New Records of Rust Disease Caused by Uromyces halstedii in North America

During springs 2014 to 2016, we observed aecia of the heteroecious rust Uromyces halstedii De Toni on native painted trillium (Liliaceae, Trillium undulatum Willd.) within an oak-dominated forest stand in Westmoreland County, southwestern Pennsylvania (PA), but could not locate telia on potential alternate hosts (Davis and Harvey 2018). However, during a fall 2017 survey, we observed rust telia on bearded shorthusk grass (Poaceae, Brachyelytrum erectum [Schreb.] Beauv.), a reported host for U. halstedii in New York but not in PA (Farr and Rossman 2018). Telia were scattered, generally linear and interveinal, dark brown to black, and erumpent. Teliospores were one-celled, stalked, brown, and measured 29 ± 4 × 15 ± 1 μm (n = 30), including pronounced apical digitate projections. Coronate apices were generally wider than the spore body. Pedicels were brown, thin walled, and typically broken at the base. These characters are consistent with a diagnosis of U. halstedii (Cummins 1971). In addition, chlorotic leaf spots containing yellow aecia were noted on Canada mayflower (Asparagaceae, Maianthemum canadense Desf.) in the same forest stand. M. canadense is a small (5 to 15 cm tall), flowering plant that forms clonal carpets within the understory of shaded forests in PA (Davis personal observations). Further, examination of herbarium material in the Arthur Herbarium (PUR) at Purdue University revealed unidentified aecia on yellow wakerobin (Trillium luteum [Muhl.] Harbison), collected in Tennessee on 6 May 2007 (PUR N22389). Aecia on T. luteum were also consistent with those reported for U. halstedii (Cummins 1971). Fungal DNA was extracted from infected leaf samples of B. erectum, M. canadense, and T. luteum. Polymerase chain reaction was performed and the 28S region sequenced using primers Rust2inv and LR6 following protocols in Aime (2006). Sequence reads were analyzed in MEGA to assess quality and accuracy. Resulting sequences for U. halstedii were compared with our previous aecial sequence for U. halstedii on T. undulatum (Davis and Harvey 2018). The three sequences (GenBank accessions MG905913 to MG905915), as well as our previous sequence from T. undulatum, shared 100% identity across ≈1,000 bp of the 28S subunit. To our knowledge, this is the first report of U. halstedii infecting B. erectum in PA and the first report of this rust fungus infecting M. canadense or T. luteum (Farr and Rossman 2018). The fact that this rust has a broader host and geographic range than previously reported may have consequences for broader conservation efforts. Controlling B. erectum within infected stands may disrupt the heteroecious U. halstedii disease cycle, reducing or preventing rust symptoms on T. undulatum, an attractive understory plant that may be threatened in PA (Davis and Harvey 2018). Likewise, controlling U. halstedii directly on T. undulatum or T. luteum could benefit the attractive nature and diversity of eastern forest understories in general. In addition, M. canadense is listed as “threatened” in Kentucky, and M. canadense var. interius is listed as “endangered” in New Jersey (USDA NRCS 2018). Although its vulnerability has not been classified in PA, our observations suggest that M. canadense may be threatened in PA by U. halstedii. Fungal voucher specimens are deposited in the Arthur Fungarium (PUR N22387 to N22389).