Artemisia tridentata Nutt. (Asteraceae), commonly called sagebrush or big sagebrush, is a coarse, hardy, silvery-gray bush growing in arid sections of the Great Basin Desert of intermountain plateau covering portions of California, Idaho, Nevada, Oregon, Utah, and Wyoming in the western United States. Sagebrush is a key component of these ecosystems, providing canopy cover, nesting habitat, and a food source for numerous species of small animals and birds (4). During a plant disease survey in the Treasure Valley Region of southwestern Idaho and eastern Oregon, symptoms and signs of rust were observed on leaves of sagebrush in July 2007. Ten of fifteen plants (~70%) observed at the site were infected. Leaf samples of sagebrush with rust were also collected from a hedge in a home garden in Canyon County, ID in May 2006 and September 2007. Symptoms on both samples included cinnamon-brown, raised uredinia, primarily on the adaxial leaf surfaces. Initially, sori were scattered, increasing in density and becoming confluent. Urediniospores were thick walled, subglobose to obovoid, golden brown, echinulate, with three +/-- equatorial germ pores, and measured 28 to 32 × 23 to 27 μm. Telia appeared late in the season (July to August) and were mostly scattered, becoming confluent and forming raised, ovoid, brown-to-dark red streaks on leaves and stems. Teliospores were brown to dark red, two-celled, averaging 45 × 26 μm, thick walled (average 0.75 to 1.5 μm), thickening at the apex, ellipsoid to broadly ellipsoid, with thin-walled, hyaline pedicels, 26 to 31 μm broad at attachment, tapering below, equal to or up to twice as long as the spore. On the basis of morphology, this pathogen was identified as Puccinia similis Ellis & Everh. (2), an autoecious rust previously reported from Arizona and Wyoming on A. tridentata and A. nova A. Nels. (3). To confirm the identification of the specimens from Idaho, an ~1,000 bp of DNA from the ribosomal 28S large subunit was amplified and sequenced with rust-specific primers (1) (GenBank No. GU168942). Since there are no sequences of P. similis available in GenBank for comparison, a sequence of the same gene was also obtained from a specimen of P. similis that had been collected on A. cana Pursh in Utah in 1995 by C. T. Rogerson and deposited in the U.S. National Fungus Collections (BPI 863644; GenBank No. GU168943). The sequences shared 100% identity and did not match any other species of rust in GenBank. To our knowledge, this is the first report of P. similis in Idaho and Oregon on sagebrush, and the first report, based on herbarium data, of this rust on A. cana in Utah. Voucher specimens from Idaho have been deposited in BPI (878064) and the Bernard Lowy Mycological Herbarium (LSUM).
References: (1) M. C. Aime. Mycoscience 47:112, 2006. (2) G. B. Cummins. Rust Fungi on Legumes and Composites in North America. University of Arizona Press, Tucson, 1978. (3) D. F. Farr et al. Fungal Databases. Systematic Botany and Mycology Laboratory, Online publication. USDA-ARS, 8 July 2009. (4) B. L. Welch and C. Criddle, USDA Forest Service Res. Pap. RMRS-RP-40. 2003.