Olive mill residues affect saprophytic growth and disease incidence of foliar and soilborne plant fungal pathogens

Olive oil mill residues constitute a major environmental problem. Although these wastes have a high fertilizer value when applied to the soil, there is concern about their use because of their antimicrobial and phytotoxic properties. Furthermore, their effect on saprophytic growth and pathogenicity of soilborne and foliar fungi is poorly known. In this study we investigated the effects of olive mill dry residue (DOR) on: (a) growth of the four crop species Lepidium sativum, Lycopersicon esculentum, Lactuca sativa and Triticum aestivum; (b) saprophytic growth of the four phytopathogenic fungi Fusarium oxysporum f.sp. lycopersici (FOL), Fusarium culmorum (FC), Sclerotinia minor (SM) and Botrytis cinerea (BC); (c) influence of soil amendment with DOR on the three plant–soilborne pathogen systems, L. esculentum–FOL, T. aestivum–FC, L. sativa–SM, and the two plant–foliar pathogen systems, L. esculentum–BC and L. sativa–BC. Residues resulted phytotoxic, both in laboratory and greenhouse bioassays, for all plant species in relation to their concentrations. L. sativum and L. sativa were the most sensitive species to the residues, followed by L. esculentum and T. aestivum. In contrast with the results observed with plant species, the performances (radial growth and hyphae density) of the tested phytopathogenic fungi were positively affected by DOR. In greenhouse bioassays, L. sativa mortality imputable to SM increased on soil amended with DOR. BC foliar disease dramatically increased on L. sativa and L. esculentum plants grown on soil amended with DOR at all used concentrations. Differently, soil amendment with DOR did not significantly affect the disease incidence of FC on T. aestivum and FOL on L. esculentum. Our study demonstrates that, in controlled conditions, undecomposed DOR affects the growth of crop species (phytotoxic effect) and phytopathogenic fungi (substrate effect), and that the interaction between these factors, in some cases, drives to an increase of fungal disease incidence.