Aspergillus flavus and aflatoxin on pistachio are the most important hazards to human health, which start in the orchard and will continue to the storage. This research selected 16 most effective yeast strains through dual culture (DC) assay according to the interaction between 376 yeast strains and A. flavus isolate P1684 in an initial screening. The antifungal ability of the strains was determined to be 94.5%, 72.08 %, and 91.31% in DC, volatile organic compounds (VOCs), and extracellular secretions (ESs) assays, respectively. In DC and VOCs assays, the YE 43-6 yeast strain showed maximum inhibition of growth, whereas powder and liquid formulation of the YE 43-10 yeast strain showed better performance. Ammonia vapor assay revealed that yeast strains significantly reduced aflatoxin production in A. flavus. Under in-situ conditions, trees were sprayed with cell/spore populations of both effective yeasts and A. flavus to determine the ability of yeast strains to compete with the pathogen. After 45 days, the fruits were harvested, and the average number of colonies per pistachio fruit unit (CFU/Nut) was determined. The highest and lowest reduction of populations were observed in YE 43-10 (85.71%) and YE 36-9 (35.18%), respectively. In both YE 43-6 and YE 43-10 strains, powder formulation was slightly more effective than liquid, and VOCs effectively prevented pistachio contamination with A. flavus and aflatoxin. پرونده مقاله