EVALUATION OF INTRASPECIFIC COMPETITION (ASPERGILLUS FLAVUS LINK) AND AFLATOXIN FORMATION IN SUSPENDED DISC CULTURE

Authors: Wicklow, Donald; Bobell, John; Palmquist, Debra

Submitted to: Mycological Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/28/2003
Publication Date: 5/1/2003
Citation: Wicklow, D.T., Bobell, J.R., Palmquist, D.E. 2003. Evaluation of intraspecific competition (aspergillus flavus link) and aflatoxin formation in suspended disc culture. Mycological Research. 107(5): 617-623. 2003

Interpretive Summary: A promising strategy for eliminating preharvest aflatoxin of susceptible crops, especially in environments conducive to an aflatoxin outbreak, involves field applications of non-aflatoxin producing strains of A. flavus or A. parasiticus, which displace the naturally occurring aflatoxin-producing strains through mechanisms of intraspecific competition. There is a need to explain the underlying mechanism(s) of competition and this study describes a novel procedure for evaluating Aspergillus interference with aflatoxin production. Substantial inhibition of aflatoxin was demonstrated in mixed cultures of Aspergillus, even where both competitors were aflatoxin-producing strains. Direct and anecdotal evidence is offered that aflatoxin inhibition results from the failure of the mixed cultures to establish a cooperative mycelial network. The suspended disc culture system provides an efficient means for evaluating the outcome of A. flavus intraspecific competition on aflatoxin production in vitro.

Technical Abstract: The ability of two non-aflatoxin producing strains of Aspergillus flavus (NRRL 32354; NRRL 29269) to interfere with aflatoxin production by A. flavus NRRL 32355 was examined using a replacement series with the suspended disc culture method (R.A. Norton, 1995). Individual glass fiber discs, affixed to a pin suspended from the caps of humidified 20 ml scintillation vials, were inoculated by adding 90 ul of a chemically defined salts (SL) medium containing 5% glucose and including A. flavus conidial mixtures in the following proportions (aflatoxin producer: non-producer = 100:0, 80:20, 60:40, 20:80, and 0:100) at a constant total density (1 x 10 4 spores/ml). Significant (p < 0.0001) reductions in aflatoxin were recorded when NRRL 32354 or NRRL 29269 represented any proportion of the inoculum mixture. Aflatoxin yield (ug/ml) values were less than expected (p < 0.0001) from the input ratios for toxigenic vs. atoxigenic conidial inoculum within the replacement series. Aflatoxin yields were also reduced (p < 0.001), with a corresponding increase in fungal growth (p < 0.001), when conidia from pairs of the following aflatoxin producing strains, NRRL 32355, NRRL 26473, NRRL 26474 or NRRL 26491, were mixed in equal proportions. It is suggested that the substantial inhibition of aflatoxin yield for inoculum mixtures results from the failure of spore germlings to establish a cooperative mycelial network. The suspended disc culture system provides an efficient means for evaluating the outcome of A. flavus intraspecific competition on aflatoxin production in vitro.