Cronartium ribicola: White Pine Blister Rust
The following is from Scharpf, Robert F. (tech. coord.) 1993. Diseases of Pacific Coast conifers, U.S. Department of Agriculture Handbook 521 (p.85).
"White pine blister rust, caused by the rust fungus Cronartium ribicola, is a textbook example of a heteroecious rust fungus. Its life cycle, symptoms, and climatic requirements are typical of most native western rust fungi.
"Hosts—C. ribicola can infect nearly all white pines and is restricted to this group. Its native North American hosts are eastern white pine (Pinus strobus), western white pine (P. monticola), sugar pine (P. lambertiana), limber pine (P. flexilis), whitebark pine (P. albicaulis), bristlecone pine (P. aristata and P. longaeva), foxtail pine (P. balfouriana), and Mexican white pine (P. strobiformis). In addition, it infects all species of the genus Ribes, its alternate host.
"Distribution and damage—The disease was introduced into North America in the early 1900's on seedlings of eastern white pine grown in Europe. It was not native to Europe, however, but is believed to have been introduced into that continent from Asia. Its introduction to North America resulted in one of our most serious outbreaks on conifers. White pine blister rust is now widely found in northeastern United States, the Lake States, and the West. In was introduced into British Columbia in 1910 and has spread from there throughout most white pine regions of Washington, Idaho, Montana, Wyoming, Oregon, and California.
"In North America, white pine blister rust has caused more damage and costs more to control than any other conifer disease. Since the 1920's, millions of dollars have been spent on the eradication of the alternate host, Ribes, and thousands of white pine stands have been severely damaged. In the western United States and Canada, some stands have been completely destroyed. When the main stem of a tree is invaded, death is only a question of time."
"It is a tribute to Northwestern foresters of the turn of the century that they recognized the importance of white pine to the ecosystem and the gargantuan threat of the blister rust. Would those Victorian-era foresters have guessed we would still be fighting the blister rust today?
"Researchers and managers determined that getting rid of Ribes - currant and gooseberry shrubs - would help prevent the spread of the rust to western white pine. Eradication of alternate host species, those surrounding plants that first contract a disease and then spread it to the important crops around them, was already being used against the black stem rust that afflicted wheat. In that case the alternate host was both the cultivated and wild barberry species. To this day, the U.S. Bureau of Plant Quarantine regulates cultivation of barberry. Against this historical and scientific background were played out the first efforts to control blister rust infection of western white pine trees.
"Canada's West Coast at Point Gray first saw the rust in 1910. From there, it radiated east to infect white pines in Washington State's Puget Sound region and Interior British Columbia by 1922. The infection reached eastern Washington and then Idaho by 1923. Because host eradication had worked with pines in the Eastern United States, and scientists were similarly fighting black stem rust successfully in wheat, the natural choice for curing western white pine of blister rust was eradication of host shrubs. But the eradication effort in the West would be different, shaped by the magnitude of the problem and by human events, specifically the Great Depression of the 1930's. The Depression released thousands of job-searchers who would become the labor force making such a huge eradication attempt feasible.
"Starting with an annual expenditure of $300,000 in 1932, the western rust control effort grew to $1.2 million in Public Works Administration (PWA) dollars. In 1934 the eradication effort received an infusion of PWA and Civilian Conservation Corps (CCC) workers. These programs, together with regular appropriations, put 13,000 men to work in the "War Against Ribes." By 1935 the CCC in the Inland Northwest alone employed 7,000 men. From 1930 to 1946, a work force averaging 2,500 men annually would dig, bulldoze, and spray Ribes shrubs on over 2.5 million acres of the 5 million-acre Inland Northwest white pine forest. All told they destroyed 444 million Ribes plants.
"Ribes along streams were treated differently from Ribes elsewhere in the forest. About 5 percent, or 36,000 acres, of these sensitive areas were treated by hand and sown with 1.8 million pounds of sodium chlorate (50 pounds per acre). Although the effort was labor-intensive, scientists now know that the number of bushes eradicated in those years still left 100 times the density that would have secured safe conditions for growing susceptible western white pine.
"The second half of the eradication story would unfold over the subsequent 20 years, from 1947 to 1968. Some 20,000 man-years of effort (1,000 men per year) removed 70 million plants from 900,000 acres. By 1949 the eradication program was ready to mount a full-scale stream and upland assault armed with a new weapon: the plant hormone herbicides 2-4-D and 2-4-5-T . For the first time a tool was available that would kill all species of Ribes and could be transported by backpack across the forest landscape. From 1949 to 1966 workers sprayed 41,000 acres of stream and forest with 13.3 million gallons of herbicide. This reduced Ribes populations to about seven bushes per acre, 10 times the efficiency of the previous eradication, but still 10 times away from what it needed to be. In the end, the eradication program had impacted almost half the 2.5 million acres originally targeted.
"Why was the war on Ribes so difficult? Understanding the shrub species may provide an answer. Sometimes the worst adversary is the one whose habits are just like yours. Like white pine, Ribes shrubs (currant and gooseberry) grow along streams, in moist open forests, in drier forest woodlands, and on subalpine woodlands. Almost all forest types that are open and contain deciduous shrubs contain Ribes and the possibility of pinekilling blister rust. The two species most commonly acting as alternate host for the rust (Cronartium ribicola) are Ribes lacustre and Ribes viscosissimum. They are found on the best growing sites for western white pine. And like the white pine, Ribes shrubs regenerate well after forest fire.
"Part of the reason the Ribes survived the CCC's herbicide attacks during the 1930's is that it is well adapted to long intervals between forest fires. Fire kills the trees and provides a chance for Ribes to regenerate from seeds placed by the previous generation - seeds used to waiting a long time for an opportunity to grow. The mother plants also regenerated after wildfire, a wait of perhaps 300 years.
"The small Ribes seeds have a hard seed coat and over time are buried by the litter of trees and shrubs that make up forest soil-building processes. One plant can produce many thousands of seeds. Not all seeds survive, but after the next fire or other disturbance such as logging, wind, or ice storm, the seeds germinate and the young plants quickly grow to sexual maturity within 7 years, producing more seed. Once established, Ribes grow to about 6 feet high and about the same width. If topped, the shrub sprouts new, quick-growing shoots from latent buds at the base of the plant. However, the tenacious plant that can live for 50 years or more may not be as fortunate as it appears. The forest itself also begins to grow after fire or disturbance. As soon as the trees outgrow the shrubs and produce shade, Ribes parent shrubs die. Mostly they leave only their seed to wait in the shady forest for the next fire.
"Although the war on Ribes was sound in concept, it needed to reduce a hazard so vast that success was virtually unattainable. That the forest professionals of the time would make such a massive attempt is powerful testimony to their understanding of the importance of western white pine to its ecosystem - and to American culture. It is also powerful testimony that society may need to rethink the prevailing attitude that humans can control nature."
White Pine Blister Rust. Includes some helpful illustrations and links.
Last Modified 2011-05-19