Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/5/2009
Publication Date: 1/15/2010
Citation: Mehl, H.L., Cotty, P.J. 2010. Variation in Competitive ability among Isolates of Aspergillus flavus from Different Vegetative Compatibility Groups during Maize Infection. Phytopathology. 100:150-159. Interpretive Summary: Aflatoxins are carcinogenic toxins produced by fungi in Aspergillus section Flavi that frequently contaminate food and feed crops. Atoxigenic biocontrol strains of A. flavus are used to limit aflatoxin contamination by competitive exclusion of aflatoxin producers, but lack of knowledge about competitive interactions among genetically diverse isolates of A. flavus limits optimization of this management strategy. We developed a pyrosequencing assay to quantify competitive differences among isolates of A. flavus from 38 different vegetative compatibility groups and assessed the influence of competitive interactions on aflatoxin contamination of maize kernels. We demonstrated that isolates vary in both competitive ability and their influence on aflatoxin contamination. However, ability to infect kernels and sporulate on kernel surfaces was not always equal, suggesting isolates may be adapted to different ecological niches. A successful biocontrol strain must become a multi-year dominant component of the A. flavus community. The results indicate that assessing both colonization and sporulation during co-infection is important when selecting isolates that are best adapted for biocontrol.
Technical Abstract: Aspergillus flavus, the primary causal agent of aflatoxin contamination, includes many genetically diverse vegetative compatibility groups (VCGs). Competitive ability during infection of living maize kernels varied among isolates from 38 VCGs. Kernels were inoculated with both a common VCG, CG136, and a test VCG; after 7 days (31ºC), conidia were washed from kernels, and aflatoxins and DNA were extracted from kernels and conidia separately. CG136 -specific SNPs were quantified by pyrosequencing; VCGs co-inoculated with CG136 produced 46 to 85 and 51 to 84 percent of A. flavus DNA from kernels and conidia, respectively. Co-inoculation with atoxigenic isolates reduced aflatoxin up to 90% and, in some cases, more than predicted by competitive exclusion alone. Conidia contained up to 42 ppm aflatoxin B1 indicating airborne conidia as potentially important sources of environmental exposure. Aflatoxin-producing potential and sporulation were negatively correlated. For some VCGs, sporulation during co-infection was greater than that predicted by kernel infection suggesting that some VCGs increase dispersal while sacrificing competitive ability during host tissue colonization. The results indicate both life strategy and adaptive differences among A. flavus isolates and provide a basis for selection of biocontrol strains with improved competitive ability, sporulation, and aflatoxin reduction on target hosts.