Screening the olive tree phyllosphere: Search and find potential antagonists against Pseudomonas savastanoi pv. savastanoi

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fmicb.2020.02051/full#supplementary-material

Olive knot (OK) is a widespread bacterial disease, caused byPseudomonas savastanoipv.savastanoi(Pss), which currently has not effective control methods. The use of naturally occurring microbial antagonists, such as bacteria, as biocontrol agents could be a strategy to manage this disease. The objective of this work was to select bacteria from olive tree phyllosphere able to antagonizePssusingin vitroandin plantaexperiments. The elucidation of their modes of action and the potential relationship between antagonism and bacteria origin has been investigated, as well. To this end, 60 bacterial isolates obtained from the surface and inner tissues of different organs (leaves, twigs, and knots), from two olive cultivars of varying susceptibilities to OK, were screened for theirin vitroantagonistic effect againstPss. A total of 27 bacterial strains were able to significantly inhibitPssgrowth, being this effect linked to bacteria origin. Strains from OK-susceptible cultivar and colonizing the surface of plant tissues showed the strongest antagonistic potential. The antagonistic activity was potentially due to the production of volatile compounds, siderophores and lytic enzymes.Bacillus amyloliquefaciensP41 was the most effective antagonistic strain and their capacity to control OK disease was subsequently assayed usingin plantaexperiments. This strain significantly reduces OK disease severity (43.7%), knots weight (55.4%) and population size ofPss(26.8%), while increasing the shoot dry weight (55.0%) and root water content (39.6%) ofPss-infected olive plantlets. Bacterial isolates characterized in this study, in particularB. amyloliquefaciensP41, may be considered as promising biocontrol candidates for controlling OK disease.

This work was funded by FEDER funds through COMPETE (Programa Operacional Factores de Competitividade), national funds through FCT (Fundacao para a Ciencia e a Tecnologia) and by Horizon 2020, the European Union's Framework Programme for Research and Innovation, within the project PRIMA/0002/2018 (INTOMED - Innovative tools to combat crop pests in the Mediterranean), and the Mountain Research Center - CIMO (UIDB/00690/2020 and UIDB/04046/2020).