Plant–Microbe Interactions, Friends or Foes: Case Study of Fagraea ceilanica Leaf Blight Caused by Geotrichum candidum in China and in vitro Screening of Antagonistic Bacteria

Fagraea ceilanica Thunb. is an important evergreen woody ornamental plant with high economic and medicinal value. It is widely distributed in China because of its strong environmental adaptability and ease of cultivation. In July 2021, a serious occurrence of leaf blight disease was observed on the leaves of F. ceilanica cultivated in outdoor space at Guizhou University, Guiyang City, Guizhou, China. We identified Geotrichum candidum as the causal agent via amplification and sequencing of the internal transcribed spacer (ITS) region, 18S ribosomal RNA (18S rDNA) and elongation factor 1-alpha (EF-1α) genes, as well as by morphological characterizations. Moreover, Koch's postulates were conducted by confirming pathogenesis of a representative strain, G. candidum GUGC1, on healthy leaves, re-isolating and identifying the pathogenic strain from diseased leaves. To our best knowledge, this is the first report of F. ceilanica leaf blight caused by G. candidum worldwide. Furthermore, plant endophytic bacteria were screened for inhibitory activity against G. candidum in vitro. The inhibition rates of three strains against the pathogen were higher than 70%, and all of them were identified as Bacillus spp. Our study provided new insights into the causal agent of F. ceilanica leaf blight, and the antagonistic bacteria identified in this study could provide the basis to develop biocontrol agents for the sustainable management of F. ceilanica leaf blight caused by G. candidum in the future. The possibility to utilize these antagonists against other, similar plant diseases, should be investigated as a key enabling component technology for ecostacking in the respective cropping systems.