Genetic Structure of Soil Populations of Aspergillus Section Flavi and Efficacy of Biocontrol of Aflatoxin Contamination in Corn . ( Under the direction of Dr

LEWIS, MARY HUNT. Genetic Structure of Soil Populations of Aspergillus Section Flavi and Efficacy of Biocontrol of Aflatoxin Contamination in Corn. (Under the direction of Dr. Ignazio Carbone and Dr. Peter S. Ojiambo). Corn is contaminated with aflatoxin, a carcinogenic mycotoxin, when aflatoxigenic strains within Aspergillus section Flavi infect corn kernels. Biocontrol using nonaflatoxigenic strains of A. flavus have been shown to have the greatest potential to control aflatoxin contamination in corn. However, factors that influence the efficacy of biocontrol agents in different locations are not fully understood. One factor affecting the effectiveness biocontrol could be the genetic structure of the native soil populations of Aspergillus section Flavi. In this study, we investigated how the genetic structure of native soil populations of Aspergillus section Flavi could impact the effectiveness of two commercially available biological control products in different locations in the southeastern US. Field trials were conducted in Alabama, Georgia and North Carolina in the 2012 and 2013 growing seasons. Biocontrol products AF36 and Afla-Guard® were applied to the corn at the VT stage. Soil samples were collected prior to and 1-week after biocontrol application and at harvest to determine the genetic structure of soil populations. In all states, A. flavus (61-100%) was the most dominant species within section Flavi, with A. parasiticus (<35%) being the secondmost frequently isolated species. A. nomius, A. caelatus and A. tamarii were detected only in Alabama, but at very low frequencies (<5%). Multi-locus sequence typing revealed that prior to biocontrol application in North Carolina in 2012, 48% of the isolates were the same haplotype as the biocontrol strain Afla-Guard, which belongs to lineage IB, while only 6% were of the same haplotype as AF36, which belongs to lineage IC. Phylogenetic analyses revealed that strains of the Afla-Guard haplotype persisted in the field throughout the study better than strains similar to the AF36 haplotype. Similar results were observed in Alabama in 2012 and Georgia in 2013, where the percentage of isolates matching Afla-Guard increased from 36% to 68% and 12% to 95% from the pre-application sampling period to the harvest sampling period in Alabama and Georgia, respectively. Many isolates shared the same multi-locus haplotype across all three states, which provides evidence of shared patterns of descent. In addition, a two-tailed binomial test of clone-corrected mating-type data showed that populations at every sampling period, except for the Alabama preapplication sampling periods in 2012 and 2013, had a recombining population structure consistent with sexual reproduction. Aflatoxin levels in harvested grain were generally low in untreated plots (2 to 104 ppb) in all locations and years. However, significant differences in aflatoxin contamination of grain between treated and untreated plots were observed in North Carolina in 2012. Here, contamination was also significantly lower in grain from plots treated with Afla-Guard than from those treated with AF36. These results seem to suggest that matching the biocontrol product to the predominant A. flavus haplotype of the native soil population can increase efficacy of biocontrol of aflatoxin contamination. The use of biocontrol strains that are genetically similar to native soil populations may be important in the sustainable use of biological control to mitigate aflatoxin contamination in corn. © Copyright 2014 Mary Hunt Lewis