Endophytic Aureobasidium pullulans (Aljabal-Oman-A) as a biocontrol tool for olive leaf spot: targeting foliar pathogens through in-vitro and in-vivo suppression

Background The production of olives is of great significance to the world, given their economic importance and nutritional benefits. However, olive cultivation faces many threats from various fungal pathogens, which cause serious diseases affecting fruits, leaves, and the entire canopy. Several fungal pathogens were reported to affect leaves and fruits of olive, including species of Alternaria , Cladosporium , Didymella and Fusarium . The aim of this study is to investigate the biocontrol potential of the endophytic fungus Aureobasidium pullulans Aljabal-Oman-A, isolated from olive leaves in Oman, against some olive pathogens. Methodology The pathogenic fungi used in this study, included Alternaria arborescens , A. tenuissima , A. alternata , Cladosporium delicatulum , Didymella sp., and Fusarium equiseti were isolated from olive leaves with leaf spot symptoms. Four isolates of the endophytic fungus Aureobasidium pullulans were isolated from asymptomatic olive leaves and fruit samples in Western Al Hajar Mountains, northern Oman, identified through phylogenetic analysis of nuclear ribosomal DNA internal transcribed spacer (ITS) region. The isolate A . pullulans Aljabal-Oman-A was found effective against the above-mentioned olive spot pathogens in dual culture and detached leaf assays, confirmed the hyphal deformation through scanning electron microscopy (SEM). The isolate Aljabal-Oman-A was further tested for the production of antifungal metabolites and screened for volatile organic compounds (VOCs). Results Experiments proved the presence of antifungal properties in A. pullulans Aljabal-Oman-A against the test pathogens, Alternaria spp., Cladosporium delicatulum , Didymella sp., and Fusarium equiseti . The dual culture tests indicated varying degrees of fungal growth inhibition, with Alternaria tenuissima being very sensitive. The inhibition percentages ranged from 19% for F. equiseti to 44% for A. tenuissima . The SEM revealed hyphal deformation and shrinkage, indicating destructive interactions due to effects of VOCs. Metabolite analysis revealed significant electrolyte leakage from the fungal isolates, which indicates antifungal activity of the isolate Aljabal-Oman-A. In addition, the biocontrol assay demonstrated a respectable decrease in disease severity by these pathogens in the presence of Aljabal-Oman-A. Conclusion The study suggests that the endophytic fungus A. pullulan Aljabal-Oman-A, residing in healthy looking plants of olive, could be used as a biocontrol agent against fungal leaf spot diseases in olive trees.