Western gall rust

Western gall rust, also known as pine-pine gall rust, is a fungal disease of pine trees. It is caused by Cronartium harknessii (formerly known as Endocronartium harknessii or Peridermium harknessii (describing the aecial phase under the now-superseded system of dual nomenclature), an autoecious, endocyclic, rust fungus that grows in the vascular cambium of the host. The disease is found on pine trees (Pinus spp.) with two or three needles, such as ponderosa pine, jack pine and scots pine. It is very similar to pine-oak gall rust, but its second host is another Pinus species. The fungal infection results in gall formation on branches or trunks of infected hosts. Gall formation is typically not detrimental to old trees, but has been known to kill younger, less stable saplings. Galls can vary from small growths on branch extremities to grapefruit-sized galls on trunks.