Inhibitory activity of olive mill waste and biocontrol bacteria against soilborne plant pathogens

Soilborne diseases, mainly caused by fungal pathogens, determine severe loss in yield of several horticultural and fruit crops world-wide. In the last decades they have been occurring with an increasing frequency and severity in nursery as well as in open field cultivations. By far, their control has been relied on the use of synthetic pesticides with consequent increasing of environmental concerns. It is therefore necessary to develop and test alternative methods, more environmentally friendly. Preventive methods, adopted even in the nursery (i.e. pathogen free soil and planting material, suppressive substrates and biocontrol agents), are considered key factors for an efficient disease control. Recently, alternative plant growing substrates containing natural organic amendments have been studied not only for agronomic properties, but also for their suppressiveness against soilborne pathogens. This study aimed at evaluating the suppressive activity against fungal infections of a selected natural amendment made up of destoned olive mill wet husk mixed with dry hygroscopic organic additives (waste wool, straw and sawdust), used after a short period of aerobic storage. In our assays the amendment was used alone or enriched with selected bacteria (Bacillus amyloliquefaciens and Burkholderia cepacia) as biocontrol agent against fungal pathogens (Fusarium oxysporum f.sp. lycopersici, Phytophthora spp. and Verticillium dahliae). In order to preliminary assess possible antifungal activity of amendment water extracts (AWE), bioassays were carried out in vitro in multiwell plates, using unsterilized or sterilized AWEs. Higher reduction of fungal biomass production was exerted by the unsterilized AWE thus evidencing the repressiveness exerted by the natural antagonist microflora contained in the amendment. The amendment was also tested for its repressiveness against soilborne fungi in pot experiments, in mixture (15% v/v) with a standard artificially contaminated substrate. The density of fungal pathogen propagules was periodically monitored by semi-selective media. The results showed a significant reduction of fungal pathogen propagules in the amended rhizosphere, particularly when the substrate was enriched with biocontrol bacteria. Further studies are in progress to optimize the suppressive activity of these innovative substrates for a more eco-compatible plant protection strategy.