Location: Peanut Research
Title: Relationship Between Soil Densities of Aspergillus Species and Colonization of Wounded Peanut Seeds Author
Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 8, 2006
Publication Date: October 18, 2006
Citation: Horn, B.W. 2006. Relationship between soil densities of Aspergillus species and colonization of wounded peanut seeds. Canadian Journal of Microbiology. 52:951-960. Interpretive Summary: Peanuts are often invaded by Aspergillus flavus and A. parasiticus, fungi that reside in soil and produce the carcinogenic aflatoxins. Peanut seeds that have been wounded by insects are particularly susceptible to invasion by aflatoxin-producing fungi. A laboratory procedure was developed in which viable peanut seeds are wounded and inoculated with soil directly from the field, then incubated under controlled conditions of temperature and relative humidity. In this study, the relationship between soil density of aflatoxin-producing fungi and peanut seed infection was examined by inoculating peanut seeds with 20 different soils. Infection of peanut seeds increased with increasing soil densities of A. flavus and A. parasiticus but was limited at higher densities by competition with related Aspergillus species that do not produce aflatoxins. Therefore, the composition of fungal populations in soil may affect the capacity of peanuts to become contaminated with aflatoxins.
Technical Abstract: The relationship between soil density of Aspergillus species and the incidence of peanut seed colonization under laboratory conditions was examined. Viable peanut seeds were wounded and inoculated with 20 soils differing in composition and density of Aspergillus species, then incubated for 14 d at 37 ºC (seed water activity = 0.92). The effect of soil density of individual section Flavi species (A. flavus L and S strains, A. parasiticus, A. caelatus, and A. tamarii), section Nigri, and A. terreus on the incidence of seed colonization was best expressed as an exponential-rise-to-maximum function. Exponential curves often rose to maximum percentages of seed colonization by section Flavi species that were well below 100% despite high species densities in some soils. A statistically significant interactive effect between soil densities of individual section Flavi species and potentially competing Aspergillus species (other section Flavi species and section Nigri) may explain the reduced incidences of seed colonization by section Flavi species. An average of two or fewer propagules of each Aspergillus species in soil at the wound site was required for colonization of 20% of peanut seeds. Other fungal species were capable of invading peanut seeds only when soil densities of section Flavi and Nigri species were low.