Evaluation of the Atoxigenic Aspergillus flavus Strain AF36 in Pistachio Orchards

The atoxigenic strain Aspergillus flavus AF36, which has been extensively used as a biocontrol agent in commercial corn and cotton fields to reduce aflatoxin contamination, was applied in research pistachio orchards from 2002 to 2005 and in commercial pistachio orchards from 2008 to 2011. AF36 was applied as hyphae-colonized steam-sterilized wheat seed (the same product and same application rate as used in cotton fields). In all orchards, applying the wheat-AF36 product substantially increased the proportion of vegetative compatibility group (VCG) YV36, the VCG to which AF36 belongs, within A. flavus soil communities. Application of the AF36 product in additional years further increased YV36 in the soil until it composed 93% of the A. flavus isolates in treated commercial orchards. Nonetheless, application of the AF36 product did not result in increased incidence of kernel decay of the nuts. For nuts harvested from commercial orchards, reductions in percentages of samples contaminated with aflatoxin from treated orchards (relative to that for untreated orchards) ranged from 20 to 45%, depending on the year. Because of the high value of pistachio nuts and the costs associated with rejection of shipments due to aflatoxin contamination, these reductions are significant and valuable to the pistachio industry.