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First Report of Uromyces halstedii Rust on Native Painted Trillium in Pennsylvania

Davis, D. D., Harvey, R. J.
Plant disease 2018 v.102 no.2 pp. 447
Aecidium, Brachyelytrum, DNA, Leersia oryzoides, Trillium, Uromyces, aeciospores, computer software, databases, grasses, hardwood forests, hosts, image analysis, indigenous species, internal transcribed spacers, leaf spot, leaves, plant pathogenic fungi, polymerase chain reaction, rust diseases, Illinois, Kentucky, Michigan, New York, Ohio, Pennsylvania, Virginia, Washington (state), Wisconsin
Between early June of 2014 and 2016, we observed chlorotic leaf spots on painted trillium (Trillium undulatum Willd.) within an oak-dominated hardwood forest in Westmoreland County, southwestern Pennsylvania. The leaf spots observed were circular, 1 to 1.5 cm diameter, occurring on both leaf surfaces, but more prominent on the abaxial surface. Clusters of yellow-orange aecia occurred within the leaf spots, and small dark red pycnia were centered within each cluster. Symptomatic leaves were collected in early June 2016 for morphological and molecular evaluations. Measurements of aecia, aeciospores, and pycnia were made using an Olympus SZ dissecting scope and an Olympus BX41 compound scope with cellSens imaging software (Olympus America) and GraphPad software. Inside aecial cup diameter averaged 232 ± 36 µm (n = 40), mean aeciospore diameter was 24 ± 4 µm (n = 37), and pycnia were 111 ± 15 µm (n = 43) wide at the base and tapered to a point. Measurements were similar to those reported for the aecial stage referred to as Aecidium trillii Burrill on white trillium (T. grandiflorum [Michx.] Salisb.) in New York (Barrus 1928). A. trillii also has been reported on painted trillium in New York (House 1925). Morphological characters corresponded to those of the aecial stage of Uromyces halstedii De Toni. To confirm that the rust was U. halstedii, PCR was performed on DNA extracted from infected leaf tissue. The internal transcribed spacer region was sequenced using primers LR6 (5′-CGCCAGTTCTGCTTACC-3′) and Rust2inv (5′-GATGAACACAGTGAAA-3′) (Aime 2006). Sequence reads were analyzed in MEGA to assess quality and accuracy and entered into NCBI Nucleotide BLAST, which revealed a 98% match with Uromyces acuminatus De Toni (accession no. GU109282.1). However, 16 base pair mismatches were observed, indicating that the two sequences did not represent the same species. Although morphological measurements indicated that the rust was likely U. halstedii, a sequence for U. halstedii had not been deposited to the BLAST database, making molecular confirmation difficult if not impossible. However, the 98% match did allow us to confirm that both sequences were from the genus Uromyces. This genus match combined with the literature reports and morphology indicated that U. halstedii was the causal rust fungus. U. halstedii has been reported on T. grandiflorum in New York and Wisconsin, T. petiolatum in Washington, and Trillium spp. in Illinois and New York (Farr and Rossman 2017). Telial hosts for U. halstedii include native grasses (Poaceae), including bearded shorthusk (Brachyelytrum erectum [Schreb.] Beauv.), Virginia cutgrass (Leersia virginica Willd.), and rice cutgrass (L. oryzoides [L.] Sw.) (McCain and Hennen 1981). Although these grasses are native to Pennsylvania, they were not observed in the collection area. T. undulatum is listed as “threatened” in Kentucky, “endangered” in Michigan and Ohio, and “exploitably vulnerable” in New York. Although its vulnerability has not been classified in Pennsylvania, our observations suggest that T. undulatum may be threatened in Pennsylvania by U. halstedii. To our knowledge, this is the first report of U. halstedii occurring on T. undulatum in Pennsylvania.