Incidence and distribution of white pine blister rust in the high-elevation forests of California

In 2004-2006, a California-wide survey was conducted to evaluate the incidence and distribution of Cronartium ribicola, cause of white pine blister rust (WPBR), in the high-elevation white pine forests. WPBR occurrence varied considerably within and between regions, and little to no disease was found in the Basin and Range, eastern Sierra Nevada and Transverse and Peninsular mountain ranges. Field surveys revealed no evidence of rust on Great Basin bristlecone, limber or the southern subspecies of foxtail pine. Rust incidence for western white pine was highest in the North Coast region (42% of trees surveyed), followed by the Klamath (18%) and northern Sierra Nevada (14%). For whitebark pine, WPBR incidence averaged 24% in the northern Sierra Nevada; this was considerably greater than other regions where whitebark pine was found infected. Latitudinal trends in WPBR incidence (i.e. greater in the north) may correspond with earlier introductions of C. ribicola into these regions, reflecting the southward spread of the pathogen since its introduction from the north in the early 1900s and environmental conditions favourable for infection. Distance from inoculum sources and potentially host phenology may also be important factors influencing the distribution of C. ribicola.