Western white pines (Pinus monticola) are five-needled pines that are native from southern British Columbia to southern California. In the northern parts of their range, including in western Washington, they can be found from sea level up to about 5,000 feet in elevation. As their range extends southward they are found at higher elevations. Western white pines are often found in sites with low-nutrient, gravelly soils, where they compete successfully with other native conifers such as Douglas-firs.

Unfortunately, these lovely trees have been under siege for over a hundred years from the white pine blister rust (Cronartium ribicola) a devastating fungal pathogen introduced from Asia. Our cool, wet weather provides ideal conditions for the pathogen to spread, and as a result we have lost up to ninety percent of our western white pines west of the Cascade Mountains. Where conditions are drier, the pathogen spreads more slowly, but still has decimated populations of susceptible pines. Western white pine is one of several white pine species native to North America, all of which are variously affected by white pine blister rust.

White pine blister rust was first observed harming pines in Russia in the 1850s, and by 1900 the pathogen was widespread throughout Europe. In the early 1900s white pine blister rust was introduced on the east coast of North America, and in 1921 infected currants were first identified in Vancouver, British Columbia. Further investigation determined that a single shipment of infected eastern white pine seedlings to a nursery near Vancouver had introduced the pathogen to the northwest in 1910. While the logging industry in the western United States was thriving at this time, nurseries were not yet established to replant in logged areas, and it was common to import seedlings from Europe. The discovery of the pathogen in 1921 sparked a frenzied search for infected plants, but the damage had already been done. In the decades that followed white pine blister rust made its way from British Columbia to Washington, Oregon, Idaho, California, and Colorado.

White pine blister rust has an interesting and complex life-cycle requiring two hosts, usually a currant or gooseberry (Ribes spp.), and a white pine. Although this pathogen can stress the currant or gooseberry host, it is generally not fatal in these plants. The full life-cycle of this rust includes five different life stages, and can take from three to six years to complete. Two stages of spores are wind-borne, facilitating the transfer between currant or gooseberry and pine hosts. These spores have been documented to travel over 200 miles from the nearest known infection, making containment all but impossible.

Early efforts to prevent the spread of white pine blister rust in North America focused on removal of currants and gooseberries from areas with white pines. This strategy proved both ecologically dubious and largely ineffective. Especially in the western United States and Canada, Ribes species are widespread and grow alongside western white pines in rugged, mountainous terrain. Many Ribes species are important nectar sources for hummingbirds, and others stabilize stream banks or steep rocky hillsides. By 1959 the US Forest Service had abandoned this strategy.

White pine blister rust is especially deadly in young trees, but infections in mature trees weakens the trees, making them more susceptible to other stressors like pine bark beetles. White pine blister rust causes cankers, leading to girdling of branches or even the whole trunk. Some individual western white pines are more resistant to infection of the rust, and breeding programs have taken advantage of these naturally resilient trees.

Family: Pinaceae

Genus: Pinus

Species: monticola

Origin: Western US and Canada

Height: Up to 200 feet tall

Location: On the west side of the Pinetum in the Washington Park Arboretum