Accelerated degradation of metam-sodium and dazomet in soil: characterization and consequences for pathogen control

Metam-sodium (MS) and dazomet (DAZ) are soil fumigants that generate methyl-isothiocyanate (MITC) in soil. The latter effectively controls soilborne plant pathogens. Recently, we have found that repeated applications of MS yield less effective pathogen control. Soil samples collected from 12 field sites in Israel without (nonhistory) or with (history) previous applications of MS or DAZ were tested for MITC generation and dissipation. Accelerated MITC degradation in history soils, as reflected by a sharp reduction in the concentration×time (C×T) values and half-lives of MITC, was evident in most of the tested history soils. Exposure to MS and DAZ in those soils resulted in a significant reduction in Verticillium dahliae, Sclerotium rolfsii and Fusarium oxysporum f. sp. radicis-lycopersici mortality compared to exposure in nonhistory soils. The chemical and biological assays were well correlated. This is the first report of accelerated degradation of MITC resulting from repeated applications of DAZ. MITC was rapidly degraded in soils which had been previously treated with either MS or DAZ, indicating induced cross-accelerated degradation with both fumigants. In two field soils, accelerated degradation of MITC capacity was still evident for 18 to 30 months after treatment. This study suggests that accelerated degradation of MITC is a likely cause of the poor pathogen control observed in commercial potato and peanut fields upon repeated application of the fumigants MS and DAZ, and emphasizes the need for rational use of pesticides to maintain their efficacy.