Antifungal properties of Eucalyptus endophytic Streptomyces strains

This study characterized three bacterial endophytes isolated from healthy Eucalyptus camaldulensis trees in Tehran, Qom, and Mazandaran provinces of Iran. 16 S rRNA sequencing identified all isolates as Streptomyces genus. Their antifungal potential was evaluated against four tomato pathogens: Botrytis cinerea, Fusarium oxysporum f. sp. lycopersici, Macrophomina phaseolina, and Rhizoctonia solani. Dual-culture assays revealed strong antagonistic activity, with inhibition rates ranging from 59% to 95% against most pathogens. However, Streptomyces sp. 3L3 showed no activity against R. solani. In volatile organic compound assays, all strains suppressed B. cinerea, while Streptomyces sp. 3L3 additionally inhibited F. oxysporum f. sp. lycopersici. In contrast, no antagonistic activity was observed against the other tested pathogens. Enzymatic assays confirmed cellulase and chitinase production in all strains, suggesting potential antagonistic mechanisms, though phosphate solubilization capacity was absent. In greenhouse experiments on tomato plants, the dual effects of pathogen control and plant growth promotion was evaluated. Among the tested strains, Streptomyces sp. QS4 exhibited the highest inhibition rate against all pathogens, except for B. cinerea, which was most strongly suppressed by Streptomyces sp. GL4. Additionally, plants inoculated with both endophytes and pathogens showed significantly less growth reduction compared to pathogen-only controls. Gas chromatography-mass spectrometry analysis identified several bioactive metabolites in the three bacterial strains, with hexadecanoic acid, di-(2-ethylhexyl)phthalate, and tetradecanoic acid being the most abundant compounds, all known to contribute to biocontrol and plant growth modulation. These findings demonstrate that the Streptomyces endophytes exhibit significant antifungal activity through multiple mechanisms, underscoring their potential as biocontrol agents.