Colorado’s oldest trees, bristlecone pines, are facing a serious threat to their existence. White pine blister rust, a disease caused by the fungus Cronartium ribicola, is spreading southward into Colorado. Hundreds of trees on national forest land and in Great Sand Dunes and Rocky Mountain National Parks are infected.
The disease originated in Asia, but was carried to Europe on ornamental plants brought home by travelers as souvenirs. The disease entered North America in 1910 through Vancouver, British Columbia, in infected pine seedlings imported to reforest areas clearcut by the lumber trade. It has spread across North America.
Only pines with five needles per bundle are susceptible. In Colorado, this includes whitebark pine, limber pine, western white pine and bristlecone pine. Further west it also includes sugar pine and foxtail pine.
Bristlecone pines are possibly the oldest living organisms on the planet. Although the Colorado species is not as long-lived as the Great Basin bristlecones in the White Mountains of California and Nevada, the oldest known tree in Colorado (on Mount Evans) has a 2,435-year tree ring record. Like bristlecones, limber pines inhabit harsh areas and contribute to the beauty of Colorado’s high country. Western white pine has great commercial value, being the pine of the lumber trade. Whitebark forests anchor mountain snowpack, preventing snow from blowing away or melting before spring runoff provides critical water for cities and agriculture. Whitebark pine nuts are an important high-calorie food source for grizzly bears. Whitebark cones do not open naturally, but the seeds are extracted and cached by Clark’s nutcrackers and squirrels. Grizzlies dig up these seeds and devour them. It is estimated that 50% of the whitebark pine forests in the Northern Rockies have succumbed to white pine blister rust.
The fungus has an elegantly elaborate life cycle. This is a simplified description: On susceptible pines the fungus forms lesions in the bark, growing outward each year. In the spring it produces spores in “blisters”—raised circular areas filled with spores that eventually burst and are spread long distances (sometimes hundreds of kilometers) by air currents. These spores can only infect plants of the species Ribes—currants and gooseberries—in which they cause minor leaf spotting. Eventually these infections produce spores that form on the underside of Ribes leaves. These spores are small and short-lived, and when dispersed in the air, travel only short distances. But these spores reinfect pines. And the cycle begins again.
It is important to note that there can be no pine-to-pine infection spread. This has led to elaborate schemes to eradicate Ribes species in areas where susceptible pines grow. Such efforts have been wildly expensive and have had limited success in the East, but were largely ineffective in the west. The attempt to control the disease in this manner has been abandoned.
There is little genetic resistance in the pines because this is an imported disease. More would be expected if the trees and the fungus had evolved in the same geographical area. Laboratory investigations into resistance have long been stymied by the fact that there is no way to grow the fungus successfully for experimentation purposes. At present the most promising efforts may be collecting seed from the small number of trees that show resistance (fewer than 5%) and using that seed to produce a new fungus-tolerant forest.
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