Combined application effect of citrus essential oils against the phytobacterium Streptomyces scabiei

Essential oils (EOs) are plant natural products typically composed of a complex mixture of molecules, many of them recognized for their antibacterial potential. EOs have different benefits in contrast to synthetic agrochemicals, like no persistence on the environment, compatibility with classical biocontrol agents and natural enemies, and no toxicity for most non-target organisms. The aim of this work was to evaluate the antibacterial effect of lemon (Citrus limon) and orange (Citrus sinensis) EOs and their combinations with mint (Mentha x piperita) or oregano (Origanum vulgare) EOs against Streptomyces scabiei, the main causal agent of the potato common scab. The EOs were characterized by GC-MS. The antibacterial activity of citrus EOs and combinations were evaluated using the broth microdilution method and the effect of the EOs on the S. scabiei envelope was analyzed by scanning electron microscopy. The lowest EO antibacterial concentrations were obtained for the combination of lemon and mint EOs, mixed in equal quantities. The application of this combination of lemon EO on S. scabiei cultures caused cell envelope damage, causing changes in the superficial hyphae anatomy and narrowing their thickness. Additionally, orange EO efficacy was enhanced in combination with oregano EO, suggesting that this binary mixture could improve their utilization as bactericidal agents against S. scabiei. These results position citrus EOs and some of their combinations as promising sustainable alternatives for potato common scab control.