Blister Rust

The infamous white pine blister rust, caused by Cronartium ribicola and having members of the Ribes genus as alternate hosts, is not as serious in the Southern Appalachians as in woodlands of the North. In the South, diseased trees are seldom found below 3500 feet, and few stands of white pine occur above that elevation. On the other hand, Ribes species, the gooseberries and currants, grow mostly at higher elevations, particularly in narrow belts along the main mountain ranges. In the geographic range of Ribes, infection of white pine is more common on moist sites and in dense, unthinned stands. Under these conditions, micro-climate is optimum for spore infection. Open, park-like stands have higher air temperatures, more air circulation, and lower relative humidity, the combination of which is not conducive to sporulation. Ribes seeds, remaining viable in the soil for many years until logging or fire allow germination, produce a brushy vegetative cover unless shaded by hardwood trees.

The life cycle of the rust involves the production of three kinds of spores. Spores from cankers on pines are released in early spring, traveling perhaps hundreds of miles to infect Ribes plants. A little later, another kind of spore is released from the infection on the currant or gooseberry foliage to spread the rust to other Ribes bushes. Now, a second fungal stage on the broadleaf shrub appears. From this infection, still another kind of spore spreads the disease, usually not more than a few hundred feet, back to the pines. These spores, however, may ride with the wind for a mile.

In a month or two after spore dispersal, small discolored spots appear on white pine needles. The next year, the tree's bark is yellow or orange at the base of the infected needle bundle. As the fungus grows, the bark swells, soon exuding light-brown gummy drops from the swollen site. Then, in the following spring, the orange spores in the blisters push through the bark at the site where the resin was exuded the previous summer to reinfest Ribes plants. Soon the bark in the blistered area becomes rough and dark, and a spindle-shaped canker develops. "Flags" of copper-colored needles in the crowns signal the presence of the disease.4

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