Relationship between soil densities ofAspergillusspecies and colonization of wounded peanut seeds

Abstract

Soil is a reservoir for Aspergillus flavus and A. parasiticus, fungi that commonly colonize peanut seeds and produce carcinogenic aflatoxins. Densities of these fungi in soil vary greatly among fields and may influence the severity of peanut infection. This study examined the relationship between soil density of Aspergillus species and the incidence of peanut seed colonization under laboratory conditions. Viable peanut seeds were wounded and inoculated with 20 soils differing in composition and density of Aspergillus species and were then incubated for 14 days at 37 degrees C (seed water activity = 0.92). The effect of soil density of individual section Flavi species (A. flavus strains L and S, A. parasiticus, A. caelatus, and A. tamarii), section Nigri, and A. terreus on the incidence of seed colonization was best expressed as a function of exponential rise to maximum. 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. Competition primarily among section Flavi species may explain the reduced incidences of seed colonization. An average of two or fewer propagules of each Aspergillus species in the 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 sections Flavi and Nigri species were low.

Analysis of protease activity in Aspergillus flavus and A. parasiticus on peanut seed infection and aflatoxin contamination

“…Firstly, seeds from pods without visible damage may be invaded by aflatoxigenic fungi when plants are drought-stressed. Secondly, pod and seed damage allows for direct fungal invasion from soil (Horn 2006). The success of fungal colonisation in the host depends on its ability to cross the external Fig.…”

“…2b) in the unwounded seeds to the same levels as in the wounded seeds where the testa barrier was broken. Seed damage was consistently associated with aflatoxin contamination (Dowell et al 1990;Horn 2006). Xu et al (2000) reported that A. parasiticus could colonise testa cells, but could not produce aflatoxin until it reached the cotyledons.…”

“…However, they can invade peanut seeds, seedlings, and pods during the pre-and post-maturity stages (Middleton et al 1994). The highest levels of A. flavus and A. parasiticus infection and aflatoxin contamination are associated with seed and pod damage and when plants are drought-stressed (Dowell et al 1990;Holbrook et al 2000;Horn 2006). However, Aspergillus can also colonise undamaged peanut pods and seeds as observed in histological studies (Xu et al 2000).…”

Monoclonal Antibodies to Hyphal Exoantigens Derived from the Opportunistic Pathogen Aspergillus terreus

“…Since the identification of A. terreus by Thom in 1918 (59), the species has been utilized in the biotechnology industry as a source of organic acids (11,20), the serum cholesterol-lowering compound lovastatin (7,62), and proteases and peptides that hydrolyze proteins (32). In contrast to its economic importance in biotechnology, A. terreus causes significant losses by spoiling food and agricultural products (2,14,31). Furthermore, A. terreus has been reported as a human pathogen and can cause superficial, cutaneous, and subcutaneous mycoses that affect the nail bed (28), outer ear canal (60), and skin (16).…”

Bioherbicides for weed control.

“…The dose-response relationship should not be assumed to be linear over a broad range of inoculum concentrations and, as reviewed (Hallett, 2005), application rates are often well beyond the linear range. Density-dependent pathogen mortality, hyperparasitism and competition for infection sites conspire to reduce efficiency at these levels (Newton et al, 1997(Newton et al, , 1998Horn, 2006). Consequently, if inadequate control is provided at a given inoculum rate, it may be more cost-effective to reconsider adjuvants, formulation and delivery systems than to simply increase the dose.…”