Biocontrol potential of Pseudomonas protegens against Heterobasidion species attacking conifers in Europe

The biocontrol potential of a commercial product based on the bacterium Pseudomonas protegens (strain DSMZ 13134) and of its cell-free filtrate (CFF) was tested under controlled conditions in vitro and on wood discs against genotypes of the three native European Heterobasidion species and on the non-native H. irregulare, all reported as destructive forest pathogens in Europe. In vitro experiments through traditional dual culture assays and by incorporating CFF into the media showed that treatments were effective in reducing significantly and regardless of the culture medium and incubation temperature both mycelial growth and rate of conidial germination of Heterobasidion spp., and that antibiosis could be the main mechanism involved in the inhibition of Heterobasidion spp. Outcomes of dual culture assays performed on two-divided Petri plates further suggest that antibiosis operates through the production of diffusible rather than volatile compounds. Based on comparative experiments on wood discs of preferential hosts of Heterobasidion spp., CFF performed significantly better than the commercial product against most of Heterobasidion spp., further confirming that the antagonistic activity hinges on antibiosis rather than on other mechanisms. While on wood discs the tested treatments seem poorly effective against the non-native H. irregulare, CFF significantly and substantially reduced infections of the native European Heterobasidion species compared to controls mostly to the level of the state of the art chemical treatment urea and of the biological treatment based on Phlebiopsis gigantea.