The distribution of Aspergillus section Flavi within the Sonoran desert of Arizona was characterized. Colony forming units (CFU) were determined for soil, plant debris and dung samples and Aspergillus section Flavi was characterized by species and strain. Aspergillus section Flavi was present in 83% of 391 samples at an average of 3,313 CFU/g. Two species were detected, A. flavus (S and L strains) and A. tamarii. CFU of section Flavi,
A. tamarii, and A. flavus differed (p=0.05) among substrates. Aspergillus flavus quantities were ten times higher in plant debris and dung than on soil. Quantities of A. tamarii were higher on plant debris than on dung and soil. Quantities of A. flavus in surface soils were weakly correlated, in most plots, with presence of vegetation. Results suggest a “fertile island” distribution of Aspergillus section Flavi in the Sonoran desert with “hot spots” containing high propagule counts in a background of soil with low numbers of propagules. Desert isolates of A. flavus like isolates from agricultural fields of Arizona produce only B aflatoxins, but the average aflatoxin-producing potential of communities in natural areas is lower due to a reduced incidence of the highly toxigenic S strain.

Throughout this study, Aspergillus section Flavi was frequently associated with desert tree legumes. Eighty-seven percent of 270 samples of debris and fruits of mesquite, ironwood, acacia and palo verde were positive for Aspergillus section Flavi, with the same two species present (A. flavus, S and L strains, and A. tamarii). Aspergillus flavus was the most common member of section Flavi in legume debris with an incidence of 87% among 3763 isolates examined. Quantities of A. flavus were highest on mesquite pods with an average of 2.3 x 10 ⁶ CFU/g. In vitro, most desert legumes allow significant growth, reproduction and aflatoxin production by A. flavus, with mesquite pods supporting production of 1 x 10 ¹⁰ propagules/g and 5,000µg/kg of aflatoxin B _1. Twenty percent of legume pods collected in the desert contained measurable quantities of aflatoxin ranging from 1 to >2500 µg/kg. The highest incidence (33%) and average aflatoxin accumulation (891 µg/Kg) was found on mesquite pods. Insect damaged mesquite pods had significantly higher aflatoxin than undamaged pods. Legume debris may be an important reservoir of aflatoxin-producing fungi and pods a significant source of aflatoxin exposure in native Sonoran desert habitats. Plant debris (especially that of tree legumes) and dung serve both as reproductive centers and reservoirs of A. flavus propagules in desert habitats. Because these reservoirs contain high densities of A. flavus with aflatoxin-producing ability, they could impact current biological control strategies using atoxigenic A. flavus strains for the management of aflatoxin contamination in agricultural crops.