Cronartium ribicola J. C. Fisch., causal agent of white pine blister rust (WPBR), is one of the most damaging pathogens of five-needle pines, forming aecial states on the trunk and branches and causing cankering, topkill, and branch dieback. Infection can predispose hosts to attack by other pests such as bark beetles, and can result in host mortality. Various species of Ribes, Pedicularis, and Castilleja are alternate hosts on which C. ribicola forms its uredinial and telial states during the mid-summer to fall. In an effort to mitigate the damage caused by white pine blister rust, the planting of ornamental species of Ribes, such as R. occidentalis, is prohibited in 14 states. Indiana currently has no restrictions on the planting of Ribes spp. Since 2010, a Cronartium sp. has been observed producing uredinia and telia on R. odoratum ‘Crandall’ H.L. Wendl. leaves in an urban environment in West Lafayette, Indiana. Symptoms include yellow-orange lesions on the leaf upper surface with uredinia on the underside. These persist from late summer until leaf drop. Telia were collected in 2011 to establish the identity of the causal agent using morphological and molecular analyses. Morphological comparisons between this specimen and other Cronartium species were made using Arthur (2). Filiform telial columns ranged from 0.5 to 1.5 mm in length. Teliospores were cylindrical to sub-ventricose, truncate on either end with one end generally tapering more than the other, and measured 9.0 to 18.6 × 37.2 to 60.0 μm (average 11.9 × 47.4 μm from 30 spores across 4 leaves). These teliospore measurements overlap those of C. ribicola and C. occidentale, but are more consistent with C. ribicola, in which the spores are wider and longer (8 to 12 × 30 to 60 μm) than in C. occidentale (9 to 10 × 27 to 56 μm). For molecular analyses, two nuclear ribosomal loci were sequenced: the internal transcribed spacer regions 1, 2, and 5.8S (ITS) and the 5′ end of the large subunit (28S) (1). The ITS sequence was 665 bp long (KF387533) and the 28S was 892 bp (KC876675). These sequences were queried to GenBank using a BLASTn search. The 28S shared 99% identity (891/892 bp) and the ITS shared 100% identity (663/663 bp) to other published C. ribicola sequences with no close matches to any other species with either locus. Both morphological and molecular methods indicate this species to be C ribicola, making this a first report of white pine blister rust on R. odoratum in Indiana. This fungus has been observed previously on R. odoratum in the northeastern United States (Connecticut, Massachusetts, Rhode Island, Vermont, and New Hampshire), the Rockies (Colorado), northwestern United States (Washington), and Canada (3). In Indiana, C. ribicola has also been reported on R. cysnobati. There are no other reports of this fungus on any other host within the state. However, the aecial host, Pinus strobus, does grow within the state, and within West Lafayette. To our knowledge, WPBR has only been observed (not reported) once in Indiana in the past 30 years (Paul Pecknold, personal communication). Further monitoring of C. ribicola hosts is needed in Indiana to determine the extent of the disease. The specimen has been vouchered in the Arthur Herbarium (PUR N6734).
References: (1) M. C. Aime. Mycoscience 47:112. 2006. (2) J. F. Arthur. Manual of the Rusts in United States and Canada. Purdue Research Foundation, 1934. (3) D. F. Farr and A. Y. Rossman. Fungal Databases Systematic Mycology and Microbiology Laboratory, ARS, USDA. Retrieved from http://nt.ars-grin.gov/fungaldatabases/ April 23, 2013.