Characterization of a fungistatic substance produced by Aspergillus flavus isolated from soil and its significance in nature.

A fungus capable of using vegetable tissues for multiplication in soil was isolated and identified as Aspergillus flavus based on morphological characteristics and sequence similarity of ITS and 28S. When grown in liquid medium prepared from the same vegetable tissues used in soil amendment, the isolate of A. flavus produced a substance capable of preventing disease development of black leaf spot of mustard cabbage caused by Alternaria brassicicola and inhibiting the germination of A. brassicicola conidia. The inhibitory substance was fungistatic, and was very stable under high temperature and high or low pH value. It was soluble in ethanol or methanol, moderately soluble in water, and insoluble in acetone, ethyl acetate or ether. The inhibitor is not a protein and has no charges on its molecule. This is the first discovery of the production of a fungistatic substance by this deleterious fungus. Results from this study suggest the possession of a strong competitive saprophytic ability by A. flavus, which in turn may explain the widespread occurrence of this fungus in soils. Production of a fungistatic substance when A. flavus was grown in medium prepared from vegetable tissues suggests the importance of antibiotic production in its competitive saprophytic colonization of organic matters in soils.