Sphacelotheca dibotrys , a new species causing smut disease of Fagopyrum dibotrys in Yunnan, China

Fagopyrum dibotrys is a perennial herb used for traditional Chinese medicine. In Jun 2017, symptoms of smut disease appeared in the ovaries of field-grown F. dibotrys in Yunnan Province, China (25.12°N, 102.76°E, altitude 1920 m). Smuts usually inhabit ovaries. The symptomatic flowers showed ovary swelling and malformation, necrotic petals with brown-to-black lesions, induced mycocecidium, and then spurted smuts. Finally, most floral organs abscised, resulting in an inability to grow. Fresh smut galls were randomly collected in July 2018 from Xianggelila (30 galls) and in July 2025 from Kunming (10 galls). Spores pale violaceous or violaceous, globose, subglobose, broadly ellipsoid or rarely ovoid, 10 to 16.5 × 8 to 16 μm (n = 349). Scanning electron microscopy revealed that most nodules on the spore surface were interconnected, forming an irregular reticulum. Additionally, spherical to broadly ellipsoid spores were occasionally observed in chains (Deml et al. 1985). The smut spores’ morphological characteristics were consistent with the genus Sphacelotheca. The size of spores (10 to 16.5 × 8 to 16 μm) was larger than of S. polygoni-serrulati (8 to 15 × 8 to 12 μm) (Schuster et al. 2016), S. reiliana (9 to 14 × 10 to 13 μm) (Parte et al. 2016), and S. polygoni-persicariae (9.2 to 15.1 × 8.3 to 14 μm) (Deml et al. 1985). DNA fragments were amplified and sequenced with ITS, LSU, RPB1, RPB2, and TEF1 (Wang et al. 2016). The sequences were deposited in GenBank (ITS, PX763181 and PX763182; LSU, PX763183 and PX763184; RPB1, PX780266 and PX780267; RPB2, PX769220 and PX769221; TEF1, PX769221 and PX769223). Maximum likelihood and Bayesian inference on PhyloSuite v1.2.2 (Zhang et al. 2020) confirmed that these isolates formed a distinct lineage within Sphacelotheca with a strong statistical support (100% ML/1.00 PP). Based on both morphological and phylogenetic, SD20250701 and SD20250702 were identified as a distinct new species in Sphacelotheca: Sphacelotheca dibotrys D.W. Sun & G. Wang, sp. nov. MycoBank number: MB861878. Etymology: The specific epithet refers to the species name of host plant, Fagopyrum dibotrys. Holotype: SD20250701. The stem of 8-week-old healthy F. dibotrys plants at a uniform flowering stage were inoculated by syringe a spore suspension (4.5×10⁵ spores/mL), while control plants received sterile distilled water. Within 3 weeks post-inoculation, flowers developed clear symptoms including flower deformation, mycocedidium formation, and the extrusion of smut spores, consistent with symptoms observed under natural field infections. In contrast, control plants remained symptomless and healthy. These results collectively fulfill Koch’s postulates. This study represents the first report of S. dibotrys associated with F. dibotrys smut disease in China. This finding will facilitate further research on defense strategies against F. dibotrys smut disease and support the development of improved management practices for this disease.