Current status of the atoxigenic strain Aspergillus flavus AF36 for controlling aflatoxin in arizona cotton

Principal investigators:
Larry Antilla¹ and Peter J. Cotty².
¹Arizona Cotton Research and Protection Council, Phoenix, AZ; ² Agricultural Research Service, USDA, College of Agriculture, University of Arizona, Tucson, AZ.

The concept of competitive displacement provides the basis for the application of a unique biological control strategy which has been used over the past several years in Arizona to limit levels of aflatoxin expression on cottonseed. The inability of certain strains of the fungus Aspergillus flavus to produce aflatoxin provides the mechanism through which such control has been made possible. The discovery of this phenomenon led to a cotton industry plan for area wide control of aflatoxin and the development of a grower owned facility designed to produce commercial scale quantities of the most commonly occurring atoxigenic strain in Arizona known as AF36. Progressive improvements in production, distribution and application of AF36 have been made since the facilities inception in 1999. To date, this program has expanded to encompass more than 150,000 acres of cotton in Arizona, Texas and southern California with data verifying displacement of aflatoxin producing fungi. A collaborative effort involving USDA, ARS and the Arizona Cotton Research & Protection Council, a state agency funded solely by the Arizona cotton industry, enabled the ACRPC to develop and implement commercial scale production capabilities and to obtain a Section 3 registration for AF36 in Arizona, Texas and southern California.

Because of declining cotton acreage and extensive crop rotation practices, Arizona cotton producers are attempting to expand both the scope of opportunity and the window of treatment timing in order to enhance the area wide influence of atoxigenic strain applications. One way to address these issues is by making atoxigenic strain applications to non-cotton crops following harvest or termination based on the theory that displacement of toxin producing strains will occur on residual organic matter occupying fallow field conditions. Several cropping scenarios are being considered in order to provide such test data including winter and spring cole crops (i.e. lettuce, broccoli, cauliflower, etc.), melons (grown primarily spring and fall), and alfalfa hay stands, which are being terminated for future rotation to other crops. In all these scenarios, pre-treatment soil sampling and analysis for fungal community composition, followed by post-treatment sampling in increments of six months to one year, are being utilized to help determine treatment effects. Although the work is field directed, microbiological analyses are critical links to assess product quality and composition of fungal communities in treated fields. These analyses are performed in the Quality Control and Assessment Laboratories at the ACRPC manufacturing facility and by the ARS lab in Tucson. The ACRPC laboratories are in the process of assessing 3,000 to 5,000 isolates in the vegetative compatibility group of AF36 during 2007. This is in addition to product quality assessments which are an integral part of the manufacturing process.

On the manufacturing front, cooperative studies with the USDA ARS Lab in Tucson are being conducted relative to the current wheat seed substrate being used for AF36. Issues such as shelf life of the inoculated product, and field loss to bird, insect and rodent predation are issues to be considered relative to atoxigenic strain product stability and durability at the field level. Studies are underway to examine commercial scale quantities of alternative substrates such as sorgum or milo as well as various additives designed to minimize product loss due to predation. ACRPC field and lab personnel will be utilized to establish replicated sampling and evaluation of efforts of the various treatments prescribed.

The durability of competitive displacement on the part of AF36 versus the highly toxic S strain continued to be demonstrated in the 2006 crop season. Despite highly diverse crop rotation patterns, producers in Yuma county have utilized AF36 over the past four years with consistent results. Average percentages of AF36 versus S strain on cottonseed from treated crops have been 76% / 3%; 80% / 6%; 72% / 3% and 76% / 4% respectively for the years 2003-2006. This is most encouraging when compared to pre-treatment levels of 4% AF36 and 51% S strain taken in 2000 before program inception.

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