Pistachio male inflorescences as an alternative substrate for the application of atoxigenic strains of Aspergillus flavus

Abstract

Aflatoxins are carcinogens mainly produced by Aspergillus flavus and A. parasiticus in susceptible crops, including pistachio. The primary inoculum sources of these pathogens are plant debris in the orchard soils. In Californian fields, one approach to controlling aflatoxin contamination is based on releasing the atoxigenic strain of A. flavus AF36 in inoculated (coated) sorghum grains (AF36 Prevail®). However, this control method can fail due to poor sporulation of the AF36 strain or sorghum grain losses due to predation. In 2008 and 2018, we showed that toxigenic and atoxigenic isolates of Aspergillus spp. frequently colonized fallen inflorescences of male pistachios. Under controlled conditions, AF36 strain profusely colonized pistachio male inflorescences when humidity was higher than 90%. However, there were significant differences (between types of inflorescence (aerial > fallen). In 2016, we considerably (P = 0.015) increased the population of AF36 on the canopies of trees when fallen inflorescences were inoculated with AF36, compared to untreated trees. In 2017 and 2018, these differences were not detected (P > 0.05) due to cross-contamination of AF36 strain between seasons and neighboring plots. In any case, the density of AF36 spores on the canopy of the inflorescence-treated trees was similar (P > 0.05) to those of treated trees with the commercial product. Here, we present a new method for applying AF36 strain based on using a natural, abundant, and uniformly distributed substrate in pistachio fields, and we discuss how it can be improved. Furthermore, our results indicate that in pistachio orchards, where biocontrol practices are not conducted, eliminating this important source of toxigenic Aspergillus inoculum is recommended.