A comparison of soil and corn kernel Aspergillus flavus populations: evidence for niche specialization

Aspergillus flavus is an opportunistic fungal pathogen that infects peanuts, cotton, corn and tree nuts. Aspergillus flavus is a major problem globally due to the production of acutely toxic and carcinogenic aflatoxins. Louisiana climatic conditions lead to annual threats of corn aflatoxin contamination. The purpose of this study was to determine the specific ability of different strains of A. flavus to infect corn. Five soil samples and 10 corn ears were collected from each of seven corn fields throughout Louisiana. In addition, Francis Deville of Monsanto Company collected 7, 6, 2, and 4 soil samples and corn ears from four additional fields in Louisiana. Six hundred twelve and 255 A. flavus colonies were isolated from the corn and soil samples, respectively. Isolates were characterized by vegetative compatibility groups (VCGs), sclerotia size, aflatoxin B1 (AFB1) production, mating type and 8 simple sequence repeat loci polymorphisms. Eighty-eight percent of corn isolates belonged to two VCGs, whereas only 5% of soil isolates belonged to the same two VCGs. Ninety-five percent of corn isolates did not produce any sclerotia, whereas 56% and 41% of soil isolates produced small and large sclerotia, respectively. The mean AFB1 production on rice for corn kernel isolates was 2314 ± 7455 ppb and 10248 ± 11430 ppb for the soil isolates. Ninety-six percent of corn isolates were in the Mat1-2 mating type whereas only 52% of soil isolates were Mat1-2. SSR fingerprints revealed 26 haplotypes in the corn sample isolates and 78 in the soil sample isolates. All characteristics differed significantly between the soil and the corn kernel populations. Differences between the corn and soil populations indicate that not all soil isolates are as capable of infecting corn and that some isolates have become specialized to infect corn. Further understanding of virulence of A. flavus is important for the development of a better biocontrol against toxigenic A. flavus and possibly more resistant hybrids of corn