First Report of the Head Smut Fungus Tilletia maclaganii Infecting Switchgrass (Panicum virgatum) in Indiana

Switchgrass (Panicum virgatum L.) is a native, perennial C₄ grass found across North America that was first used as a forage for grazing livestock and, later, as food and shelter for wildlife and for soil conservation. More recently, breeding efforts have focused on utilizing switchgrass as an herbaceous energy crop (HEC). In June 2017, a rusty brown powdery spore mass was observed to be associated with seed heads of ‘Shawnee’ switchgrass in mid-elongation, growing in a Purdue HEC research plot at Roann, Indiana. Swollen, dark seedheads were collected and submitted to the Purdue Plant and Pest Diagnostic Lab. Microscopic examination of cross-sectioned, discolored spikelets revealed ovaries replaced with fungal sori consistent with a smut fungus. For each of three specimens collected, the species identity of the suspected smut fungus was determined by teliospore morphology as well as polymerase chain reaction (PCR) amplification and Sanger sequencing of the internal transcribed spacer (ITS) region utilizing DNA extracted from teliospores from single spikelets and the primer pair ITS5 and ITS4. Teliospores (n = 50 per specimen) measured 16.5 to 29.6 µm (mean = 21.8 ± 0.4 µm) × 15.1 to 25.4 µm (mean = 20.1 ± 0.4 µm). The maximum spore diameter was somewhat larger than the maximum of 22 µm noted in the original description by Vánky, but the mean falls within his range. Teliospores were variable in shape from subglobose to ovoid, ellipsoidal, or partially irregular, and pale yellow to orange-brown or orange-yellow in color with a thick, finely verrucose cell wall (1.7 to 3.8 µm; mean thickness = 2.7 ± 0.1 µm) that appeared serrate in profile. Based on characteristic teliospore morphology, the fungus was identified as the head smut fungus, Tilletia maclaganii Clinton (Vánky 2012). The morphological determination was confirmed by DNA analysis of the ITS region. Teliospores from each of the three specimens yielded an identical ITS sequence (674 nucleotides; GenBank Tilletia_maclaganii_17-928 MH256490, Tilletia_maclaganii_17-994 MH256491, Tilletia_maclaganii_17-1055 MH256492) and, following a BLASTn search, shared 100% nucleotide identity (638/638 nucleotides) to that of the ITS sequence for T. maclaganii (JF745116) isolated from the switchgrass cultivar ‘Shelter’ in New York State (Layton and Bergstrom 2011). Head smut (T. maclaganii) has been reported to be responsible for decreasing the yield of switchgrass (Thomsen et al. 2008) and has been previously reported from Iowa (Vánky 2012), New York (Layton and Bergstrom 2011), and Texas (Kenaley et al. 2019). To the best of our knowledge, this is the first report of head smut on switchgrass in Indiana (Farr and Rossman 2018).