Xylella fastidiosa: A Serious Threat to the Mediterranean Olive Heritage.

fastidiosa, a quarantine organism included in the EPPO list A1, is a gram negative bacterium which has long been known to affect a large number of plant species (ca 300), primarily grapevine (Pierce disease) and citrus (Citrus Variegated Chlorosis), mainly in the American continent. In October 2013, it was reported for the first time in the Euro-Mediterranean region, on olive trees in Apulia, which is the main olive-producing region of Italy (Saponari et al., 2013). Its presence is highly associated to the olive quick decline syndrome (OQDS), a severe disease which is killing thousands of Olive trees (Olea europaea), mainly the ancient specimen which have been landmarks of the Mediterranean region for thousands of years. However, associated to OQDS are also fungal species belonging to the genus Phaeoacremonium and Phaemoniella, which may play a role in the severity of the symptoms. A great variability among olive varieties in OQDS severity, associated to the presence of X. fastidiosa, has been assessed in South Italy. Based on such variability a different impact of the infection could be envisaged in each country based on pathogen susceptibility of native olive varieties. The new X. fastidiosa strain found in Italy and named “CoDiRO” belongs to the subspecies pauca; it infects more than 20 host species, primarily olive, cherry, polygala and oleander but not grapevine and citrus spp.. The high level of genetic identity with the X. fastidiosa strains from Costa Rica could explain the potential origin of the CoDiRO strain from Central America, from which millions of ornamental plants are annually imported without targeted inspections at the EU entry points. Indeed, after the first finding in EU, several new interceptions of the bacterium occurred in other European countries (e.g. France, The Netherlands) mainly on ornamental coffee plants imported from Costa Rica and Honduras. In 2015, X. fastidiosa subsp. multiplex, was intercepted in France on Polygala myrtifoliaand other ornamentals. Apart the movement through infected propagating material, in nature this pathogen can be vectored by insects. The assessed vector of the Apulian strain is the spittlebug Philaenusspumarius, a polyphagous Auchenorrhyncha insect widely distributed in the Euro-Mediterranean region, which is rapidly disseminating the bacterium in olive groves (Cariddi et al., 2014). At least two more species, Neophilaenuscampestris and Euscelislineolatus, proved to be capable of harbouring the bacterium, although there is no evidence so far that they can transmit the infection (Elbeaino et al., 2014). There is no record of successful eradication of X. fastidiosa once established outdoors due to its broad range of plant hosts and vectors. However, control measures in EU are mainly based on the prevention of introduction of the pathogen in free areas and on the containment of the outbreak where the pathogen is not well established yet. The severe threat posed by X. fastidiosa in Italy, which is a new strain mainly affecting olive trees, prompted the Italian Ministry of Agriculture to appoint a special Commissioner to deal with this phytosanitary “emergency” and to establish a national scientific committee for advising technical decisions.

CIHEAM of Bari has developed and applied in the official monitoring programme efficient and innovative monitoring and detection methods, some of which have been already included in the official monitoring of the pathogen in Italy: OQDS-suspected trees are identified by photointerpretation of high resolution aerial images, field data area acquired through an application (XylApp) and sent to a central server (XylWeb) for their rapid storage and analysis (D’Onghia et al., 2015). Moreover, on site pathogen detection has been developed in plants and ‘spy insects’ by real time LAMP (loop mediated isothermal amplification) and DTBIA (Direct tissue blot immuno assay) in order to avoid the movement of infected plant material in pathogen-free areas. Apart the uprooting of infected plants in the buffer zones, where the pathogen should be absent, vector control is also mandatory based on an integrated management approach against all life stages of the insect. Furthermore, plant nurseries are submitted to regulatory restrictions concerning the production and movement of Xf-host plants.

The experience gained on X. fastidiosa in Italy should be soon considered by all Mediterranean countries for taking an immediate action primarily for the safeguard of the olive trees in the whole region, posing attention to ornamental plant species, which are main carriers of most undesirable pests because scarcely regulated by the law. To the aim efficient early surveillance programmes of the infection, funds for developing specific research and strong information campaigns oriented to the whole civil society should be soon taken into account.

Anna Maria D’Onghia

Head of the Division on Integrated Pest Management. CIHEAM-Mediterranean Agronomic Institute of Bari, Italy