Postharvest biocontrol ability of Pseudomonas synxantha against Monilinia fructicola and Monilinia fructigena on stone fruit

The biocontrol properties of the endophyte Pseudomonas synxantha DLS65 were tested in vitro and in vivo against Monilinia fructicola and Monilinia fructigena, causal agents of postharvest brown rot of stone fruit. P. synxantha cells significantly reduced the mycelial growth of both pathogens on Potato Dextrose Agar (PDA), and strongly inhibited the Monilinia fructicola growth on Peach Extract Agar (PEA). Cell-free culture filtrates inhibited the pathogens on PDA and PEA to lesser extent. The production of volatile organic compounds (VOCs), with in vitro inhibitory effects on mycelial growth, was also observed. P. synxantha significantly reduced brown rot incidence and severity on peach fruit artificially inoculated with M. fructicola after 5 d at 25 °C. Moreover, P. synxantha more significantly reduced incidence and severity after 10 d at 10 °C and after 20 d in cold storage at 0 °C in comparison to control fruit, even if its activity was never comparable to that of the synthetic fungicide Scholar® (fludioxonil). Similarly, P. synxantha exhibited an excellent antagonistic activity against M. fructigena on fruit at 10 and at 0 °C, and a weak biocontrol activity at 25 °C. Competition for nutrients and space, production of diffusible toxic metabolites and VOCs may play a role in the antagonism of P. synxantha toward M. fructicola and M. fructigena, especially at the lowest temperatures of storage. For that reason, this strain of P. synxantha could be suggested as active ingredient for the setting up of bioformulates against Monilinia species representing a limiting factor for stone fruit production.