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.