Pilot project on Xylella fastidiosa to reduce risk assessment uncertainties

Abstract

Xylella fastidiosa (X. fastidiosa) has a very broad host range, including many cultivated and wild plants common in Europe. There is, however, scant information on the potential hosts of X. fastidiosa in the natural European flora, as a wide range of these plants have never been exposed to the bacterium. Investigations carried out in the recent years (2013-2015) in Apulia (southern Italy) where X. fastidiosa was first recorded on olive trees, determined that: (i) the strain associated with this outbreak, denoted CoDiRO, is genetically related to the subspecies pauca, representing a variant classified as “sequence type 53”; (ii) CoDiRO is consistently associated with infections occurring under natural conditions on several hitherto unknown hosts of X. fastidiosa. The aim of the studies conducted in this pilot project was to assess the host range of the Apulian strain of X. fastidiosa by artificial inoculations and exposure to infective vectors, of selected cultivars of major perennial crops and some forest species. Diagnostic tests, isolation and symptom evaluation were performed to assess bacterial colonization in the different hosts and the development of symptoms associated withX. fastidiosa infections. The overall results of molecular assays and bacterial isolation tests carried out up to 14 months post-inoculation clearly differentiated the plant species in which rapid colonization of the plants occurred from those that did not support X. fastidiosa multiplication and movement. Bacterial inoculation of olives, oleanders and Polygala myrtifolia plants resulted in systemic colonization by the bacterium, and symptoms resembling those observed under natural infection conditions were observed. Indeed, the results relative to different olive cultivars confirmed the high susceptibility of this crop to strain CoDiRO and the consistent association of the infections with the appearance of symptoms of dieback and desiccation of the inoculated plants. Conversely, inoculated plants of citrus, grapes and Quercus ilex were never found to be systemically infected nor did they develop any suspicious symptom. In addition, field experiments confirmed, although with a different transmission rate, that infective Philaenus spumarius w as able to transmit the bacterium to the host plants used in the field experiment (olive, oleander and Polygala myrtifolia). Specifically, serological and molecular assays readily detected the bacterium in these host plants as soon as six months after caging the infective vectors, when the plants had not yet shown any symptom. In agreement with the results of the artificial inoculation, none of the citrus, grape or Q. ilex plants so far tested positive for X. fastidiosa upon exposure to infective P. spumarius.