Submitted to: Mycopathologia
Publication Type: Abstract only
Publication Acceptance Date: 10/14/2003
Publication Date: 2/11/2004
Citation: 163:48 Interpretive Summary: Not required for "abstract" only.
Technical Abstract: Soil is the source of primary inoculum for Aspergillus flavus and A. parasiticus, fungi that produce the highly carcinogenic aflatoxins in agricultural commodities. Aflatoxigenic fungi commonly invade peanut seeds during maturation and the highest concentrations of aflatoxins are found in insect-damaged seeds. A laboratory assay was developed in which sterile, viable peanut seeds were wounded and inoculated with different soils (n = 20) from cultivated and fallow fields and from forested land. The effect of fungal density in soil on percent seed infection best fits a logarithmic function showing a rapid rise in infection with increasing fungal density followed by saturation at a particular level of infection. Coefficients of determination (r2) were significant (P < 0.001) for all species in section Flavi: A. flavus L strain (0.70), A. flavus S strain (0.91), A. parasiticus (0.55), A. tamarii (0.85), A. caelatus (0.59) and A. alliaceus (0.83, linear function). Species from other sections in the genus, A. niger (0.75) and A. terreus (0.93), showed similar logarithmic regressions. Other species of fungi were rare on peanut seeds when inoculated with cultivated soils and they were detected only on seeds inoculated with soils containing extremely low densities of section Flavi species and A. niger. Nearly all wounded seeds became infected with one or more species from section Flavi at > 400-500 CFU/g of soil. An estimated 1-3 propagules at the wound site were sufficient for 50% infection of peanut seeds. Maximum percent infection by individual species often occurred at considerably less than 100% despite high soil densities. The order of species according to maximum infection levels approximated the frequency at which species are found in peanuts in the field: A. flavus > A. parasiticus > A. tamarii and A. caelatus. Competition by A. niger and other members of section Flavi, as evidenced by high densities in soil, may be responsible for limiting infection by aflatoxigenic species.