Chaetoglobosin D isolated from the endophytic fungus MG2 exhibited a good control to apple tree Valsa canker

Apple tree Valsa canker (AVC), predominantly infected by the pathogenic fungus Cytospora mali, poses a severe threat to apple. Exploring biocontrol resources is of significant importance for the management of AVC. Chaetomium globosum MG2 is an endophytic fungus isolated from the bark tissue of Chaenomeles cathayensis, and exhibits promising biocontrol efficacy against AVC. In this study, the bioactive compound was isolated from strain MG2, and its structure was elucidated and identified to be chaetoglobosin D based on the extensive spectroscopic analyses. The EC₅₀ value of chaetoglobosin D against C. mali mycelial growth was 0.339 μg/mL. The hyphae exhibited distorted morphology, shrinkage, and cytoplasmic extrusion, ultimately resulting in cell death. The conidial germination of C. mali was completely inhibited after treatment with 2 μg/mL chaetoglobosin D, ultimately leading to protoplast extrusion. Simultaneously, the protective and therapeutic efficacies of chaetoglobosin D in the indoor detached twigs against AVC were 91.15 % and 88.26 %, respectively. Chaetoglobosin D exhibited comparable efficacy to the commercial fungicide pyraclostrobin in controlling AVC by C. mali. Further research revealed that chaetoglobosin D induced an increase in chitinase activity, elevated cell membrane conductivity, elevated MDA levels, depleted ergosterol content and enhanced the accumulation of reducing sugars in C. mali, which led to severe damage to the hyphal cell walls and cell membranes. Moreover, chaetoglobosin D was discovered to possess inhibitory effect on nine other pathogenic fungi. This is the first report on the antifungal activity of chaetoglobosin D against C. mali. It provides valuable insights for the development of novel green fungicides for controlling AVC.