Identification of the Nut Rot Pathogen Affecting Castanopsis carlesii Based on Morphological and Phylogenetic Analyses

In recent years, Castanopsis carlesii, a keystone species in southern China’s forest ecosystems with high ecological and economic importance, has faced growing challenges from severe nut rot diseases. Gnomoniopsis (Gnomoniaceae, Diaporthales, Sordariomycetes, Ascomycota) represents a significant fungal genus that causes leaf spots, branch cankers, and fruit rot diseases. In this study, rotten nuts of C. carlesii were collected from Fujian Province, and fungal isolates were obtained using the tissue isolation method. Morphological characterization and molecular phylogenetic analysis, based on the combined sequences of the internal transcribed spacer region of rDNA (ITS), the translation elongation factor 1-alpha (tef1) gene, and the partial beta-tubulin (tub2) gene were used to identify these isolates. As a result, new isolates from diseased nuts of C. carlesii formed a distinct clade with Gnomoniopsis, and morphologically differentiated from the other species; hence, G. flava sp. nov. is proposed herein. Furthermore, pathogenicity tests involving three isolates of G. flava were conducted on healthy nuts of C. carlesii, confirming its role as the causal agent of this new plant disease. This study not only advances our understanding of species diversity within Gnomoniopsis but also lays the groundwork for developing control strategies for C. carlesii nut rot disease.