Summary of Awards to Date

Biology of aflatoxigenic fungi and biocontrol of aflatoxins in orchards of nut crops in California.

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Oct. 1, 2009 - Sep. 30, 2014

Award Number


Funding Agency

Center for Produce Safety


Themis J. Michailides, Ph.D.
University of California Kearney Agricultural Center


Aflatoxins (produced by Aspergillus flavus and A. parasiticus) are potent liver carcinogens and are widely regulated by governments who have set very low tolerances for aflatoxins in food and feed. Contamination of nuts crops with aflatoxins has become a huge issue relevant to food safety in the last several years. Recently, nut loads have been rejected frequently due to excessive aflatoxin contamination levels (far more the regulated tolerance of 10 parts per billion). Some other industries have used and registered atoxigenic (not producing aflatoxin) of A. flavus and thereby have successfully reduced aflatoxin contamination. We have used very successfully the atoxigenic strain AF36 in pistachio and fig orchards. We propose here to determine the density of A. flavus and A. parasiticus and the ratio of toxigenic to atoxigenic strains in nut crop orchards and specifically the incidence of the atoxigenic strains of A. flavus. Also we will compare isolates of A. flavus collected during 2009 to 2011 (recent populations) with isolates collected from nut crops during 1992 to 1994 (historic populations) to determine whether there was a shift of strains in recent populations towards strains highly toxigenic (producing higher amounts of aflatoxins than normal). Indeed, recent analyses of nut loads exported to Europe were found contaminated with unusually higher levels of aflatoxins than previously encountered, suggesting perhaps that there was a shift in A. flavus/parasiticus towards highly toxigenic strains. Representative aflatoxigenic strains from recent populations both from pistachio and almond orchards will be tested for amounts of aflatoxins they produce using HPLC and the levels compared with those of historic A. flavus to determine if there is a shift towards highly toxigenic strains. We also propose to apply a mixture of the most commonly encountered atoxigenic strains in a pistachio and almond orchard at Kearney Ag Center and determine establishment of these strains, survival, displacement of the aflatoxigenic toxigenic strains, and reduction of aflatoxin contamination in nuts. Samples of soils will be collected from plots of treated and non treated replicated plots before and 3 months after the application of the atoxigenic strains to determine displacement of the toxigenic strains. Similar determinations in the year following the first year of application can help determine how well the strains will survive and used as a guide whether application of the atoxigenic strains need to be done yearly or every other year. Nuts will be collected and isolations be made to determine the incidence of atoxigenic strains landed on the nuts and contamination of nuts with aflatoxins. Therefore an anticipated output of this project is that it is possible that a successful technology used to reduce aflatoxins in row crops (cotton, peanut, and corn) could be transferred and applied in nut tree crops that are also susceptible to aflatoxin contamination. Secondly, the atoxigenic strains that will be used in this project are naturally occurring in California orchards and their registration will be easier than strains introduced from other States or abroad.