Technical Abstract:
Insect-damaged peanut seeds are highly susceptible to contamination by carcinogenic aflatoxins produced by A. flavus and A. parasiticus, fungi belonging to Aspergillus section Flavi.  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 L and S strains, A. parasiticus, A. caelatus, A. tamarii and A. alliaceus.  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. Optimal temperatures for seed colonization were lower for A. parasiticus and A. caelatus than for A. flavus.  A. parasiticus was the only species from section Flavi to colonize peanut seeds at 15 C after extended incubation (35 d).  Cool soil temperatures relative to temperatures of aerial crop fruits may explain why A. parasiticus is more frequent in peanuts than in corn and cottonseed.  Other fungal genera (Penicillium, Fusarium and Clonostachys) colonized seeds primarily at water activities and temperatures suboptimal for section Flavi species and A. niger.  The inoculation of wounded viable peanut seeds with soil provides a model system for studying the infection process, the interactions between fungi, and those factors important in aflatoxin formation.