| Summary: | <i>Euonymus alatus</i> (Celastraceae) is widely cultivated in China for its economic value and landscape benefits. <i>Euonymus alatus</i> dieback occurs due to members of <i>Cytospora</i> and has become one of the most severe diseases affecting its cultivation in China. In this study, we examined the causal agent of bough dieback on campuses of University Road, Beijing, China. Among the strains, three were morphologically consistent with <i>Cytospora</i>, showing hyaline and allantoid conidia. Based on phylogenetic analyses of the concatenated actin (ACT), internal transcribed spacer (ITS), RNA polymerase II second largest subunit (RPB2), translation elongation factor 1-alpha (TEF1-α) and beta-tubulin (TUB2) gene sequences, along with morphological and physiological features, we propose <i>C. haidianensis</i> as a novel species. It was confirmed as a causal agent of dieback of <i>E. alatus</i> by pathogenicity tests. Mycelial growth of <i>Cytospora</i> <i>haidianensis</i> occurred at pH values ranging from 3.0 to 11.0, with optimum growth at 8.3, and at temperatures from 5 to 35 °C, with optimum growth at 19.8 °C. We also tested the growth of <i>C. haidianensis</i> in the presence of six carbon sources. Sucrose, maltose and glucose were highly efficient and xylose was the least. The ability of <i>C. haidianensis</i> to grow at 19.8 °C may help to explain its occurrence causing dieback of <i>E. alatus</i> in Beijing during the autumn season.
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