How to recognize blister rust infection on whitebark pine

Abstract

White pine blister rust is a disease of five needle pines. The fungus (Cronartium ribicola) infects trees through the needles. It grows down the interior of the needle and into the stem, producing fusiform cankers. Susceptible white pines that are exposed to the disease become infected with one or more cankers. Most die, with just a few remaining canker-free. The disease is native to Eurasia. It was introduced into western North America in 1910 near Vancouver, BC. Foresters did not notice it until the autumn of 1921. Within 25 years the disease had expanded over most of the range of western white pine (Pinus monticola), whitebark pine (P. albicaulis), limber pine (P. flexilis) and sugar pine (P. lambertiana). The fungus needs two hosts. It is perennial in the inner bark of white pines, and annual on the leaves of currant and gooseberry bushes (Ribes spp.). The fungus moves from currants and gooseberries to pine by a very small spore, too small to be visible without a microscope. These spores usually move during the fall, or anytime during extended cool, moist conditions. The fungus moves from the pine to currants and gooseberries in the spring by another type of spore that is small, but can be seen with a hand lens. The first blister rust canker recorded on whitebark pine was in 1922 at the University of British Columbia arboretum. Infection of natural stands was soon observed. Foresters quickly realized that whitebark pine was several times more susceptible to blister rust than western white pine. Nonetheless, a few whitebark pine trees do not become infected. The following photos and descriptions of blister rust on whitebark pine (figs. 1 through 19) are to help identify the disease. Forest workers armed with this knowledge can find canker-free trees to start developing a new variety of whitebark pine that is resistant to blister rust. At times some resistance reactions can even be observed (figs. 18 and 19). Abstract—Color photographs show how white pine blister rust can be identified. In addition, the photographs show how pines resistant to the fungus could be identified. Such trees could be used to develop a new variety of whitebark pine that is resistant to blister rust.