Microbial Degradation of Fenamiphos After Repeated Application To a Tomato-Growing Soil

Abstract

Soil was obtained from a tomato-growing field near Bowen, Queensland, where the nematicide fenamiphos was reported to have lost its effectiveness following 14 years of regular use. A bioassay using the fungal-feeding nematode Aphelenchus avenae demonstrated that nematicidal activity in this soil was lost in 2 wk, whereas similar soil that had never been treated with fenamiphos retained significant nematicidal activity for at least 6 wk. Results of further experiments with the bioassay suggested that microorganisms were responsible for enhanced degradation of fenamiphos. Significant nematicidal activity was retained for 6 wk following autoclaving or gamma-irradiation, but was lost in untreated soil in less than 2 wk. Furthermore, nematicidal activity of fenamiphos in mixtures of autoclaved and non-autoclaved tomato-growing soil decreased as the proportion of non-autoclaved soil was increased. Despite evidence that enhanced microbial degradation was occurring, the degradative organisms were not isolated. Twelve fungal and 70 bacterial isolates were obtained from the soil by enrichment with fenamiphos, but none degraded the nematicide when added individually or as mixtures to autoclaved soil.