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University researchers are working to produce white pine seedlings resistant to the destructive blister rust fungus.
Getting the red out of white pines
University foresters are in a race to save the majestic Eastern white pine from a deadly fungus
By Nina Shepherd and Deane Morrison
Aug. 29, 2006
In the cool understory of the University of Minnesota's Cloquet Forestry Center, scientists are in a race against time to save Minnesota's most iconic tree, the magnificent Eastern white pine. Found in old growth forests throughout the state and along the North Shore, the towering timber with its feathery boughs is a favorite nesting site for the bald eagle. But the same towering heights and straight trunks that make the white pine picture perfect also made it irresistible to loggers in the early 1900s. Today the white pine faces a worse threat from a fungus called blister rust, a disease imported from Europe on seedlings in the early 1900's. Like something out of a science fiction novel, blister rust needs two hosts to survive: gooseberry bushes and white pines. Gooseberry leaves harbor the fungus until it releases spores into the air. The spores settle on white pine seedlings and enter via the needles, causing an oozing, rust-colored infection that eventually chokes the tree and prevents it from absorbing water. Infected branches and doomed trees turn a bright red that makes them stand out in the forest like burning flags. In the University's largest outdoor laboratory, the Cloquet Forestry Center, Andrew David and his research team are on the front lines of the battle to save the white pine. David, an associate professor in the department of forest resources at the University's North Central Research and Outreach Center in Grand Rapids, runs a white pine breeding program through the Minnesota Tree Improvement Cooperative (MTIC). The team's work rests on the old-fashioned horticultural technique of grafting. The team takes advantage of healthy mature trees that grow along the North Shore of Lake Superior and that have been regularly exposed to blister rust. Twigs from these healthy trees are grafted onto stems of white pine seedlings grown in Cloquet. The resulting seedlings are--for breeding purposes, at least--genetically identical to their North Shore parents. The grafting is thus, in effect, a way of transporting trees from the North Shore to Cloquet.
"We've given up trying to find a super tree," says David. "Rather, we're looking for trees with increased resistance with the ultimate goal of producing seeds."The grown-up seedlings become part of the Cloquet Forestry Center's "breeding arboretum," which now has more than 400 trees in various stages of growth. David's team cross-pollinates these trees, harvests dozens of seeds from each parent and transports them to greenhouses on the St. Paul campus. There the seedlings are raised and vigorously tested for resistance to rust, using a screening test developed in large measure by plant pathology professor Robert Blanchette. By looking at which seedlings survive the rust and which don't, the researchers can identify superior parents. Those trees will eventually be grafted to create a "seed orchard" containing only highly resistant trees, which will interbreed. Their seeds will be used in reforestation efforts throughout the state. "We've given up trying to find a super tree," says David. "Rather, we're looking for trees with increased resistance with the ultimate goal of producing seeds." If a parent tree is resistant to the rust, then a large proportion of its offspring--perhaps 75 ot 80 percent--will not die from rust if exposed, says Bob Stine, a professional forester and coordinator of the Cloquet Forestry Center. (A "super tree," or tree with immunity to rust, would produce offspring that all survive rust.) Trees that resist the rust must have not just one gene but a combination of genes that together confer resistance. The program is the quintessential long-term project, one that can only be completed by foresters of the future. One reason is that the white pines take two growing seasons to produce cones. For example, a tree that was pollinated in spring 2006 will not produce seeds until fall 2007. Also, the goal is much bigger than finding one or a few resistant trees and populating our forests with just their offspring. "It doesn't do a lot of good to have just a few genetic varieties," says Carrie Pike, manager of MTIC. "We want genetic diversity. We want breadth in the genetic pool." Planting many trees that share the same genetic makeup may allow them all to resist blister rust, but it could also leave them vulnerable to some other disease or parasite that may wipe them out. "Through various breeding schemes we can tell which combinations of trees yield offspring with the right combination of genes that can be passed on reliably from generation to generation," says Stine.