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. Plant Disease [Internet].
Switchgrass (
Panicum virgatum L.) is a perennial C
4 grass native to North America. Over the last two decades switchgrass has received considerable attention for its potential as a bioenergy-biofuel feedstock crop and model system to examine the intraspecific genetic divergence in plants. In April 2017, we observed and collected panicles from the switchgrass accession ‘West Bee Caves’ (WBC3), which was being grown within a four-way mapping population (
Milano et al. 2016) planted in 2015 in Temple, TX (UTM: 14 N 657505.8E, 3435580.0N). Infected plants had conspicuously swollen spikelets with reddish- to deep-purple glumes, and, when agitated, spikelets released a dry, red-brown mass of teliospores. Cross-sectioning of spore-laden spikelets revealed the complete replacement of the ovary with fungal sori, consistent with infection by the smut fungus
Tilletia maclaganii (Berk.) G.P. Clinton or
T. pulcherrima Ellis & Galloway (Tilletiaceae). Likewise, in April 2018 within the same mapping population,
Tilletia-infected panicles were collected from WBC3, ‘Alamo’ (AP13), and an F
1-hybrid between ‘Summer’ (VS16) × WBC3 planted in Austin, TX (UTM: 14 N 622078.5E, 3362019.3N), as well as AP13 planted in Kingsville, TX (UTM: 14 N 610479.6E, 3047839.0N). The site in Austin is located ∼100 km south-southwest of Temple, TX, and the site in Kingsville is ∼400 km south of Temple. Seven voucher specimens (
n = 3 Austin, 2 Kingsville, and 2 Temple) were deposited in the Cornell Plant Pathology Herbarium (CUP 068303 to 068309). Thereafter, the
Tilletia sp. was determined by teliospore morphology as well as polymerase chain reaction (PCR) amplification and sequencing of the internal transcribed spacer (ITS) region using primers ITS5 and ITS4. DNA extracts for PCR were procured from single-teliospore cultures or direct lysis of teliospores from single spikelets. Teliospores (
n ≥ 25 per voucher specimen,
n = 200 total) lacked a sheath and were pale yellow to orange-brown in color, mostly globose to broadly ellipsoid, rarely irregular, and measured 19.1 to 31.0 × 14.1 to 27.0 µm (mean = 21.5 ± 0.4 × 23.8 ± 0.4 µm). Cell walls were finely verrucose, 1.7 to 3.8 µm thick (mean = 2.7 ± 0.1 µm), and serrate in profile. Teliospore morphology was consistent across host-site combinations and best fit the description of the species
T. maclaganii (
Vánky 2012). DNA analyses confirmed our morphological-based identification, because ITS sequences were identical across all seven specimens (674 nucleotides; GenBank MH397299 to MH397305) and shared 100% nucleotide identity with authenticated ITS sequences of
T. maclaganii from Indiana (GenBank MH256491) and New York State (JF745116;
Layton and Bergstrom 2011). To the best of our knowledge, this the first report of the head smut fungus in Texas (
Farr and Rossman 2018) and expands the southern geographic range of
T.
maclaganii in North America to below 28° N latitude. The head smut fungus is an economically important pathogen of switchgrass, particularly when switchgrass is grown across large acreage for biomass, commercial seed, and/or lignocellulosic ethanol (
Thomsen et al. 2008).
T. maclaganii persists as a systemic parasite within switchgrass plants from which pathogen spread can occur via windborne teliospores, teliospore-infested seed, or asexual propagation of infected plants. Thus, the documentation of
T. maclaganii in widespread locations in Texas suggests that management of head smut may become necessary for switchgrass feedstock production in the southern Great Plains.