Colonization of wounded peanut seeds by soil fungi: selectivity for species from Aspergillus section Flavi

Abstract

Soil is a source of primary inoculum for Aspergillus flavus and A. parasiticus, fungi that produce highly carcinogenic aflatoxins in peanuts. Aflatoxigenic fungi commonly invade peanut seeds during maturation, and the highest concentrations of aflatoxins are found in damaged seeds. A laboratory procedure was developed in which viable peanut seeds were wounded and inoculated with field soil containing natural populations of fungi, then incubated under different conditions of seed water activity and temperature. Densities of Aspergillus section Flavi in soil used for inoculating seeds were low relative to the total numbers of filamentous fungi (<1%). Aspergillus species from section Flavi present in soil included A. flavus morphotypes L and S strains, A. parasiticus, A. caelatus, A. tamarii and A. alliaceus. Wounding was required for high incidences of fungal colonization; viability of wounded seeds had little effect on colonization by Aspergillus species. Peanut seeds were colonized by section Flavi species as well as A. niger over broad ranges of water activity (0.82–0.98) and temperature (15–37 C), and the highest incidences of seed colonization occurred at water activities of 0.92–0.96 at 22–37 C. A. parasiticus colonized peanut seeds at lower temperatures than A. flavus, and cool soil temperatures relative to temperatures of aerial crop fruits might explain why A. parasiticus is found mostly in peanuts. Other fungi, dominated by the genera Penicillium, Fusarium and Clonostachys, colonized seeds primarily at water activities and temperatures suboptimal for section Flavi species and A. niger. Eupenicillium ochrosalmoneum frequently sporulated on the conidial heads of section Flavi species and showed specificity for these fungi. The inoculation of wounded viable peanut seeds with soil containing natural populations of fungi provides a model system for studying the infection process, the interactions among fungi and those factors important in aflatoxin formation.