Suppressive biomasses and antagonist bacteria for an eco-compatible control of Verticillium dahliae on nursery-grown olive plants

Two organic amendments (OMW-M1 and OMW-M2), based on olive mill wastes (OMWs) subjected to advanced processes of aerobic static storage or composting, were tested for their suppressive activity against Verticillium dahliae , the causal agent of olive Verticillium wilt. OMW-M1 and OMW-M2 drastically inhibited the pathogen growth in vitro and then were further tested in suppressive pot experiments. The amendments, mixed at 15 % (v/v) with a nursery standard plant-growth matrix, were tested alone or in combination with two biocontrol bacteria (Bacillus amyloliquefaciens and Burkholderia cepacia ) selected from suppressive soils. All mixtures were artificially contaminated with V. dahliae microsclerotia (MS), the density of which was periodically monitored by either a semi-selective medium or a specific real-time PCR technique. In plant-less pot assays conducted in a growth chamber, OMW-M1 was the most effective amendment, reducing V. dahliae MS density by 100 % after 90 days with respect to the untreated control. In nursery experiments with pot-growing olive plants, OMW-M1, particularly when combined with the biocontrol bacteria, confirmed its strong suppressive activity reducing up to 100 % the density of V. dahliae MS in the rhizosphere behaving even better than a commercial biofungicide (Trichoderma asperellum TV1) used as a control. The best combined treatment also reduced plant mortality and increased root and shoot extension. We conclude that organic amendments from stabilized olive mill by-products showed positive agronomic and phytosanitary properties on pot-growing olive plants and, particularly when enriched with biocontrol agents, they are potentially suitable for use in sustainable agriculture.