Aspergillus flavus control: do we need to worry about spores?

The toxic metabolite produced by Aspergillus flavus and Aspergillus parasiticus represents a significant morbidity and mortality factor to wildlife feeding on contaminated grain. A 10% Clorox® bleach solution has often been... [ view full abstract ]

The toxic metabolite produced by Aspergillus flavus and Aspergillus parasiticus represents a significant morbidity and mortality factor to wildlife feeding on contaminated grain. A 10% Clorox® bleach solution has often been recommended for Aspergillus spp. control. Although this concentration may kill the fungal colonies, it is unclear whether spores remain viable, thereby allowing repopulation. Our objective was to determine the concentration of Clorox® bleach solution needed to effectively kill spores of A. flavus. Aspergillus flavus was plated onto 16 Petrie dishes containing potato dextrose agar and incubated at room temperature (23 C) for 4 days. A 1-cm diameter sterile paper disk soaked to saturation with each treatment (autoclaved water control, 10, 12, and 14% Clorox bleach solution from a bottle of 8.25% active ingredient of sodium hypochlorite) was placed over an actively growing A. flavus colony in each Petrie dish. Twenty-four hours later, spores from each dish were extracted from the zone covered by the paper disk and placed into individual agar Petrie dishes as follows: water control—5 Petrie dishes of 4 spores each (20 total spores); treatments of 10%, 12%, and 14% bleach solution—25 dishes (100 spores) each, and incubated at room temperature for 5 days. Fungal colonies exposed to a bleach-saturated paper disk were killed (100%) at each treatment level, while the control group remained unaffected. None of the spores from the control group were killed, while 10%, 12%, and 14% bleach solutions killed 89%, 96%, and 100% of the spores, respectively. We conclude that although A. flavus colonies may be killed by 10% Clorox® bleach solution, such a concentration is not sufficient to reliably kill spores, which would lead to repopulation. We discuss the implications of A. flavus control, based on our findings.