Substrate-dependent isothiocyanate toxicity

The impending loss of methyl bromide as a fumigant has stimulated interest in isothiocyanates (ITCs), antimicrobial volatiles produced in Cruciferous tissues, for soil-borne pathogen control. We tested benzyl-, butyl-, ethyl-, and phenylethyl-ITC for relative efficacy against Fusarium oxysporum f. sp. dianthi in one sand and three soil substrates. Ethyl-ITC was consistently the most efficacious in all soil types. Benzyl-ITC was generally the least efficacious, and butyl- and phenylethyl-ITCs were variable, appearing relatively efficacious in some soils, and relatively non-efficacious in others. Lethal-dosage-50s (LD50s) for the tested ITCs were dependent on substrate. ITC persistence in the different substrates varied as well. Persistence, however, did not correlate with LD50. Correlations between substrate characteristics, LD50s and ITC persistence suggests that substrate pH is related to phenylethyl-ITC LD50 and benzyl-, butyl- and ethyl-ITC persistence. This study demonstrates the importance of soil type in ITC toxicity to soil-borne pathogens.