Ceramothyrium siamense sp. nov. (Chaetothyriaceae, Chaetothyriales) From Ficus elastica (Moraceae, Rosales) in Northern Thailand

Ceramothyrium (Chaetothyriaceae, Chaetothyriales) comprises mainly foliicolous fungi characterized by superficial ascomata covered by a mycelial pellicle, bitunicate asci, and hyaline, pluriseptate ascospores, and is predominantly distributed in tropical and subtropical regions. During an investigation of leaf‐associated microfungi on Ficus elastica Roxb. ex Hornem. (Moraceae) in northern Thailand, an undescribed species of Ceramothyrium was discovered. Maximum likelihood (ML) and Bayesian inference (BI) analyses based on concatenated ITS and LSU sequence data place C. siamense within Ceramothyrium , forming a distinct and well‐supported lineage sister to C. aquaticum and C. phuquocense . Notably, these phylogenetically closest relatives are known only from their asexual morphs, whereas C. siamense is represented by a sexual morph. Morphologically, C. siamense is distinguished by superficial, nonsetose ascomata of moderate size compared to other species in the genus, relatively smaller bitunicate asci, and oblong–ellipsoid, hyaline ascospores with 3–5 transverse septa, lacking longitudinal septa and arranged 2–3 seriately. A morphological comparison of sexual Ceramothyrium species is provided.