.S. on both pine and angiosperm hosts, with particular emphasis on white pine blister rust, pinyon blister rust, and western gall rust. Collections incorporate host and fungal tissue for DNA and biogeographic analyses of all partners in the pathosystems. One goal of these studies is to compare the range extension and host interactions of introduced exotic (white pine blister rust) and endemic (pinyon blister rust and western gall rust) pathogens.

Potential for cryptic speciation in heteroecious, autoecious, and facultatively autoecious pine stem rust fungi

Sample collection, geographic location, and genetic analysis of spore color-mutant autoecious gall rust fungi (infect pines only) around the periphery of the Great Basin where they form small, relict, genetically-isolated populations. In many cases, these organisms exist in isolated locations under constraints to range extension that remain unknown. As with more well-documented pine stem rusts, like the western gall rust fungus, these pathogens exhibit the ability to expand their geographic and host ranges rapidly, acting biologically like invasive exotics. Studies of these fungi hold promise for understanding the modes and trajectories of pathogens within rapidly-changing or threatened forested ecosystems.

Diversity within pine stem rust fungi of Mexico and Central America

Because both pine hosts and pine-infecting stem rust fungi extend as far southward as the Gulf Coast of Nicaragua, diversity and speciation within these organisms, as compared to their nearest relatives in the U.S. and Canada, is an important key to understanding of Cronartium and Peridermium species evolutionary history and pathogenic potential. Recent trips to Mexico and Central America have documented rusts similar to but possibly phylogenetically-divergent from North American taxa. DNA sequence analyses currently underway will be used to better understand speciation and diversity within these species.