Digitaria isanensis sp. nov. and a reinstated taxon of Digitaria (Poaceae) from Thailand Boonchuang Boonsuk, Pranom Chantaranothai and Trevor R. Hodkinson B. Boonsuk and P. Chantaranothai (pranom@kku.ac.th), Applied Taxonomic Research Center, Dept of Biology, Faculty of Science, Khon Kaen Univ., TH-40002 Khon Kaen, Tailand. – T. R. Hodkinson, School of Natural Sciences, Trinity Centre for Biodiversity Research, Trinity College Dublin, Dublin 2, Ireland. Digitaria isanensis is described as a new species apparently endemic to sandy soils in open areas of northeastern Tailand. In addition, D. thwaitesii var. tonkinensis is reinstated and presented as a new record for Tailand. Descriptions and illustrations of both taxa are provided. Digitaria Haller, the crab grass genus contains approxi- mately 260 species distributed mainly in tropical and warm temperate regions (Clayton and Renvoize 1986, Shouliang and Phillips 2006, Clayton et al. 2012). Te first report of Digitaria in Tailand was published by Bor (1962) who listed 12 species. Tree years later, he published a second list with nine additional species, of which D. microstachya Henrard, D. setigera Roth and D. tomentosa (J. König ex Rottler) Henrard were new records for Tailand (Bor 1965). Te last synopsis of Digitaria in Tailand was made by Nanakorn and Norsaengsri (2001) who enumerated 26 species. Since then, new specimens of Digitaria have been collected in Tailand including collections by the first author. While preparing the taxonomic treatment of Digitaria for the ‘Flora of Tailand’, we examined recently collected specimens from the provinces of Loei, Udon Tani, Kalasin, Nakhon Ratchasima, Surin, Yasothon and Ubon Ratchathani and found a new species which we name D. isanensis. In addition, we argue for recognizing D. thwaitesii (Hack.) Henrard var. tonkinensis Henrard, which has frequently been treated as a synonym of D. violascens Link (Veldkamp 1973, Lazarides 1980, Shouliang and Phillips 2006). It differs from D. violascens in several ways including its habit, longer pedicels, larger spikelets and hairs on spikelets. Tis taxon is also reported as a new record for Tailand. Descriptions and illustrations based on Tai collections are provided. Measurements of morphological characters were taken from fresh and dry specimens (from BK, BKF, KKU and QBG). Spikelet characters were measured and investigated using stereo light microscopy (Nikon SMZ-445). Digitaria isanensis B. Boonsuk, Chantar. & Hodk. sp. nov. (Fig. 1) Digitaria isanensis has similar spikelets to D. petelotii Henrard but differs in being a small annual creeping to decumbent grass which is 10–30 cm tall, having glabrous cauline internodes, lanceolate leaves which are 1.5–4.0 cm long and inflorescences usually with a single branch. Type: Tailand, Kalasin province, Yang Talat district, Hua Na Kham subdistrict, elevation 150 m a.s.l., 28 Sep 2010, B. Boonsuk 430 (holotype: KKU!, isotypes: BK!, BKF!, K!, QBG!). Etymology Te specific epithet refers to the locality of all known collections of this species, i.e. northeastern or ‘isan’ region of Tailand. Description Plant annual. Culms creeping to decumbent, 10–30 cm tall, rooting at the lower nodes; nodes and internodes glabrous. Sheaths sparsely pilose or glabrous, 1–3 cm long. Ligules membranous, 0.4–0.6 mm long, erose. Blades linear, 1.5–4.0 cm  2–4 mm, chartaceous, rounded at base, acuminate at apex; abaxial and adaxial surfaces sparsely pilose or glabrous. Inflorescence branches 1–3, usually 1, solitary or digitate, the largest branch 2.5–6.0 cm long; rachis ribbon-like, winged, 0.7–0.9 mm wide, its margins serrulate. Pedicels terete, spicular, their abscission cupuliform to dis- coid. Spikelets ternate, homomorphous, elliptic, 2.1–2.3  0.6–0.7 mm; pubescent, hairs whitish, acute, verrucose,  2014 Te Authors. Nordic Journal of Botany  2014 Nordic Society Oikos Subject Editor: Torbjörn Tyler. Accepted 18 March 2014 Nordic Journal of Botany 32: 811–814, 2014 doi: 10.1111/njb.00531 811