Olive mill composted residues as new resource for bio-control of plant pathogens

Olive mill wastes (two and three-phase olive husks, olive mill wastewater) might cause unfavourable impact on plants, soil structure, microbial population and activity, aquatic ecosystems and even on air media because of the phenolic, fatty acid, mineral salts content and high COD and BOD5. In the last decade, many pilot and industrial scale composting trials on agricultural residues (including solid and liquid olive wastes) proved the reliability of this microbial biotechnology to allow the transformation of these by-products into organic fertilizers (cured composted residues) with no phytotoxicity, and able to improve soil fertility and plant production (Alfano et al., 2008). More, olive mill composted residues showed positive properties which suppress soilborne plant pathogens (Lima et al., 2008). In fact, cured composts consistently reduced the growth in vitro of Verticillium dahliae and other important fungal pathogens. The inhibitory activity decreased or disappeared when the cured composts was autoclaved before its use. The inhibitory activity seem to involve the beneficial residual microbial population selected spontaneously during the composting process. In other trials performed on young olive plants grown on soil artificially contaminated by Verticillium dahliae microsclerotia (MC), when the antagonistic fungus Trichoderma viride was enriched with 15% (w/w) of OMW cured compost, a significant reduction in the density of Verticillium dahliae MS in the soil, was observed. In conclusion the application of good quality cured composts with suppressive characteristics for the control of fungal pathogens of some important vegetal crops in organic and integrated agriculture systems seem to be a very promising strategy.