Soft Rot Pectobacteriaceae (SRP) bacteria as Pectobacterium and Dickeya are phytopathogenic agents responsible of blackleg disease on several crop as potato, affecting the yield and depressing the seed production quality. However, neither conventional nor biocontrol product are available on the market to control this disease. In this study Pseudomonas PA14H7, a bacteria isolated from potato rhizosphere, was selected as a potential antagonist agent against Dickeya. In order to understand the mechanism involved in this antagonism we managed to identify the main active(s) molecule(s) produced by PA14H7. In that way, cell-free supernatant (CFS) of PA14H7 cultures were extracted and analyzed by LC-MS, GC-MS and NMR. We have correlated biological activity in extracted CFS-PA14H7 to the presence of 7-hydroxytropolone (7-HT) complexed with iron. Even if this molecule is produced by other Pseudomonas and mostly known for its antibacterial and antifungal activities, it is the first description of its involvement as an effective molecule against pectinolytic bacteria. We have synthesized 7-HT in order to determine the real amount produced by PA14H7 and to compare its activity at the same concentration with that of CFS-PA14H7. This study aims to understand the origin of the antagonist activity of PA14H7 as a potential biocontrol agent against potato blackleg.