Submitted to: Mycologia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 7, 2004
Publication Date: June 1, 2005
Citation: Horn, B.W. 2005. Colonization of wounded peanut seeds by soil fungi: selectivity for species from aspergillus section flavi. Mycologia. 23:425-450. Interpretive Summary: The carcinogenic aflatoxins produced by the molds Aspergillus flavus and A. parasiticus are common contaminants of peanuts. Peanuts are invaded by aflatoxin-producing molds before harvest under conditions of late-season drought and warm soil temperatures. The cost of managing aflatoxins is often very high, averaging $25 million annually to the peanut industry in the southeastern United States alone. The highest levels of mold infection and aflatoxin contamination are associated with seed damage caused primarily by insects. For this study, a laboratory technique was developed in which living peanut seeds were wounded and the wounds were inoculated with soil directly from the field. Wounding was required for high levels of infection. Even though A. flavus and A. parasiticus made up less than 1% of the total molds in soil, wounded peanut seeds were preferentially invaded by A. flavus and A. parasiticus over broad ranges of seed moisture and temperature. A. parasiticus invaded peanut seeds at lower temperatures than A. flavus, and cool soil temperatures relative to temperatures of above-ground crops such as corn and cotton may explain why A. parasiticus is found mostly in peanuts. The inoculation of wounded living peanut seeds with soil containing natural populations of aflatoxin-producing molds provides a model system for studying the infection process, the interactions between molds, and those factors important in aflatoxin formation. This information is necessary for devising new strategies for controlling aflatoxin contamination of peanuts.
Technical 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. Aspergillus species from section Flavi present in soil included A. flavus 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. Despite the low soil densities of section Flavi relative to the total numbers of filamentous fungi (< 1%), wounded peanut seeds were preferentially 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). The highest incidences of seed colonization occurred at water activities of 0.92-0.96 at 30-37 C. A. parasiticus colonized peanut seeds at lower temperatures than A. flavus, and cool soil temperatures relative to temperatures of aerial crop fruits may explain why A. parasiticus is found mostly in peanuts. Other fungi, dominated by the genera Penicillium, Fusarium and Gliocladium, 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 strong 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 between fungi, and those factors important in aflatoxin formation.