ECOLOGICAL BASIS AND PROSPECTS FOR BIOFUMIGATION
JA KIRKEGAARD1, M SARWAR1 and JN MATTHIESSEN 2
1CSIRO Plant Industry, GPO Box 1600, Canberra 2601, Australia; 2CSIRO Entomology, Private Bag P0 Wembley, WA 6014 Australia
The use of chemical fumigants for soil-borne pest and disease control is coming under increasing environmental scrutiny world-wide. Most notably, the international phase-out of ozone-depleting methyl bromide is stimulating a reappraisal of fumigant use, and driving the search for alternatives and options that cause less ecological disruption or that help limit overuse. While fumigants with less noxious residues may fill some of the void, such as the methyl isothiocyanate-generator metham sodium, there is a widespread desire to utilize approaches that are less aggressive. A need also exists for less costly methods of soil disinfestation in less intensive production systems and in developing countries.
Biofumigation refers to the suppression of soil-borne pests and pathogens by biocidal compounds (principally isothiocyanates) released in soil when glucosinolates in brassicaceous crop residues are hydrolysed . Hydrolysis occurs rapidly when the glucosinolates and the enzyme myrosinase, located in different cells within the intact plant, come into contact upon tissue disruption. Brassicaceous green manure and rotation crops or seed meal amendments have been reported to suppress a range of soil-borne pests and pathogens, although the results are variable. Most of the field evidence for suppression is empirical, rarely having supporting data of the glucosinolate types or concentrations present in the incorporated tissues. Glucosinolate profiles and concentrations vary greatly between species and cultivars and are influenced by environmental factors. Furthermore, glucosinolate hydrolysis products also vary widely in toxicity to different organisms, some of the plant-derived isothiocyanates being inherently more toxic than the methyl isothiocyanate derived from synthetic metham sodium . There is clearly scope to systematically develop lines with enhanced biofumigation potential [3,4].
Interest in developing biofumigation as a pest and disease management alternative, or adjunct to chemical fumigants has been heightened by concerns about the sustainability of fumigant-dependent intensive production systems. There is also considerable attraction in harnessing biofumigant rotation crops such as rapeseed (canola) for suppressing noxious organisms such as fungal root pathogens in broad-scale cereal production systems where chemical fumigation is impractical.
Assessing the biofumigation potential of different brassicas requires a knowledge of the type and concentration of glucosinolates present in the plant tissues and information on the toxicity of their various hydrolysis products to target organisms. We have recently carried out a systematic study to compare the glucosinolate production of diverse field-grown brassicas, including glucosinolates which have often been ignored [3,5]. Associated studies have been carried to measure the toxicity of pure isothiocyanates and tissue hydrolysis products against a variety of organisms . The process aims to determine biofumigation potential and to investigate the factors influencing that potential. Understanding the nature of this variation is essential to assessing and developing opportunities to harness and enhance the desired biofumigation effects.
A fundamental understanding of the chemistry, genetics, biological activity and factors influencing biofumigant effects is emerging. As the genetics of glucosinolate production within brassicas, the profile and distribution of glucosinolates within different tissues and the biological activity of different isothiocyanates in soil becomes clearer, there will be increased scope to manipulate plants to optimise biofumigation potential. It may provide a more ecologically acceptable means of suppressing soil-borne pests and diseases than reliance on heavy doses of synthetic fumigants.