Submitted to: Mycotoxin Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2011
Publication Date: 2/1/2012
Citation: Hua, S.T., Mcalpin, C.E., Chang, P., Sarreal, S.L. 2012. Characterization of toxigenic and atoxigenic Aspergillus flavus isolates from pistachio. Mycotoxin Research. Vol 28:67-75. Interpretive Summary: Aspergillus flavus Link is an economically important fungus which produces carcinogenic compounds known as aflatoxins in agricultural crops such as corn, peanuts, cotton and tree nuts. Many isolates of this fungus are able to produce hepatocarcinogenic aflatoxins (AFs) and a neurotoxin called cyclopiazonic acid (CPA). Aflatoxin contamination of food continues to be a regulatory deterrent to the growers and processors. The U. S. Food and Drug Administration has set a maximum level of 20 ppm of aflatoxin B1 (AF B1) in food intended for human consumption. The Commission of the European Community has set a more restrictive level of 2 ppm for AFB1. In this study we found only 21 % of the California (CA) A. flavus isolates produced AFB1, but 84% produced CPA. The most toxigenic one was the S strain, CA28 which produced the amount of AFB1 ranging from 2 to 137 fold higher than other isolates. The greater aflatoxin-producing potential of the S strain isolates could make this type of A. flavus the most likely contributor to AF contamination in pistachio.. Genomic profiling by a retrotransposon DNA probe revealed fingerprint patterns that were highly polymorphic, which correlates well with VCGs ( vegetative compatibility group). A better understanding of the characteristics of A. flavus strains in pistachio orchard may facilitate the development of effective biocontrol strategies.
Technical Abstract: Thirty eight Aspergillus flavus isolates collected from a pistachio orchard in California were analyzed for production of aflatoxin (AF), cyclopiazonic acid (CPA), vegetative compatibility groups (VCGs) and mating types. All toxigenic isolates produced both AFB1 and CPA. Twenty-one percent of the isolates produced AFB1, 84% produced CPA and half formed sclerotia on at least one of three tested media. Among the aflatoxigenic isolates, four produced small (S) sclerotia and four produced large (L) sclerotia. The most toxigenic one was the S strain, CA28 which produced 164 'g AFB1 from 5 ml PDA fungal culture. The other toxigenic strains produce AFB1 ranging from 1.2 'g to 80 'g per 5 ml fungal culture. Only four L strain of CA isolates were toxigenic. The isolates formed 26 vegetative compatibility groups (VCGs), six of which had two or more isolates and 20 contained single isolates. Genomic profiling by a retrotransposon DNA probe revealed fingerprint patterns that were highly polymorphic. The predicted VCGs (Pred-VCGs) based on a similarity coefficient > 80% matched the VCGs of multiple isolates determined by complementation. However, two of the Pred-VCGs showed discrepancies with corresponding VCGs. All isolates within a VCG had the same mating-type gene of either MAT1-1 or MAT1-2. Uncorrected and VCG-corrected MAT1-1 and MAT1-2 among the isolates were equally distributed.