White pine blister rust, also known simply as blister rust (Cronartium ribicola), is a lethal stem rust fungus of five-needled or " white" pines. It is a non-native disease that is thought to have originated in Asia and come to the U.S. from Europe early in the 20th century. There are five susceptible species of pines in the US, but limber pine (Pinus flexilis) is the only pine species in Rocky Mountain National Park at risk. Because limber pines have not evolved with this disease, they have no resistance and die once infected.
Blister rust has a complex life history. Until quite recently scientists believed it spent part of its life in its alternate host Ribes (currants and gooseberries), but could not overwinter there. Spores from Ribes must infect pine needles in order to overwinter. Spores from pines cannot infect other pines, however. They must infect the alternate host. Thus Ribes and at least one species of five-needled pine must both be present for blister rust to survive.
In 1930, five western national parks- Rocky Mountain, Glacier, Grand Teton, Mount Ranier, and Yellowstone - declared "war" on the disease by trying to destroy all the Ribes plants in these parks. This campaign was intended to save the pines at the expense of the native Ribes species. It resulted in the destruction of uncounted numbers of native plants, the spraying of over half a million gallons of the herbicide 2-4-5T in national parks, and a huge effort by large crews of workers for four decades. In 1971 the effort was deemed to be ineffective and discontinued.
Recently researchers at the U.S. Forest Service's Rocky Mountain Research Station lab in Moscow, ID have found that sickletop lousewort (Pedicularis racemosa) and scarlet paintbrush (Castilleja miniata) can also be alternate hosts for blister rust. Because both species are widely distributed across the west, including Rocky Mountain National Park, these species, and perhaps others not yet identified as alternate hosts, could have provided refuge for the disease during the "Blister Rust Wars" even if Ribes had been obliterated from our landscape. Hopefully future management of blister rust will never again result in an attempt to sacrifice one or more native species to save another. In any case, these new findings will make controlling this destructive forest disease even more challenging in the future.