When walking through the woods, have you have noticed odd growth forms on the branches and stems of lodgepole pine trees? Chances are you’re looking at western gall rust, a native disease of pine trees caused by the fungus Peridermium harknessii. Unlike other rust fungi, Peridermium harknessii does not need an alternate host to complete its life cycle and infection occurs directly from pine to pine. While most two and three-needle pines are considered hosts, ponderosa pine and lodgepole pine are the most susceptible to infection locally. [1]
As the name implies, galls (abnormal swelling) are formed on the branches and stems of infected hosts. Branch galls are usually round or spherical in shape while stem galls appear as flat or sunken diamond shapes often with concentric ridges (hip cankers). In the spring, watch for white to orange pustules (aecia) full of yellow-orange spores (aeciosprores) which form in bark cracks on galls [1]. As the aecia rupture, spores are released and dispersed by the wind to infect susceptible hosts.
New infections typically occur on the current year’s shoots or needles and can cause branch mortality within a few years. Infected seedlings are highly susceptible to early mortality due to the gall fully encompassing the stem (girdling). On more mature trees, galls rarely girdle the main stem but can cause significant structural defects resulting in weakened trees that often die from snapping off at the site of the stem infection.
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