IN-VITRO SUPPRESSION OF FUNGAL ROOT PATHOGENS OF CEREALS BY BRASSICA TISSUES

The superior growth of wheat following Brassica crops compared to that following non-Brassica crops may be due to the suppression of soilbor ne fungal pathogens by volatile isothiocyanates (ITCs) released in the soil during hydrolysis of glucosinolates contained in Brassica tissue s. We investigated the effects of volatile compounds released from the root, shoot and seed meal tissues of canola (Brassica napus) and Indi an mustard (Brassica juncea) on the mycelial growth of five soilborne pathogens of cereals-Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Pythium irregulare and Bipolaris soroki niana. Three isolates of each species, originally collected from the r oots of wheal (Triticum aestivum) and barley grass (Hordeum leporinum) in southern Australia, were exposed to volatiles released in vitro wh en sterile water was added to freeze-dried Brassica tissues. The root and shoot tissues of both Brassica species were more suppressive at fl owering than maturity and mustard tissues were generally more suppress ive than canola. The degree of fungal suppression by the various Brass ica tissues was related to the concentration and type of isothiocyanat es released, which varied with Brassica species, tissue age and tissue type. There were significant differences in the sensitivity of the fu ngal species and among isolates of each species. Gaeumannomyces and Rh izoctonia were generally the most sensitive to the volatiles released, Pythium and Bipolaris the least. The results indicate that the effect iveness of fungal suppression by Brassica crops will depend upon the s pecies, age and type of Brassica tissue, which influence the type and concentration of isothiocyanates evolved, and the sensitivity of the p athogen.