Submitted to: Peanut Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 5, 2005
Publication Date: January 15, 2006
Citation: Dorner, J.W. 2006. Combined effects of biological control formulations, cultivars, and fungicides on preharvest aflatoxin contamination of peanuts. Peanut Science. 31:79-86. Interpretive Summary: Aflatoxin contamination of peanuts is a serious economic problem for peanut producers and processors and a food safety concern for consumers around the world. Contamination, which results from growth in peanuts by the molds, Aspergillus flavus and A. parasiticus, can occur in the field under drought conditions or in storage when peanuts are not maintained at safe moisture levels. Technology for biological control of aflatoxin contamination was previously developed that is based on competitive exclusion. It is carried out by applying strains of A. flavus and A. parasiticus that cannot make aflatoxin to peanut soil where they compete with naturally-occurring, toxin-producing strains. A three-year study was conducted to test the efficacy of a newly developed, more efficient biocontrol formulation with the formulation that was used in previous studies. In addition, two peanut cultivars and two fungicides commonly used to control leaf spot disease were evaluated for their effect on aflatoxin contamination. Results showed that the new formulation, which is prepared by coating barley with spores of the nontoxigenic A. flavus, was just as effective as the previous formulation, which is prepared by growing the A. flavus on rice. The new formulation established the desired strain in the soil and produced the same reduction in aflatoxin contamination as the old, more expensive formulation. In comparing the two cultivars, Georgia Green was found to be more resistant to aflatoxin contamination than Florunner, whereas the two fungicide treatments had no effect on aflatoxin contamination. The spore-coated formulation technique is more attractive for commercialization of this technology because the process is more economical and efficient than is the process of growing the mold on rice or other small grain.
Technical Abstract: A three-year field study was conducted to determine the effect of biological control formulations of nontoxigenic strains of Aspergillus flavus and A. parasiticus, peanut cultivars, and fungicides on preharvest aflatoxin contamination of peanuts. Formulation treatments consisted of: (1) no biocontrol treatment; (2) the fungi cultured on rice via solid-state fermentation; (3) conidia of the fungi coated onto the surface of rice; and (4) conidia coated onto the surface of wheat (year one) or hulled barley (years two and three). Experiments consisted of factorial combinations of the four formulation treatments, two peanut cultivars (Florunner or Georgia Green), and two fungicide treatments (chlorothalonil [Bravo] or combinations of chlorothalonil and tebuconazole [Folicur]). Florunner and Georgia Green peanuts were each planted in 32 individual plots consisting of six rows 15.2 m in length. Biological control formulations, consisting of a mixture of nontoxigenic strains of A. flavus (NRRL 21882) and A. parasiticus (NRRL 21369), were applied to the same plots in each of the three years at a rate of 56 kg/ha (50 lb/acre). Foliar applications of fungicides were made as recommended for control of leaf spot with one treatment being full season applications of chlorothalonil and the other being two applications of chlorothalonil followed by four applications of tebuconazole and remaining applications of chlorothalonil. Only in year two of the study was late-season drought sufficient to produce preharvest aflatoxin contamination. Aflatoxin in Georgia Green peanuts with no biocontrol treatment averaged 242 ppb, which was significantly (P < 0.01) lower than that in Florunner (1101 ppb). All three biocontrol formulations produced significant reductions in aflatoxin averaging 81.4%. There was no interaction between cultivar and biocontrol treatment, and no differences were observed between the two fungicide treatments. Analysis of soil for populations of A. flavus and A. parasiticus throughout the study showed that all formulations, except the conidia-coated wheat in the first year, were effective in delivering competitive levels of the nontoxigenic strains. In the third year, which did not result in significant aflatoxin contamination, analysis of peanuts for fungal colonization showed no significant differences among biocontrol treatments (including control) for total amounts of A. flavus and A. parasiticus in peanuts. However, the incidence of toxigenic strains in peanuts was significantly reduced by all three biocontrol formulations.