Ochratoxin A (OTA) is a potent mycotoxin produced by Aspergillus and Penicillium spp., which contaminates many crops, including pistachios. Pistachios contaminated with OTA may be subjected to border rejections resulting in significant economic losses to the United States agricultural revenues. The current study examined prevalence of OTA in California grown pistachios and identified its causal agents. OTA was detected in 20% samples from 2018-21 (n=809), with 18% samples exceeding the European Union regulatory limit of 5 µg/kg. Fungi potentially responsible for OTA contamination were isolated from leaves, nuts, and soil collected from 14 pistachio orchards across California. A total of 1,882 isolates of Aspergillus section Nigri and 85 isolates of section Circumdati were recovered. Within section Nigri, 216 (11.5%) isolates were identified as potential OTA producers using a boscalid-resistance assay. Phylogenetic analyses of partial gene sequences for β-tubulin and calmodulin genes resolved section Circumdati into four species: A. ochraceus (33%), A. melleus (28%), A. bridgeri (21%), and A. westerdijkiae (19%). Aspergillus westerdijkiae produced the highest levels of OTA in inoculated pistachios (47 µg/g), followed by A. ochraceus (9.6 µg/g) and A. melleus (3.3 µg/g). Aspergillus bridgeri did not produce OTA. OTA production by section Circumdati was optimal from 20 to 30ºC. All 216 boscalid-resistant isolates from section Nigri were identified as A. tubingensis and representative isolates (n =130) produced 3.8 µg/kg OTA in inoculated pistachios. This is the first detailed report on OTA contamination and causal fungi in California pistachios and will be helpful in devising effective management strategies.
Keywords: Aspergillus section Circumdati; Aspergillus section Nigri; Ochratoxin A; pistachio.