Endophytic and epiphytic fungal community associated to olive tree differ in antagonistic activity against Pseudomonas savastanoi pv. savastanoi

Abstract

The Olive knot (OK) caused by the Pseudomonas savastanoi pv. savastanoi (Psv) is an important disease, causing severe damages and yield losses in olive trees worldwide (Olea europaea L.). In a previous study we have isolated this bacterium from the phyllosphere of olive tree, together with many fungal species. In these complex communities, microorganisms compete with each other for space and resources, promoting survival of the best-adapted individuals. This has prompted interest in the exploitation of these microorganisms for OK control. In this study, 48 fungal species from the endo- and epiphytic communities of olive twigs were screened for the growth inhibition of the phytopathogen Psv under in vitro conditions. The time course of interspecific interactions (24, 48, 72 and 144h) was studied in potato dextrose agar and olive leaf + twig extract (OLTE), by assessing a clear zone of growth inhibition around fungal colony. Results showed that epiphytic community was the main reservoirs for antagonistic fungi. Almost 75% of the tested epiphytes inhibited Psv growth, being Dothiorella iberica, Aspergillus felis and Aspergillus brasiliensis the most prominent species. The proportion of antagonists within endophytic community was lower (11%), being the most efficient Epicoccum nigrum and Rhinocladiella similis. The antibacterial activity was observed to be significantly (p<0.01) affected by growth medium and time of interaction. Higher growth inhibition was found in the OLTE culture media, showing that inhibition of these endophytic and epiphytic fungi was specifically enhanced by the host plant extract. Most of the fungi tested (up to 64%) from both microenvironment showed higher antibacterial activity in the first 24 hours of interaction, whereas only 16% and 19% strongly inhibited Psv after 48 and 144 hours of interaction, respectively. Altogether, the results indicate that D. iberica, E. nigrum and A. felis, are the best candidates for the biocontrol of olive knot. These potential biological agents should be considered and further evaluated under natural conditions