ABSTRACTPestalotioid fungi have been frequently studied with respect to their morphology, molecular phylogeny, and pathogenicity.Monochaetiais a pestalotioid genus that is morphologically characterized by 5-celled conidia with single apical and basal appendages. In the present study, fungal isolates were obtained from diseased leaves ofFagaceaehosts in China in 2016 to 2021 and identified based on morphology and phylogenetic analyses of the 5.8S nuclear ribosomal DNA gene with the two flanking internal transcribed spacer (ITS) regions, the nuclear ribosomal large subunit (LSU) region, the translation elongation factor 1-α (tef1) gene, and the β-tubulin (tub2) gene. As a result, five new species are proposed here, namely,Monochaetia hanzhongensis,Monochaetia lithocarpi,Monochaetia lithocarpicola,Monochaetia quercicola, andMonochaetia shaanxiensis. In addition, pathogenicity tests for these five species andMonochaetia castaneaefromCastanea mollissimawere conducted with detached leaves of Chinese chestnut. Results demonstrated that onlyM. castaneaesuccessfully infected the hostC. mollissimaand caused brown lesions.IMPORTANCEMonochaetiais a pestalotioid genus, with members that are commonly known as leaf pathogens or saprobes; some strains were isolated from air, in which case their natural substrate is so far unknown.Fagaceaerepresents an ecologically and economically important plant family that is widely distributed in the Northern Hemisphere, including an important tree crop species,Castanea mollissima, which is widely cultivated in China. In the present study, diseased leaves ofFagaceaein China were investigated, and five newMonochaetiaspecies were introduced based on morphology and phylogeny of combined ITS, LSU,tef1, andtub2loci. Additionally, six species ofMonochaetiawere inoculated onto healthy leaves of the crop hostCastanea mollissimato test their pathogenicity. The present study provides significant data on the species diversity, taxonomy, and host range ofMonochaetiaand enhances our understanding of leaf diseases ofFagaceaehosts.