Switchgrass (Panicum virgatum L.) is a native perennial grass with potential as a biofuel crop. The smut fungus, Tilletia maclaganii (Berk.) Clint., is associated with significant biomass reduction in switchgrass in the Midwest (4), but has not been reported in the northeast United States in more than 60 years (New York in 1890 and Pennsylvania in 1946) (2,3). From 2007 to 2010, smutted panicles were observed on the majority of plants in stands of several switchgrass cultivars at the USDA-NRCS Plant Materials Center in Big Flats (Chemung County), NY; in production fields of several switchgrass cultivars near Meadville (Crawford County), PA; and in an ornamental bed of switchgrass in Ithaca (Tompkins County), NY. Smutted panicles emerged 3 to 4 weeks prior to healthy panicles, had a compact, club-shaped appearance, and enlarged florets with swollen ovaries that readily released a powdery mass of odorless, rusty orange-to-dark brown teliospores when pinched. The entire caryopsis of every floret within a panicle was smutted and the infected plants appeared stunted, indicative of systemic infection. The fungus from each location was identified as T. maclaganii based on host, habit, and teliospore morphology (3). Teliospores were pale yellowish brown to reddish brown, varied from globose to slightly irregular in shape, and averaged 21 μm (18 to 25 μm) in diameter. The exospore was thick (2 to 3 μm), finely verrucose, and no sheath was present. True sterile cells, pale yellow and 10 to 18 μm in diameter, were sparsely present. Teliospores germinated and formed large (40 to 60 × 3 to 6 μm), nonconjugating basidiospores within 20 h on 2% water agar (WA). Occasionally, we also found the floret-infecting species T. pulcherrima (1) on switchgrass at very low incidence in Big Flats, NY, but it was easily distinguished from T. maclaganii. Stratified seeds (3 g) of ‘Shelter’, washed and found to be free of teliospores, were dusted with 0.04 g of teliospores of T. maclaganii isolate Tm001NY09 (Cornell Plant Pathology Herbarium Accession CUP-67931) harvested from infected ‘Shelter’ in Big Flats, NY in 2009. Inoculated and noninoculated seeds were sown in seedling trays, transplanted, and evaluated at panicle emergence. There were no symptoms on plants from noninoculated seeds. Symptoms on inoculated plants were consistent with field observations and teliospores were reisolated from infected panicles and cultured on 2% WA. Teliospores harvested from a single panicle infected with Tm001NY09 were used for culturing and DNA extraction. The fully annotated sequence of the rDNA internal transcribed spacer and 5.8S regions of this isolate were deposited in GenBank (Accession No. JF745116). Smut outbreaks in New York and Pennsylvania suggest that T. maclaganii must be managed effectively if switchgrass production is to be sustainable in the Northeast.
References: (1) L. M. Carris et al. Plant Dis. 92:1707, 2008. (2) R. Durán and G. W. Fischer. The Genus Tilletia. Washington State University, Pullman, WA, 1961. (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/, May 3, 2011 (4) P. M. Thomsen et al. Online publication. doi:10.1094/PHP-2008-0317-01-RS. Plant Health Progress, 2008.