Suppression of soil-borne plant pathogens in growing media amended with espresso spent coffee grounds as a carrier of Trichoderma spp

Espresso spent coffee grounds (SCG) are a high-value organic waste material often reported as a suitable soil amendment in horticulture. The aim of the present work was to evaluate the suppressiveness characteristics of SCG by the analysis of antagonistic microorganisms colonizing SCG and by a bioassay with cress grown on peat amended with 10% SCG and infested with the damping off causal pathogens Sclerotinia sclerotiorum and Phytophthora nicotianae. Among culturable microorganisms, mesophilic bacteria were the main forms colonizing SCG followed by actinomycetes. Fungal species isolated from SCG were represented by Trichoderma atroviride and, at a lesser extent, T. citrinoviride and Aspergillus sp. In vitro assays revealed that sterilized SCG were able to strongly inhibit mycelial growth of both S. sclerotiorum and P. nicotianae. By contrast, T. atroviride representative isolates Ta3 and Ta4, isolated from SCG, were able to rapidly colonise sterilized SCG. This result indicates that this species is insensitive to antimicrobial compounds present in SCG and are probably involved in the mechanism of soilborne pathogen suppression. Both Trichoderma isolates were characterised for their antagonistic activity. In dual culture assays, T. atroviride Ta3 and Ta4 inhibited the mycelial growth of both S. sclerotiorum and P. nicotianae and produced toxic volatile and non-volatile metabolites, having complementary inhibitory effects on growth of the target pathogens. Results from the bioassay revealed that sterilized peat-SCG substrate significantly reduced damping-off of cress due to P. nicotianae but failed to suppress that caused by S. sclerotiorum. When the sterilized peat-SCG substrate was enriched with T. atroviride Ta3 and Ta4 a consistent decrease of damping off caused by the two pathogens was recorded. SCG was found to be a valuable source of nutrients and not phytotoxic on cress at 10% peat substitution rate, but decreased plant growth. Present results indicate that SCG, a suitable component of mixed-peat or fortified substrates, are capable to provide disease suppression and represents an optimal substrate to support growth and activity of antagonistic Trichoderma.