Causing Leaf Spot Disease on × Ramat and Screening of Its Potential Antagonistic Bacteria.

× Ramat. is a famous perennial herb with medicinal, edible, and ornamental purposes, but the occurrence of plant diseases can reduce its value. A serious disease that caused leaf spots in appeared in 2022 in Tongxiang City, Zhejiang Province, China. The leaves with brown spots were collected and used for pathogen isolation. By completing Koch's postulates, it was proven that the isolate had pathogenicity to infect . It was determined that the pathogen isolated from chrysanthemum leaves was , through morphology and a multilocus sequence analysis method using a combination of the internal transcribed spacer gene (ITS), beta-tubulin gene (TUB2), and translation elongation factor 1-alpha gene (TEF1-α). This is the first report of disease caused by in the world. Through dual culture assay on PDA plates, 12 strains of bacteria with antagonistic effects were selected from 231 strains from the phyllosphere, among which D65 had the best inhibitory effect on growth. In addition, the components of a strain D65 fermentation broth were profiled by SPME-GC-Q-TOF analysis, providing a foundation for further application and research of biological control.