Biofumigation: environmental impacts on the biological activity of diverse pure and plant-derived isothiocyanates

Four pure isothiocyanates (methyl, 2-propenyl, benzyl and 2-phenylethyl isothiocyanate), hydrolysing tissue of two brassicas rich in either 2-propenyl or 2-phenylethyl isothiocyanate, and the methyl isothiocyanate-generating pesticide metam-sodium were tested in vapour exposure tests for biological activity against a model soil insect both in vitro and in the presence of three contrasting soils and under four temperatures from 5 to 20 degrees C. The purpose was to develop an understanding of the factors controlling isothiocyanate release and maintenance in soil in order to identify advantageous attributes to seek in utilising brassicas for isothiocyanate-based biofumigation. Methyl isothiocyanate, structurally the simplest and the most volatile, was the most biologically active isothiocyanate under all conditions. It was less affected by the presence of soil and by lower temperature than the longer-chain aliphatic 2-propenyl isothiocyanate. The activity of the less volatile aromatic isothiocyanates was reduced much more by soil, with a decline up to many thousand-fold in the presence of soil with high organic matter content at lower temperature. Metam-sodium closely reflected the methyl isothiocyanate results. The results indicate that brassicas rich in aliphatic isothiocyanates are more likely to have the potential to exert stronger isothiocyanate-based biofumigation effects than those similarly rich in aromatic isothiocyanates.