Suppression of chlorothalonil degradation in soil after repeated application

Mechanisms of degradation suppression of chlorothalonil after repeated application were investigated. Under laboratory conditions, soil was treated sequentially with 40 mg/kg dry weight of chlorothalonil. This resulted in the degradation suppression of chlorothalonil and accumulation of the metabolite 4‐hydroxy‐2,5,6‐trichloroisophthalonitrile (TPN‐OH) in soil. After pretreatment with three kinds of chlorothalonil metabolite, only TPN‐OH was found to suppress chlorothalonil degradation in soil. After the pretreatment of TPN‐OH, residual chlorothalonil in soil at 1 week after application was 9 to 11% higher than those with pretreatment with other metabolites and control. The TPN‐OH pretreatment also suppressed soil respiration and TPN‐OH was more persistent in soil than chlorothalonil, indicating that toxicity of residual TPN‐OH may suppress the degradation of chlorothalonil in soil.