Mode of action of pesticides on aflatoxin biosynthesis and oxidase system activity.

The effects of nine pesticides on the biosynthesis of aflatoxin and oxidase activity in wild-type Aspergillus flavus and mutant strains of A. parasiticus avr-1 (w 49) and A. parasiticus ver-1 (wh 1) were investigated. In A. parasiticus, phosphonic acid derivative (lancer) reduced the formation of aflatoxin B2 but B1, G1 and G2 and anthraquinones (versicolorin A, versiconal hemiacetal acetat and averufin) accumulated. Phosphorothioic acid derivatives (pirimiphos-methyl and pyrazophos) reduced the formation of aflatoxin B2 and G2 but B1 and G1 and anthraquinones accumulated. Phosphorodithioic acid derivatives (dimethoate and malathion) blocked aflatoxin B2, reduced B1 and G2 but G1 and anthraquinones accumulated. Phosphoric acid derivative (profenfos) inhibited the formation of all aflatoxins, versicolorin A and versiconal hemiacetal acetate but averufin accumulated. The phenylurea derivatives (linuron and pencycuron) at concentrations of 500 and 1000 ppm inhibited all aflatoxin but anthraquinones accumulated. On the other hand, the dicarboximide derivative (iprodione) inhibited the whole pathway in the mutant strains of A. parasiticus. The oxidase system in wild-type A. flavus was active in the conversion of averufin and versicolorin A into aflatoxin B1. Most organophosphate and phenylurea derivatives may competitively increase or decrease the oxidase enzymes, however, profenfos and iprodione blocked the enzymes between averufin and versicolorin A.