Glucosinolates, isothiocyanates and biofumigation : a potential alternative method of controlling soilborne pests

The systematic phase-out of methyl bromide in the United States has begun and will be complete by 2005. As the predominant soil fumigant, loss of MeBr will have a dramatic and immediate impact on agriculture. Viable alternative methods of controlling soilborne plant diseases are required. Glucosinolates (GS), found in Brassicaspp. L. and other members of the Brassicaceae family, are naturally occurring secondary metabolites that exist in intact plant tissues. When tissue is damaged, myrosinase, normally physically isolated from GS, is released and catalyzes GS into the volatile isothiocyanates (ITC). These pungent chemicals have been shown to control some pests in laboratory experiments.

This study investigated, the application of Brassica spp. as a potential alternative for controlling soilborne pest. Three sets of experiments were conducted: 1) Indian mustard (B. juncea) and allyl ITC inhibition of Sclerotium roflsii Sacc. mycecial growth were investigated., 2) In a three year study, Brassica spp. were used as cover crops and incorporated into soil (biofumigation) to determine effects on tomato fruit yield. 3) Intact GS from seed extracts were separated using capillary electrophoresis and individual ITC were assayed to determine their toxicity to Botrytis cinerea Pers.: Fr., Penicillium expansum Link, S. roflsii, Pythium myriotylum Drechs., and Agrobacterium tumefaciens(Smith and Townsend) Conn.

The concentrations of Indian mustard needed to produce 50% and 90% inhibition (IC₅₀ and IC₉₀) were 0.74 and 0.98 g‧L headspace volume. Allyl ITC IC₅₀ and IC₉₀ were calculated at 1.6 and 4.5 µmol‧L^-1, respectively. The concentrations of Ally ITC produced by the Indian mustard were lower than could account for the total inhibition demonstrated by the Indian mustard.

For the 2000 crop, tomato plots biofumigated with fall raab cv. "Salade," demonstrated 40% increase in marketable fruit yield when compared to control plots (P< 0.01). Indian mustard biofumigated plots showed on 18.2 % increase in marketable fruit yield (PThe IC₉₀ for methyl ITC, allyl ITC, phenethyl ITC, cheirolin, erucin and brassinin were calculated for each of the pathogens tested. Brossinin had the lowest IC₉₀ value for B. cinerea at 26 µMoles mL^-1. Phenethyl ITC had the strongest level of inhibition against P. myriotylum, S. rolfsii, and P. expansum with IC₉₀ values of 11 -, 27-, and 22 26 µMoles mL^-1, respectively. Agrobacterium tumefaciens was less sensitive to the compounds tested. Erucin was projected to provide 90% inhibition at 126 26 µMoles mL^-1. However, only allyl ITC and phenethyl ITC achieved 90% inhibition within the tested concentration range, with IC₉₀ values of 395- and 213 µ/Moles mL^-1, respectively.

While there is still a vast amount of research to be done in this area, the potential for Brassica spp. and other GS containing plants to be used to control soilborne pests is great. With the information discovered to date, biofumigation using these plants could provide organic and "green" growers with measurable control of some soil pests.