| Summary: | <i>Populus laurifolia</i> is one of the most valuable tree species in the world and an important silvicultural tree species in the Xinjiang Uygur Autonomous Region, China. In July 2017, an unreported brown leaf spot disease was observed on <i>P. laurifolia</i> in Altay City, Xinjiang. The causal agent of this leaf spot disease was isolated, and Koch’s postulates were performed to confirm its pathogenicity. Based on a morphological characterization and phylogenetic analyses, the causal organism was identified to be a fungal species, <i>Coniolariella gamsii</i>. The optimum mycelial growth conditions for <i>C. gamsii</i> are on PLPDA (<i>Populus</i> leaves potato dextrose agar) medium, at 28 °C, in the dark. The sporulation time when using PLPDA medium (12 days) is much less than that for PDA medium (25 days). Pathogenicity tests revealed that <i>C. gamsii</i> can also infect two other <i>Populus</i> species (<i>P. bolleana</i> and <i>P. tomentosa</i>). This is the first report of <i>C. gamsii</i> causing brown leaf spot disease on <i>P. laurifolia</i>, and the optimum culture and sporulation conditions have also been optimized for the first time. This study provides a theoretical basis for the diagnosis of this disease and the monitoring of the disease’s occurrence and epidemic status.
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