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United States Department of Agriculture

Agricultural Research Service

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Research Project: Aflatoxin Control in Pistachios, Almonds, and Figs: Biocontrol Using Atoxigenic Strains

Location: Food and Feed Safety Research

2011 Annual Report

1a.Objectives (from AD-416)
For pistachios: Apply the atoxigenic strain AF36 in commercial pistachio orchards. Complete the survival studies for atoxigenic strains previously applied in research orchards. Identify spatial patterns associated with aflatoxin contamination in pistachio orchards using processor library samples. Determine the incidence of AF36 among A. flavus isolates obtained from commercial pistachio orchards in 2006. For almonds: Determine the incidence of atoxigenic strains among A. flavus isolates naturally occurring in almond orchards at various locations. Initiate studies on biocontrol of aflatoxin-producing fungi in a drip-irrigated almond orchard using the AF36 strain of A. flavus. Determine establishment/survival of AF36 in an almond orchard and displacement of toxigenic A. flavus and/or A. parasiticus. Provide data from the biocontrol for an application to be submitted to the EPA to obtain an experimental use permit (EUP) to treat major acreage of almond orchards in “aflatoxin-hot” areas to reduce aflatoxins (no-cost objective). For figs: Follow the survival and spread of atoxigenic A. flavus strains previously applied in a research fig orchard. Prepare for application of AF36 in commercial fig orchards.

1b.Approach (from AD-416)
For pistachios: Continue research on biocontrol of aflatoxin-producing fungi using atoxigenic A. flavus strains: (1) apply the atoxigenic strain AF36 (currently working with EPA in order to apply AF36 in commercial pistachio orchards); (2) determine the survival of the atoxigenic strains, including the atoxigenic strain AF36 previously applied as a wheat formulation in two research pistachio orchards; (3) use the results from the aflatoxin analyses of “library” samples to identify spatial patterns of aflatoxin contamination in California. Library samples, which consist of 20 pounds of nuts taken at the processing plant as nuts are being unloaded, represent a valuable research tool because it is easy to collect numerous nut samples from many commercial orchards representing extensive acreage. The resulting information will assist in deciding which commercial orchards would be best for applying AF36; (4) determine the incidence of AF36 naturally occurring in commercial pistachio orchards with a history of high aflatoxin contamination. For almonds: The density of A. flavus and A. parasiticus and the ratio of toxigenic to atoxigenic strains will be determined in almond orchards and specifically the incidence of the AF36 atoxigenic strain of A. flavus. AF36 will be applied in experimental plots to obtain data on its survival, displacement of the toxigenic strains, and reduction of aflatoxin contamination in almonds. Data of survival of the atoxigenic strain(s) and the displacement of toxigenic A. flavus and/or A. parasiticus will be used to expand the application of registering AF36 from pistachios to include almonds. For figs: Continue research on biocontrol of aflatoxin-producing fungi using the atoxigenic Aspergillus flavus strain AF36. AF36 has been applied in a fig orchard in California, resulting in AF36 becoming the dominant A. flavus strain where it was applied. The survival and spread of the previously applied atoxigenic strain AF36 (no AF36 will be applied in 2007) will be measured in order to determine how many years after treatment AF36 will still be the dominant strain and the extent that AF36 will move outside the treated areas. All results will be statistically analyzed and summarized in preparation for applying AF36 large scale in commercial fig orchards.

3.Progress Report

Work is carried out at the Kearney Agricultural Center of the University of California in Parlier and in commercial orchards. In early July, 2008, June, 2009, and June, 2010, wheat infected with AF36 were applied in 15 commercial pistachio orchards for a total treated area of 3,000 acres. Soil samples were collected from 7 pairs of orchards (treated orchards and adjacent nontreated orchards), and nut samples were collected from 3 pairs of orchards. The frequency of the atoxigenic AF36 in the soil increased substantially in the treated orchards after applying the wheat-AF36 product. Prior to the first application of the wheat-AF36 product in these orchards, only 2% of the Aspergillus (A.) flavus isolates from the soil of these orchards belonged to vegetative compatibility groups (VCG) YV36 (the VCG for the isolate AF36). But this increased until 92% of the isolates were VCG YV36 in the soil collected in September 2010 from treated areas. This demonstrates that the application of the wheat-AF36 product was successful in substantially increasing AF36 in the A. flavus/A. parasiticus populations in these orchards. In addition, application of the wheat-AF36 product did not result in increased incidences of kernel decay. Early split nuts (the main source of aflatoxin contamination in pistachio orchards) did not have significantly more decay by A. flavus in the treated areas than in untreated areas (both less than 0.1% compared to 3.0% decay by A. niger). Furthermore, nuts from the orchards treated with the wheat-AF36 product were less likely to be contaminated with aflatoxin than those from nearby untreated orchards in 2010. For the nut samples from the 2010, 97.1% of the ones from the treated orchards had no aflatoxin contamination, whereas 94.8% from the untreated orchards had none. In conclusion: after applying the AF36 starting in 2008, its level in soil increased from the naturally occurring 2% to 92% in 2010; application of AF36 did not increase kernel decay of pistachio nuts; for the 2010 harvest, 97.1% of the ones from the treated orchards had no aflatoxins, and 94.8% of those collected from untreated orchards (so there was an increase of nuts without aflatoxin contamination in the treated orchards); there are a number of atoxigenics that occur naturally in California pistachio orchards with AF36 being the most common atoxigenic. For figs: No new information to report. Progress is monitored by emails, periodic reports, phone conversations, and conference calls including, when useful, representatives of both IR-4 and industry.

Last Modified: 4/17/2014
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