Submitted to: Proceedings of the International Groundnut Workshop
Publication Type: Proceedings
Publication Acceptance Date: September 1, 2006
Publication Date: November 5, 2006
Citation: Dorner, J.W. 2006. Development of biocontrol technology to manage aflatoxin contamination in peanuts. Proceedings of the International Groundnut Workshop. Interpretive Summary: none required
Technical Abstract: A biopesticide, afla-guard®, has been developed for the control of aflatoxin contamination in peanuts. Biological control of aflatoxigenic strains of Aspergillus flavus and A. parasiticus in soil is achieved by introducing and establishing a dominant population of a non-aflatoxigenic strain prior to the onset of conditions (late-season drought) that result in preharvest aflatoxin contamination. Early studies showed that this form of biological control, achieved by competitive exclusion, was possible using non-aflatoxigenic strains of both A. flavus and A. parasiticus. However, several of the strains tested produced other mycotoxins, such as cyclopiazonic acid, or mutagenic aflatoxin biosynthetic precursors, such as O-methylsterigmatocystin. Whereas application of these strains reduced aflatoxin concentrations in peanuts, they also increased the concentrations of the metabolite(s) they produced. Finally a strain of A. flavus was identified that did not produce any toxic or carcinogenic metabolites, yet effectively reduced preharvest aflatoxin contamination of peanuts. It was subsequently shown that this strain has a deletion of the entire aflatoxin gene cluster, thus essentially eliminating the future possibility of genetic reversion or recombination to become aflatoxigenic. An important achievement in the commercialization of this concept was the development of a technique for producing the large amounts of high quality inoculum needed for field application. The unique method involves coating seeds of hulled barley with conidia of the biocontrol strain. Seeds are coated at the rate of approximately 1 × 106 conidia per gram, ensuring good growth and sporulation by the fungus upon uptake of moisture on the soil surface. The process is accomplished without the need of sterilization or drying of barley since water is not introduced. Flow-through seed-coating equipment allows the production of several tons of afla-guard® per hour. After the biopesticide received EPA Section 3 registration, large-scale studies were carried out in Georgia and Alabama to monitor the efficacy of the product for reducing aflatoxin contamination in commercially-produced peanuts. Results showed that application of afla-guard® produced a shift in the toxigenicity of A. flavus soil populations from a mean of 85.1% toxigenic strains in untreated fields to a mean of 5.4% toxigenic strains in treated fields. Analyses of farmers’ stock peanuts delivered at seven different locations from central Georgia to southeastern Alabama showed that the mean aflatoxin concentration of 11.7 ppb in peanuts from treated fields (n = 404) was significantly (P < 0.001) reduced by 85.2% compared with the mean of 78.9 ppb in peanuts from untreated fields (n = 178). Furthermore, two different shelling plants produced edible grade shelled lots from treated and untreated peanuts that had undergone routine storage conditions for approximately six months. At both plants, highly significant (P < 0.001) reductions in mean aflatoxin concentrations were associated with treated peanuts. No shelled lots of peanuts treated with afla-guard® were rejected for edible use for having an aflatoxin concentration above 15 ppb, despite the fact that at one plant the rejection rate for untreated peanuts was 48.4% while at the other plant the rejection rate was 15.8%. From an economic standpoint, those differences meant increases in shelled stock value of treated peanuts at the two plants of 13.0 and 4.3%, respectively.