Celtis sinensis is a highly versatile species that is commonly cultivated in the southern regions of China. In June 2022, leaf spot disease was detected in C. sinensis in Nanjing, Jiangsu, China. Based on morphological characteristics, three isolates were determined to be of the Alternaria species. A phylogenetic analysis of combined ITS, GAPDH, TEF1-α, RPB2, and Alt a 1 sequences identified the three isolates we obtained as Alternaria alternata and A. koreana. We determined the pathogenicity of A. alternata and A. koreana in C. sinensis leaves using inoculation through in vivo experiments. Symptoms of inoculation onset in indoor pots were in accordance with those observed in the open country. A. alternata and A. koreana can grow at 15–35 °C, with the best growth occurring at 25 °C. The findings from fungicide susceptibility experiments demonstrated that A. alternata and A. koreana were the most sensitive to prochloraz, which could offer an effective approach for future prevention and control measures against A. alternata and A. koreana. This study provides the initial step toward further research on A. alternata and A. koreana as pathogens of C. sinensis and provides the stage for future control strategy development.
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