4.5.1
BRASSICACEAE WITH HIGH CONTENT OF TIOFUNCTIONALIZED GLUCOSINOLATES AS GREEN MANURE TO CONTROL SOIL-BORNE FUNGI

LM MANICI, L LAZZERI, G BARUZZI, O LEONI and S PALMIERI

Istituto Sperimentale per le Col, Bologna, Italy

Background and objectives
Glucosinolates are a class of about 120 secondary plant metabolites present in Brassicaceae organs where they seem to play a defensive role. Glucosinolates and the endogenous enzyme myrosinase, kept separate in intact plant cells, come into contact after mechanical wounding or pathogen attack, generating hydrolysis-derived products with a well known biological activity. Our in vitro studies on the fungitoxic activity of enzyme hydrolysis-derived products of 12 different glucosinolates, mainly isothiocyanates, showed that their inhibition activity varied significantly depending on their side-chain structure. In particular, the thiofunctionalized glucosinolates had a fungicidal-fungistatic activity 10-20 times higher than alkenyl or hydroxy-alkenyl glucosinolates [1]. In addition, the fungal sensitivity varied widely and, in particular, Pythium and Phytophthora were the most sensitive genera followed by mycelia sterilia fungi [1]. The amount of glucosinolates and myrosinase in Brassicaceae plant organs suggests the possibility of controlling soilborne pathogens by plugging them into the soil. We report the results obtained on Pythium soil inoculum with plants chosen for their particular qualitative and quantitative content of glucosinolates in fresh tissues: Iberis amara cv. isci10, Rapistrum rugosum cv. isci3, and Eruca sativa cv PRISCA, selected for a high content of thiofunctionalized glucosinolates, and Crambe abyssinica cv. Mario, containing hydroxy-alkenyl glucosinolates.

Materials and methods
Fresh tissues of Brassicaceae, collected at the flowering stage, characterized by a special qualitative and quantitative glucosinolate content, were mixed in pots with a naturally infested soil, collected after a long period of strawberry cultivation. The amount of fresh tissue per pot was calculated on the basis of mean field production of each crop; sunflower was chosen as a control. Untreated soil was included as a reference. Pots were maintained in the greenhouse at 26-28C under natural light and watered once a week. Pythium spp. and total fungi were recorded with the soil dilution-plate method using selective media at the start and after 3 weeks, 6 weeks and 2 months.

Results and conclusions
In naturally infested soil with fresh tissue of biocidal plants, the number of total fungal propagules increased as in the control; it increased seven- to eight-fold compared to untreated soil, and this difference was maintained throughout the trial time. In the control treated with glucosinolate-free tissues, Pythium (which can survive as saprophyte on plant debris) increased three- to four-fold in comparison with untreated soil. In treated soil we observed that all Brassicaceae reduced the Pythium inoculum significantly. In particular, Iberis and Rapistrum reduced the number of Pythium propagules below that determined in untreated soil. Rucola and Crambe always reduced by about 50% the number of propagules determined in the control. The inhibition of Pythium spp. observed in soil, when it was treated with plants containing thiofunctionalized glucosinolates, confirms our results previously obtained in vitro [1]. This finding, and the high sensitivity to thiofunctionalized glucosinolates of some other important soilborne fungal pathogens, suggests the possibility of benefiting from the use of green manure together with the suppression of Pythium and, presumably, of other soilborne pathogens.

References
1. Manici LM, Lazzeri L, Palmieri SJ, 1997. Journal of Agricultural and Food Chemistry 45, 2768-2773.