White Pine Blister Rust (Cronartium ribicola) is an invasive fungal species that was accidentally introduced into N. America in the early part of the 20th century. The disease can cause serious damage to all the American white pine species such as; Western White Pine, Sugar Pine, Limber Pine, and Whitebark Pine. Originating in Asia the pathogen, has little or no effect on their local pines because of co-evolution between the fungus and Asian pines. Blister Rust in parts of northern Montana has killed off entire stands of Western White Pine and Whitebark Pine, both of which are extremely important to the habitats and food sources for several species including the Clark's Nutcracker and the Grizzly Bear. The Forest Service has been working for several years on genetic improvement projects to make more resistant trees to this particular disease but that information is for a another posting.
The pathogen itself C.ribicola is heterecious, which means that it needs two host species to complete its life cycle; an aecial host and a telial host. Aecial hosts are pine trees while telial hosts are those plants found in the currant or Ribes families. So what happens is the disease gets passed from pine tree to currant bush and back again... over and over. One idea to get rid of the fungi was to eradicate all naturally growing currants in our pine stands but it was an impractical theory in practice.
The earliest identifier of C. ribicola infection is flagging, or patches of needles on the limbs turning a flaming red-orange color. There are other pine diseases that can cause this same "symptom" so it is often overlooked. The disease is usually first confirmed by finding cankers on infected branches. These cankers usually are a swollen and cracked portion of the limb that show a yellow orange substance between the cracks. If the infection is just limited to one or two limbs, those limbs can be pruned and there is hope for the tree itself, but if the canker if found on the main stem of the tree there is little help for it. Within 1-2 years the entire tree will turn bright red as the bole dies. Once infected the tree is also prone to other pest attacks as well. Because this agent passes so quickly between the two host species it takes very little time for an entire stand to be wiped out.