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White Pine Blister Rust
White pine blister rust is one of the most serious diseases affecting American forests. Stands of white pines can be easily devastated by the disease, which is caused by the fungus Cronartium ribicola.
The pathogen was introduced to the United States via trees imported from Europe in the late 1800s and to Canada a few years later. Europe was not the natural home of this fungus either, however, but spread there from Asia long ago. In its native home, Cronartium ribicola is not such a serious problem because the pine trees there evolved with the fungus, those varieties with the best resistance to pine blister rust living to produce trees genetically similar to them. Due to the relatively recent introduction of the fungus to North America, such natural selection has not yet taken place, so there is no stable balance between host and pathogen.
Cronartium ribicola exhibits a very complex lifecycle that requires as many as four years and a second host to be completed. The other host for the fungus may be any of several different types of currants or gooseberries. Accordingly, if these plants grow in close proximity to pines, serious timber losses can be incurred. In an attempt to help stem the spread of white pine blister rust, many states passed laws over the course of the twentieth century prohibiting or restricting the cultivation of certain species of the plants, though they have since been repealed in some areas.
The earliest sign that a pine tree has become infected with Cronartium ribicola is usually the presence of small yellow or red spots on pine needles, but these are not easy to observe. Oftentimes the infection is not conspicuous until one or two years after the initial infection when perennial cankers begin to swell from the branches. The cankers slowly work their way around the circumference of the branch and succeed in killing it when girdling is complete.
Additional Confocal Images of White Pine Blister Rust
White Pine Blister Rust at High Magnification - Cankers on branches located close to the stem of a tree with white pine blister rust cause significant damage by spreading to the trunk. Infected trees viewed from a distance may appear stunted or yellowed, and dead branches or dead tops may be noticeable
Nathan S. Claxton, Shannon H. Neaves, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.