Distribution of Aspergillus section flavi among field soils from the four agroecological zones of the Republic of Benin, West Africa

Certain members of Aspergillus section Flavi produce carcinogenic and immunotoxic metabolites called aflatoxins. These fungi perennate in soils and infect maize grain in the field and in storage. The distribution of Aspergillus section Flavi across the four different agroecologies of Benin Republic was determined. The four agroecological zones range from humid equatorial tropics in the south to the dry savanna near the Sahara desert in the north. Soil samples collected in 1994 to 1996 from 44 different maize fields in Benin were assayed over 3 years (88 samples total) for fungi in Aspergillus section Flavi. All soils tested contained A. flavus. Isolates (1,454 total) were collected by dilution plate from the soils and existed in populations ranging from <10 to >200 CFU/g of soil. CFU counts did not differ from year to year or change significantly with cropping systems within a zone, but differed significantly among zones. Incidence of A. flavus strain isolations varied from south to north, with greater number of CFU of L strain isolates in southern latitudes and higher numbers of CFU of S strain isolates found in the north. The L strain isolates occurred in 81 of 88 samples, whereas S strain isolates were in only 41 of 88 soil samples. Of 96 L strain isolates tested, 44% produced aflatoxins. Only B toxins were produced, and toxigenic isolates averaged over 100 mug of aflatoxin B-1 per 70 ml of fermentation medium (similar to1.4 ppm). All S strain isolates produced both B and G aflatoxins, averaging over 557 mug of aflatoxin B, per 70 ml (8 ppm) and 197 mug of aflatoxin G, per 70 ml of fermentation medium (2.8 ppm). A. parasiticus and A. tamarii were present in less than 10% of the fields and were not associated with any particular agroecological zone.