Submitted to: National Cotton Council Beltwide Cotton Conference
Publication Type: Proceedings
Publication Acceptance Date: 2/15/2006
Publication Date: 4/1/2006
Citation: Garber, N.P., Cotty, P.J. 2006. Timing of herbicide applications may influence efficacy of aflatoxin biocontrol. National Cotton Council Beltwide Cotton Conference. San Antonio, TX. p. 1-6. Interpretive Summary: Aflatoxins are toxins produced by fungi. These toxins can contaminate many crops, including cottonseed and corn. There is a method for preventing aflatoxin contamination that utilizes a fungus to competitively exclude aflatoxin producers. To be useful, this biological control must work in agricultural fields where farmers commonly use chemical herbicides to control weeds. Laboratory tests were run by Nicholas Garber of the University of Arizona and Peter Cotty of the Agricultural Research Service in order to assess potential influences of herbicide use on the biological control fungus. Many common herbicides were found to reduce the efficacy of the biocontrol fungus. The results suggest that aflatoxin management through biological control will be most effective when biological control fungi are applied after all herbicide applications have been made.
Technical Abstract: 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 A36 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® 1.6E, 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® Ultra applied to the product in an aerosol spray that mimicked exposure that might occur from an herbicide application in the field. Bueno® 6 was used because it was the most toxic to AF36 in the first trial, and Roundup® Ultra was used because of the frequency with which it is used in Arizona. The second trial revealed that at field-use concentrations, AF36 product exposed to either Bueno® 6 or Roundup® Ultra produces fewer spores than untreated controls, but product exposed to Bueno® 6 produced the fewest spores. Reductions in AF36 sporulation after exposure to several commonly used herbicides indicates that AF36 applications should be made after all herbicide applications have completed.