First Report of the White Pine Blister Rust Fungus, Cronartium ribicola, Infecting Ribes inerme in North-Central Utah

Cronartium ribicola Fisch. has not been found infecting any of the five-needle white pines (Pinus subgenus Strobus) in Utah, despite being established on both white pine and Ribes hosts in the other 10 western states, defined as those west of the 102° meridian. Krebill (1964) reported C. ribicola infecting limber pine 53 km north of the Utah border in the Wasatch Mountains of southern Idaho. Apparently, this is the closest to the northern border of Utah the pathogen had spread to that date. Thirty-six years after Krebill’s report, Smith and Hoffman (2000) reported finding no evidence of C. ribicola infecting either Ribes or Pinus hosts in their host range-wide sampling locations in Utah. However, in September 2005, while scouting for signs and symptoms of blister rust in Utah, coauthor B. W. Geils collected what appeared to be white pine blister rust-infected Ribes inerme Rydb. leaves at a location 257 km south of Krebill’s Idaho discovery. The original rust-infected R. inerme voucher specimens, acquisition no. KPK-1084, are archived in the Kamal Prasad Kaphle Fungal Herbarium in the Mirov Laboratory at our Institute of Forest Genetics. No five-needle pines were found at or near the new discovery site. The infected Ribes leaves were located in Duchesne Co., Utah, at 2,505 m elevation just east of Highway 191, 2.9 km south of Argyle Ridge (the southern boundary of the Ashley National Forest), at a road turnout near a tributary of Willow Creek, a perennial southward-flowing stream (39.86133°N, 110.76528°W). Since pinyon pine occurs nearby, Geils hypothesized that the pathogen might be the pinyon rust, C. occidentale Hedgc., Bethel, & N. Hunt, and was hesitant to conclude that he had found white pine blister rust in Utah, since the fungal fruiting structures on Ribes are virtually indistinguishable between C. occidentale and C. ribicola. We obtained a sequence of the rust fungus infecting the Utah R. inerme leaves, and can confirm that the pathogen is C. ribicola, not C. occidentale. DNA extracted from the scraped telia present on dried samples of Ribes leaves was amplified and sequenced using basidiomycete-specific primers ITS1F and ITS4B (Gardes and Bruns 1993). BLASTN analysis of the 783-bp sequence (GenBank accession no. KX574672) showed that it differs by only three nucleotides (0.38%) from the matching segment of a C. ribicola ITS reference sequence (DQ533975). Now that C. ribicola has been confirmed on Ribes in Utah, the pathogen might also be established there on pine, although there is no evidence of that at present. © The American Phytopathological Society.