The western gall rust fungus (Endocronartium harknessii) induces globular, woody stem galls on several pine species throughout the West. The pictured galls infected a ponderosa pine (Pinus ponderosa) growing near the shore of Baum Lake (Lassen County CA).
The galls occur on stems and exposed roots. They continue to grow larger year after year. As the galls increase in size they damage the branches, needles (leaves) and cones beyond the gall until they kill the branch.
In late spring or early summer bright orange-yellow spores form on the surface of the gall. Over two or three weeks the spores are dispersed. The spores do not require an intermediate host. Although the spores can move directly from pine to pine, they do need specific humidity and weather conditions to germinate.
Although they do not cause the galls, insects occasionally invade the galls and kill the rust gall.
I plan to check these galls throughout the season in hopes of seeing the yellow spores.