Influence of field application of an atoxigenic strain of Aspergillus flavus on the populations of A. flavus infecting cotton bolls and on the aflatoxin content of cottonseed

An atoxigenic strain of Aspergillus flavus was applied to soils planted with cotton in Yuma, Arizona, to assess the ability of the atoxigenic strain to competitively exclude aflatoxin-producing strains during cotton boll infection and thereby prevent aflatoxin contamination of cottonseed. In both 1989 and 1990, the atoxigenic strain displaced other infecting strains during cotton boll development. Displacement was associated with significant reductions in the quantity of aflatoxins contaminating the crop at maturity. Although frequency of infected locules differed between years (1% versus 25%), in both years displacement occurred without increases in the amount of infection as measured by the quantity of locules with bright-green-yellow-fluorescence (BGYF). In the low infection year (1990), locules exhibiting BGYF were analyzed individually for both incidence of the applied strain and aflatoxin content. In the high infection year (1989), infected seed from each replicate plot (32 total) were pooled and analyzed for both aflatoxin and incidence of the released strain. Results of the latter analyses indicate an inverse relationship (r = 0.71, P < 0.001) between aflatoxin content and the percent seed infected by the applied strain. In 1990, quantities of A. flavus on mature crop surfaces did not differ between treated and untreated plots. When reisolated from the infected crop the applied atoxigenic strain retained the atoxigenic phenotype. Most infecting strains belonging to other vegetative compatibility groups did produce detectable quantities of aflatoxin B1 in liquid fermentation. The applied atoxigenic strain spread from treated plots to untreated controls at different rates in the two years and accounted for 7 and 25% of A. flavus strains isolated from infected locules in untreated control plots in 1990 and 1989, respectively. The results suggest that the aflatoxin-producing potential of A. flavus populations associated with crop production can be reduced in order to reduce aflatoxin contamination.