Leaf spots starting as small pale brown circular to subcircular leaf spot that later become pale centrally with a black periphery followed by a reddish-brown halo, 1–1.5 × 0.6–2 mm; Internal mycelia indistinct; Conidiomata acervular, subepidermal, isolate or in groups, circular, 85–200 μm diam; Conidiophores subcylindrical reduced to phialidic conidiogenous cells, 14–20 × 3–5 μm, subhyaline and smooth. Conidia falcate, 20–30 × 3–4 μm, aseptate, guttulate, hyaline, smooth. Appressoria ovoid to clavate, 6–11 × 4–6 μm, pale brown. In culture: Relative fast-growth (90 mm diam in 8 days at 25 ± 2 °C), aerial mycelium sparse, felty, colony predominantly immersed, pale purplish gray to greyish-brown to pale mouse gray, with or without diurnal zonation and pattern of dense strands of dendritic mycelium irradiating from the central area, humid; greenish black reverse with diurnal zonation; sporulation abundant.

Comments: C. riograndense is morphologically close to C. falcatum Went, as described by Sutton (1992), a fungus commonly found in associations with grasses and other species of monocotyledons. Nevertheless, the fungus on Tradescantia has relatively smaller conidia (20–30 × 3–4 μm for C. riograndense as compared to 19.9–27.2 × 3.9–4.9 μm for C. falcatum). There are only two records of C. falcatum on members the Commelinaceae: on Dichorisandra sp. and Zebrina pendula Schinzl., in Florida (Farr and Rossman 2014). Additionally there is one record of an undetermined species of Colletotrichum sp. on T. fluminensis in Florida and Texas (Farr and Rossman 2014), but they are obscure records without any information on the fungus other than its occurrence. Genetically C. riograndense is a member of the “spaethianum clade” of the Colletotrichum genus (Cannon et al. 2012), along with other falcate-spored species, this clade is distinct from superficially morphologically similar species such as C. falcatum in the “graminicola clade”. A robust Bayesian inference genetic analysis of the spaethianum clade is presented in Fig 4. This analysis shows clearly that C. riograndense ICMP 20083 is a distinct taxon in this clade, and is most closely related to C. bletillum G. Tao, Z. Y. Liu & L. Cai and C. incanum H.C. Yang, J.S. Haudenshield & G.L. Hartman.

BRAZIL: Rio Grande do Sul, São Marcos, on living leaves of Tradescantia viz. fluminensis, 13 Jul. 2008, D. M. Macedo, HOLOTYPE specimen: VIC 31366, ex-holotype culture: COAD 928 = ICMP 20083.