BIOFUMIGATION POTENTIAL OF BRASSICAS - III - IN-VITRO TOXICITY OF ISOTHIOCYANATES TO SOIL-BORNE FUNGAL PATHOGENS

Isothiocyanates (ITCs) released from Brassica crops or seed meal amend ments incorporated into soil have the potential to suppress pest and d isease organisms in soil. We investigated in vitro toxicity of six ITC s to the mycelial growth of five cereal root pathogens (Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Bipo laris sorokiniana, Pythium irregulare) by either adding them to the he adspace above, or dissolving them in the growing media. Four alkenyl a liphatic ITCs (methyl-ITC, propenyl-ITC, butenyl-ITC, pentenyl-ITC) an d two aromatic ITCs (benzyl-ITC and 2-phenylethyl-ITC) were tested. Ar omatic ITCs were less toxic in the headspace experiments due to their lower volatility which reduced the headspace concentration, but were m ore toxic than the aliphatic ITCs when dissolved in the agar. In both experimental methods, the toxicity of the aliphatic ITCs decreased wit h increasing length of the side chain although there was little differ ence between methyl-ITC and propenyl-ITC in the headspace experiment. The fungi differed in sensitivity to the ITCs. Gaeumannomyces was the most sensitive, Rhizoctonia and Fusarium intermediate and Bipolaris an d Pythium the least sensitive. Pythium was 2-16 times more resistant t han the other fungi to the ITCs dissolved in agar and, in contrast to the other fungi, was more sensitive to the aliphatic ITCs than the aro matic ITCs. Suppression of some fungi by propenyl ITC and 2-phenylethy l ITC, principal products of glucosinolate hydrolysis in Brassica tiss ue, was superior to that of the synthetic fumigant methyl-ITC, suggest ing an important role for these compounds in the pest suppression pote ntial of brassicas. The variation in toxicity of different ITCs to the fungi (up to 7 fold) suggests there is significant scope to enhance t he biofumigation potential of brassicas by selecting those which produ ce large quantities of GSL precursors to the most toxic ITCs for the t arget organism.