ABILITY OF SOME SOIL FUNGI TO DECREASE THE ACTIVE PRINCIPLE CONCENTRATION OF BIOFUMIGATION BY BRASSICA CARINATA SEED MEAL

Biofumigation is a low-impact alternative to chemical disinfestation of soil, based on the glucosinolate-myrosinase defensive system of Brassicaceae plants, ploughed into the soil as green manure or seed meal. The enzymatic degradation of glucosinolates by myrosinase occurring into the soil delivers volatile isothiocyanates, which are very effective against several soil pathogens. The combination of this technique with a biological control agent like the filamentous fungus Trichoderma, selected for tolerance to allyl-isothiocyanates, in previous trials gave interesting synergic effects in controlling sugar beet damping-off by Pythium ultimum. On the other hand, it was found that a non pathogenic isolate of Aspergillus flavus was able to actively reduce the concentration of allyl-isothiocyanate in soil during biofumigation, thus hampering its effectiveness. A screening was then carried out in vitro among different non-pathogenic fungal isolates in order to investigate their interaction with allyl-isothiocyanate. The results showed differential responses: in particular some isolates of Trichoderma were able to decrease allyl-isothiocyanate concentration similarly to A. flavus, while the same Trichoderma isolate used in the biofumigation experiment against P. ultimum, was not. These findings suggest that the soil fungal community could be one of the causes of the reduced isothiocyanate concentration often found in biofumigated soil with respect to expectations. Therefore, Trichoderma isolates, selected for their antagonistic behaviour against the pathogens, if used in combination with biofumigation, should be carefully evaluated not only for their tolerance to isothiocyanates, but also for their ability to interact with these molecules.