Biocontrol of aflatoxins in peanuts

Aflatoxins remain the most important mycotoxin problem in the world, and peanuts are a major source of these toxins. Control of aflatoxin levels in peanuts on a commercial scale is possible by colour sorting and aflatoxin assays on shelled peanuts, and this is widely practised in developed countries. However, this procedure is expensive. In consequence, a great deal of effort has been devoted to developing methods for reducing aflatoxin formation in peanuts, but traditional approaches using plant breeding have been unsuccessful. Current approaches in the USA and Australia centre on biocontrol or, more correctly, competitive inhibition, using nontoxigenic strains of A. flavus and/or A. parasiticus, added to peanut soils to compete with the naturally occurring toxin forming strains. The nontoxigenic strains to be used must be carefully chosen. They must be incapable of producing toxins, and competitive under field conditions. Ideally, the strains should be taken from soils or nuts in the same general area as they are distributed. Because A. flavus is a known (though weak) human pathogen, the introduction of nontoxigenic spores on the large scale will require care, and may face difficulties with regulatory authorities.. It is likely that distribution over plants or soil by spraying or dusting will never be permitted. In Australia, we have chosen to spread the spores directly onto soil on a carrier, by means of standard farm machinery such as fertiliser spreaders. After looking at alternatives, the carrier system we chose is cracked barley, to which the spores of the selected nontoxigenic isolates are added in molasses. This paper outlines the steps in choice of nontoxigenic strains, from laboratory to glasshouse, then small and larger scale field trials. Varying success has been achieved, on some occasions aflatoxin levels in harvested peanuts have shown a 95% reduction in aflatoxin compared with controls. It is expected that this process will be in use commercially soon, both in Australia and the United States.