Identification and biological control of a pathogenic Aspergillus niger causing soft rot in Citrus unshiu Marc.

The citrus industry is of significant global economic importance but faces substantial postharvest losses from soft rot disease. While chemical fungicides are commonly used for control, their application has raised concerns about pathogen resistance and environmental hazards. This concern underscores the urgent need for safe and sustainable alternatives. The current research focused on isolation and identification of primary pathogen responsible for soft rot in mandarin (Citrus unshiu Marc) and evaluated the efficacy of the antagonistic yeast Debaryomyces nepalensis as a biological control agent. A fungal pathogen was isolated from diseased citrus fruits, identified as Aspergillus niger through morphological and molecular analyses, and its soft rot pathogenicity was confirmed by inoculation experiments, leading to 100% disease incidence within nine days. The biocontrol efficacy of a D. nepalensis and a commercial agent ('LvKangWei') containing Bacillus amyloliquefaciens was tested against A. niger-induced soft rot. Both treatment with 'LvKangWei' and D. nepalensis significantly reduced disease severity, with a control efficacy of 31.54% and 22.98% respectively at 15 days post-inoculation. The research extends the study of effect of the yeast biocontrol agents that was associated with the induction of host defense responses, as evidenced by a significant increase in CAT, POD, and SOD activity and the maintenance of total sugar content compared to infected control fruit. Our findings demonstrate that D. nepalensis is an effective biocontrol agent against A. niger-induced soft rot in mandarin.