Effect of inoculum rate of biological control agents on preharvest aflatoxin contamination of peanuts

Studies were conducted during 1994 and 1995 in the environmental control plot facility at the National Peanut Research Laboratory to determine the effect of different inoculum rates of biological control agents on preharvest aflatoxin contamination of Florunner peanuts. Biocontrol agents were nontoxigenic color mutants of Aspergillus flavus and Aspergillus parasiticus that were grown on rice for use as soil inoculum. Three replicate plots (4.0 x 5.5 m) were treated with 0, 2, 10, and 50 g/m of row (0, 20, 100, and 500 lb/acre, respectively) of an equal mixture of the color mutant-infested rice in 1994, and the same plots were retreated in 1995. Aflatoxin concentrations were determined by high performance liquid chromatographic analysis of all peanuts. Treatment means for total kernels in 1994 were 337.6, 73.7, 34.8, and 33.3 ppb for the 0, 2, 10, and 50 g/m treatments, respectively. Regression analysis indicated a trend toward lower aflatoxin concentrations with increasing rates of inoculum (R-2 = 0.40; P < 0.05). For the same repeated treatments in 1995 aflatoxin concentrations in total kernels averaged 718.3, 184.4, 35.9, and 0.4 ppb. Regression analysis revealed a stronger relationship between inoculum rate and aflatoxin concentrations (R-2 = 0.66; P < 0.05) in the second year of treatment. Compared with untreated controls, the 2, 10, and 50 g/m treatments produced respective reductions in aflatoxin of 74.3, 95.0, and 99.9% in the second year. The data indicated not only a treatment-related effect, but also that a higher degree of control might be achieved when plots or fields are retreated with biocontrol agents in subsequent years.