Variation in allyl isothiocyanate production within Brassica species and correlation with fungicidal activity

Bassica nigra (black mustard) and B. juncea (Indian mustard) genotypes were tested for pathogen suppression and release of allyl isothiocyanate (AITC) , a fungitoxic volatile produced in mustard tissue after enzymatic hydrolys is of allyl glucosinolate (sinigrin). In bioassays, 28 genotypes of B. nigr a and 35 genotypes of B, juncea were screened for inhibition of the potato pathogens Helminthosporium solani and Verticillium dahliae by volatiles rel eased from macerated leaf tissue. Release of AITC from plant tissue was qua ntified by gas chromatography; isothiocyanate profiles were determined by h eadspace analysis. All mustard genotypes produced compounds that suppressed radial growth of both fungi. Growth suppression and AITC release differed significantly (P < 0.001) among genotypes of B. nigra and B. juncea. Mustar d treatments releasing >1.2 mg AITC/g plant tissue were fungicidal to both pathogens. Headspace analysis confirmed that allyl glucosinolate was the ma jor glucosinolate in all genotypes of B. nigra tested; most genotypes also produced 2-phenylethyl-isothiocyanate (ITC). Brassica juncea genotypes prod uced variable amounts of AITC and other volatiles with antimicrobial activi ty, including 2-phenylethyl-ITC, benzyl-ITC, and 3-butenyl-ITC. Evaluating mustards from geographically diverse locations allowed selection of mustard genotypes that may be useful in breeding programs designed to develop dise ase-suppressing green manure cultivars.