TIMING OF HERBICIDE APPLICATIONS MAY INFLUENCE EFFICACY OF AFLATOXIN BIOCONTROL

Biological control of aflatoxin producing fungi must function within a complex crop production system that includes management schemes directed at diverse pests. One biological control of aflatoxin producers involves applying a strain of Aspergillus flavus that does not produce aflatoxins (atoxigenic strain) on a sterile food source (e.g. sorghum, wheat or barley seed) to field soils. The current study used the atoxigenic strain Aspergillus flavus AF36 formulated on steam sterilized wheat seeds (AF36 product). AF36 is the atoxigenic strain used commercially to limit contamination of cottonseed in Arizona and Texas. Proper timing of applications allows this atoxigenic strain to grow, sporulate and disperse to other organic matter associated with the developing crop in time to competitively exclude aflatoxin producers. Competitive exclusion of aflatoxin producers from the crop results in decreased aflatoxin contamination. Atoxigenic strain applications are frequently made to fields where chemical methods are used to limit weed growth. Interactions between herbicides and AF36 have not previously been investigated. AF36 product efficacy in the field is dependant upon production of spores of AF36 on the applied colonized wheat seed. Thus fungus ability to produce spores was quantified after exposure to the common herbicides: Buctril ® 4EC, Bueno ® 6, Caparol ® 4L, Direx 4L ® , Goal ® 1.6 E, Gramoxone ® Extra, Prowl ® 3.3 EC and Roundup ® Ultra. The AF36 product was immersed in varying concentrations of herbicide, and subsequent spore production was quantified following incubation under conditions ideal for AF36 development. Spore production was greatly reduced when AF36 product was exposed to high concentrations (10X-100X recommended use rates) of all eight herbicides. Six herbicide treatments reduced sporulation greater than 30% when the product was exposed to recommended use rates: Gramoxone ® Extra, Buctril ® 4EC, Bueno ® 6, Caparol ® 4L, Prowl ® 3.3 EC and Roundup ® Ultra. A second set of trials was conducted with Bueno ® 6 and Roundup