Olive mill wastes: A source of resistance for plants against Verticillium dahliae and a reservoir of biocontrol agents

•OMW compost, GR9, activity against Verticillium dahliae is mainly due to biotic factors.•Blastobotrys sp. FP12, Arthrobacter sp. FP15 were isolated from plants grown in GR9.•FP12 and FP15 reduced V. dahliae disease severity on eggplants.•FP12, FP15 and GR9 reduced V. dahliae microsclerotia germination. Verticillium dahliae is a destructive vascular wilt soil fungus distributed worldwide. The disease causes serious economic losses to a large number of crops, since there are no chemical treatments to control it. In parallel, the olive producing countries face every year the challenge of managing the olive mill wastes (OMW). Recycling through composting constitutes a feasible and agronomically valuable strategy of dealing with the OMW. Therefore, it is of vital importance for the Mediterranean countries, the main olive producers in the world, to promote through scientific research the usage of OMW composts in agriculture. In the present study, the microbial nature involved in the suppressiveness of an OMW compost amendment (GR9) against V. dahliae was investigated. It was revealed that heat sterilization of GR9 resulted in partial loss of its suppressiveness pointing out the presence of microorganisms antagonistic to V. dahliae. For this purpose, several microbes were isolated from the rhizosphere of eggplants grown in the compost and tested in vitro against V. dahliae. One bacterial and one yeast like fungus, identified as members of the Arthrobacter and Blastobotrys genera, respectively, were selected for further evaluation under glasshouse conditions. The ability of the microbial agents to reduce Verticillium wilt symptoms was demonstrated in a single root and split root experiment; indicating the possible triggering of induced systemic resistance. Furthermore, it was observed that application of the two microorganisms or OMW compost reduced the percentage of V. dahliae microsclerotia germination and the number of hyphae per germinated microsclerotium in planta.