Assessment of Paraburkholderia phytorfirmans PsJN biocontrol potential against Xylella fastidiosa 'De Donno' strain in olive

Numerous attempts have been made to test the use of endophytic bacteria to control diseases caused by Xylella fastidiosa (Xf), but promising results obtained in vitro have often proved inadequate once transferred to the field. A new encouraging possibility came from the observation that the bacterium Paraburkholderia phytofirmans PsJN, widely studied for its ability to defend plants from biotic and abiotic stresses, was able to reduce the symptoms caused by Xf in grapes affected by Pierce’s disease. Our study aimed at testing the effectiveness of PsJN as a biocontrol agent in the Xf strain ‘De Donno’/olive pathosystem. Although in vitro tests showed the absence of competitive inhibitory effects of PsJN on Xf ‘De Donno’ growth or its ability to form biofilm, several trials were started both in greenhouse and in Xf-contaminated orchards. Although different approaches were tested to deliver PsJN in olives, high rates of successful isolations and positive detection, achieved with a SYBR? -Green-based qPCR assay ad hoc developed in this study, were obtained only upon needle inoculation of 1- to 2-year-old shoots, in which PsJN proved to remain viable for >500 days. Current observations in open-field trials, so far limited to a single season, have not revealed significant differences in the reduction of OQDS symptoms or Xf concentration in therapeutic treatments, between plants treated or not with PsJN, nor reduction of the new infections upon preventive applications. Despite the evidence that PsJN can colonise the xylem vessels, time course diagnostic tests clearly showed that it moves slowly away from the point of inoculation and its concentration decreases significantly over time. This absence of systemic colonisation suggests a possible induced response of the plant, which will need to be analysed in detail, to optimise treatments and trigger a cascading effect on Xf. In this direction, preliminary results of PsJN impact, on the resident microbiome diversity indices, in presence/absence of Xf, have been gathered using a WGS approach