2012 Annual Report
1a.Objectives (from AD-416):
1. Advance biocontrol technologies through strain selection, formulation, and adaptation to agronomic practices. Influences of agronomic practices on long-term effects of atoxigenic strain treatments will be determined. Improved criteria for selection of atoxigenic strains and strain mixtures will be sought and a collection of atoxigenics of potential value in target regions developed. Inexpensive substrates and formulations for atoxigenic product will be evaluated. Atoxigenic strain technology will be further adapted to aflatoxin management in commercial maize and tree crops.
2. Characterize adaptive differences among Vegetative Compatibility Groups (VCGs) and strains of Aspergillus flavus including adaptation to host, environment, and ecosystem. Adaptations of A. flavus to life-strategy, host, and environment will be characterized and variability within and between VCGs assessed. It will be determined if cotton and maize differentially influence the composition of A. flavus communities and if VCG composition is location dependent.
3. Use advanced molecular tools to characterize genetic basis for adaptive divergence among aflatoxin-producing and closely related fungi and to develop SNP databases and practical methods for monitoring temporal shifts in compositions of A. flavus communities. Quantitative Pyrosequencing assays will be developed that distinguish specific fungi or groups of fungi in field and laboratory samples. Relationships among morphotypes and vegetative compatibility groups will be clarified while molecular methods for identification are developed. Genome-wide comparisons will be used to identify both adaptive features and paths to adaptation among VCGs successful on different hosts.
4. Identify agronomic, environmental, and ecological factors (e.g. hosts) that favor development of highly toxic fungal community structures and practices that favor selection and retention of atoxigenics. Factors that favor dominance and dispersal of the S strain of A. flavus and that result in loss of A. flavus with reduced aflatoxin-producing potential will be determined in areas with severe levels of contamination attributable to the S strain. This will be done by examining relationships between climatic, biological, ecological, and geographical factors and the spatial and temporal distribution of Aspergillus section Flavi.
1b.Approach (from AD-416):
Develop improved formulations and production techniques to address problems in
commercial practice of biological control and to increase efficacy against aflatoxin producing fungi. Through field tests and retrospective analyses characterize influences of agronomic practices on biological control and use recommendations. Apply geostatistical and epidemiological tools to development of a model to predict aflatoxin contamination after crop maturation. Collect representative A. flavus from crops in Texas and Arizona and characterize strain specialization, adaptive traits, and optimal atoxigenic strains for distinct cropping systems. Develop both a SNP database for differentiating distinct A. flavus strains and a molecular technique for quantifying strain incidence in environmental samples. Select elite biocontrol strains based on improved knowledge of A. flavus adaptations and responses to relevant environments and ecological niches.
In order to support the national program action plan component on foodborne contaminants, increased efficacy of agents for the biological control of mycotoxins was sought by several studies in commercial agriculture. A commercial field study is following the fate of the biocontrol agents (non-toxigenic strains of fungi) during rotations out of treated target crops revealing both the longevity of beneficial influences during commercial agronomic practices and differential influences of rotation crops. New manufacturing procedures for reducing the energy requirements for production of biocontrol products and new cost effective formulations were pursued and the new formulations were evaluated in field trials. Sixteen genetic groups of atoxigenic biocontrol strains from South Texas were compared in North Central Texas for ability to move from the soil up to the corn crop and to overwinter between seasons and over crop rotations. The work is identifying differences among genetically distinct individuals in potential efficacy as biocontrol agents and should lead to selection of the next generation of highly effective biocontrol agents. Isolates belonging to several genetic groups of Aspergillus (A.) flavus were compared for ability to compete with each other on corn, cotton, sorghum, and soybean. Frequencies of single nucleotide polymorphisms (a commonly found change in deoxyribonucleic acid, DNA, sequence) were quantified with pyrosequencing (a method for sequencing DNA) in order to determine relative success during competition on each host. Isolates differed in competitive ability and these differences were dependent on which host was infected. The results suggest that hosts play important roles in selecting which aflatoxin-producing fungi dominate fungal communities and insights are provided on how to select optimal atoxigenic strains for biological control of aflatoxins in diverse crop rotations. Fungal populations associated with commercial corn in South Texas were sampled in collaboration with elevators distributed from the Rio Grande Valley to the border with Oklahoma and the process of applying genetic techniques to identify specific groups was initiated. Improved methods for assessing the aflatoxin producing potential of fungi in the laboratory were developed which will permit more reliable identification of atoxigenic strains and better assessment of the most important causes of contamination. Causal agents of the outbreaks of lethal aflatoxicoses (disease from ingesting aflatoxin) in Kenya over the past decade were found to be caused by a previously not described species of the fungus Aspergillus. Identification of specific genetic traits of this new species will permit monitoring and thus prevention of the movement of this deadly fungus from the highly affected regions in Kenya and will permit research into management of this distinct etiological (disease causing) agent. Overall the accomplishments on the project provide the basis for developing improved management for aflatoxin contamination and a basis for improving the most effective aflatoxin management tool, biological control with atoxigenic strains of A. flavus.
Crops influence aflatoxin contamination by altering population structures of crop associated fungi. Aflatoxins are potent cancer causing toxins that frequently contaminate several important crops including corn, peanut and cottonseed. The ability of fungal populations associated with crops to produce aflatoxins varies widely but it is unknown what causes particular aflatoxin producers to dominate in specific areas. A team of scientists working in the Agricultural Research Service laboratory at the University of Arizona in Tucson used sophisticated genetic analyses to reveal that crops exert selection on which aflatoxin producing and closely related fungi dominate. Crops do this by influencing the success of certain fungi during competition. This is the first empirical demonstration that specific genetic groups of aflatoxin-producing fungi receive advantage from specific crops and will provide bases for designing optimal crop rotations and for selection of elite biological control agents for aflatoxin reduction in diverse cropping systems.
Chang, P., Abbas, H.K., Weaver, M.A., Ehrlich, K., Scharfenstein, L.L., Cotty, P.J. 2012. Identification of genetic defects in the atoxigenic biocontrol strain Aspergillus flavus K49 reveals the presence of a competitive recombinant group in field populations. International Journal of Food Microbiology. 154:192-196.
Probst, C., Cotty, P.J., Callicott, K.A. 2011. Deadly strains of Kenyan Aspergillus are distinct from other aflatoxin producers. European Journal of Plant Pathology. 132:419-429.
Probst, C., Cotty, P.J. 2012. Relationships between in vivo and in vitro aflatoxin production: reliable prediction of fungal ability to contaminate maize with aflatoxins. Fungal Biology. 116:503-510.