Antibacterial activity of carob (Ceratonia siliqua L.) extracts against phytopathogenic bacteria Pectobacterium atrosepticum

• The leaf extracts of carob (Ceratonia siliqua) showed antimicrobial activity in vitro against P. atrosepticum.
• Potato (Solanum tuberosum, var, nicola) tuber rot tissues were analyzed by LC–MS and GC–MS.
• Secondary metabolites were identified including phytoalexins and fatty acids.
• The combinations of extracts of carob leaves in the presence of potato tuber extract were evaluated.
• The possibility of use of antimicrobial extracts in combination in treating soft rot caused by Pca.

Acetone and ethanol extracts of carob (Ceratonia siliqua L.) leaf and pods were evaluated for their in vitro inhibitory ability against the pectinolytic Gram negative Pectobacterium atrosepticum (Pca, CFBP-5384) bacteria, the causal agent of potato soft rot. Potato (Solanum tuberosum, var nicola) tuber rot tissues obtained after 5 day bacterial inoculation was analyzed by LC–MS and GC–MS to study Pca pathogenicity. Trans/cis N-feruloylputrescine was identified in potato tuber after 5-day inoculation with Pca in a dark moist chamber. Although glycoalkoloid (α-chaconine and α-solanine) production increased due to Pca soft rot infection, it was not a resistance-determining factor. Many secondary metabolites were identified including the phytoalexins solavetivone and fatty acids responsible for plant defence responses. Acetone extract of carob leaf (FCA) exhibited the strongest inhibitory effect (IC50 = 1.5 mg/ml) and displayed synergistic antimicrobial effect in the presence of infected potato tuber extract (Pdt-Pca extract) against Pca. This synergy could be used in an integrated control program against potato soft rot pathogens, thereby reducing chemical treatments.