Biocontrol Activity of Actinomycetes Strains against Fungal and Bacterial Pathogens of Solanum lycopersicum L. and Daucus carota L.: In Vitro and In Planta Antagonistic Activity

Plants are affected by various biotic and abiotic stresses due to climate change. Tomato and carrots are important crops that are attacked by various pathogens. Fourteen plant growth promoting actinomycetes bacteria (PGPB) of the genus Streptomyces sp. and Nocardiopsis sp. were selected for the biocontrol of several common fungal and bacterial pathogen strains. Antifungal activity was assessed against Fusarium oxysporum f. sp. radicis-lycopersici (FORL) and Rhizoctonia solani (RHS). Antibacterial activity was evaluated against Pseudomonas syringae, Pseudomonas corrugata, Pseudomonas syringae pv. actinidiae, Pectobacterium carotovorum subsp. carotovorum, In vitro antifungal and antibacterial antagonistic activities were evaluated by dual culture method. Fungal-bacterial interaction zones were also analysed by scanning electron microscopy (SEM). From strains with good biocontrol potential, cell-free culture filtrates (CF) were produced and investigated for their in vitro antifungal and antibacterial activity. The most effective strains were also combined in consortium and utilized for the seed treatment for in planta pre-emergence biocontrol assays on both S. lycopersicum and D. carota. For each pathogenic strain, four experimental conditions were compared: CNT (without PGPB/infection), PGPB (with PGPB/no infection), PGPB+INF (with PGPB/infection), INF (with infection/no PGPB). The PGPB strains H12 (Streptomyces albidoflavus) and H14 (Nocardiopsis aegyptica) showed good in vitro antifungal (inhibition > 50%) and antibacterial (inhibition > 10 mm) control. The SEM micrographs showed deterioration of fungal filaments and modification of hyphal structures. The CFs of H12 and H14 were also able to inhibit FORL and RHS in vitro growth (minimum inhibitory concentration of 0.2-0.8%). In planta biocontrol assessments showed that the consortium was effective in reducing the infection effects of both fungal and bacterial pathogens. Consortium allowed normal plant development compared to the control. These results confirm the usefulness of actinomycetes strains as a bio-control agent and can therefore be an alternative to chemicals used in agriculture.