Impacts of Competition between Aspergillus Section Flavi Species on Aflatoxin Contamination of Crops

Abstract

Aspergillus section Flavi species are important causal agents of aflatoxin contamination in crops including maize, groundnuts, cottonseed, and tree nuts. These species co-occur in agricultural fields where they compete for nutrients and space and contaminate crops with aflatoxins. Aflatoxin contamination of food and feed impacts the health of humans and animals causing liver damage, stunting in children, immune suppression, and death. There is a need to understand the interactions of aflatoxin-producing species and the role of abiotic and biotic factors in modulating competition between fungi in Aspergillus section Flavi. Furthermore, interactions between atoxigenic strains of Aspergillus, aflatoxin-producing fungi, environmental parameters, and crop hosts need to be studied in detail in order to optimize the aflatoxin biocontrol technology for diverse cropping systems. Thus, the overall goal of this dissertation research was to characterize the impacts of competition between Aspergillus section Flavi species on aflatoxin contamination in crops. The first objective (Chapter 2) was to quantify the influence of temperature on interactions among species of Aspergillus section Flavi. Competition experiments between highly aflatoxigenic S-type species, A. aflatoxiformans (AA) and the unnamed taxon known as the Lethal Aflatoxicosis Fungus (LAF), and L-type species, A. flavus L morphotype (AF) and A. parasiticus (AP), were conducted on maize kernels at 25°C and 30°C. Sporulation by L-type isolates was reduced during competition with S-type fungi at 30°C, while relative quantities of conidia produced by S-type species either increased or did not change during competition. Results indicate that both species interactions and temperature can shape population structure of Aspergillus section Flavi. The second objective (Chapter 3) was to characterize the influence of crop host on outcomes of competition between AF, AP, AA, and LAF during infection and colonization of maize and groundnuts. The two hosts differentially influenced outcomes of competition between species with AF generally more competitive on maize and LAF more competitive on groundnuts. However, isolates within a species varied in competitive ability and in their response to hosts. The third objective (Chapter 4) was to characterize a non-aflatoxigenic A. parasiticus isolate using molecular methods and evaluate its ability to reduce aflatoxin production during co-infection with aflatoxin-producing fungi. Whole genome sequencing revealed an insertion and a premature stop codon in the aflE gene. Atoxigenic A. parasiticus reduced aflatoxins when co-inoculated with aflatoxin-producing species on groundnut. The final objective (Chapter 5) was to assess the ability of an applied biocontrol strain of atoxigenic A. flavus to disperse into the upper canopies of pistachio trees in Arizona. The applied biocontrol strain AF36 was detected at all canopy heights, but overall propagule densities were greatest in the upper canopy. Overall results from this study provides insights on Aspergillus species interaction and the role of abiotic and biotic factors which have a potential to shape Aspergillus species population and community structure and therefore aflatoxin contamination of crops. Results from this study will help development of improved aflatoxin management strategies.