Xylella fastidiosa is a vector-transmitted bacterial plant pathogen associated with serious diseases in a wide range of plants. It causes Pierce’s disease in grapevine, which is a major problem for wine producers in the United States and South America. X. fastidiosa was detected on olive trees in Apulia, southern Italy, in October 2013, the first time the bacterium has been reported in the European Union. Since then it has also been reported as present in Corsica and the Provence-Alpes-Côte d'Azur region of southern France. Controls are in place to prevent the bacterium from spreading.
Numerous species of xylem sap-sucking insects are known to be vectors of the bacterium. X. fastidiosa also has a broad range of host plants, including many common cultivated and wild plants.
In March 2017 EFSA published a statement on the susceptibility of olive varieties to the Apulian strain of Xylella fastidiosa. A systematic literature search identified 21 references providing results of primary research studies on the susceptibility of olive plants to X. fastidiosa subsp. pauca strain ST53 (also known as CoDiRO).
EFSA has recently addressed a number of other questions related to the role of X. fastidiosa in the ongoing olive dieback outbreak in Apulia, southern Italy. EFSA’s experts concluded in August 2016 that currently there is no scientific evidence to indicate the presence of multiple types of X. fastidiosa in Apulia. They then reviewed new evidence regarding the categorisation of Vitis (grapevine), Citrus, Quercus ilex (holm oak) and Phoenix roebelenii (ornamental dwarf palm) as host plants of X. fastidiosa.
Earlier in 2016 EFSA published a report by the Italian National Research Council on the host range of X. fastidiosa CoDiRO, which confirmed that CoDiRO causes olive dieback.
In February 2016 EFSA’s database of host plants was updated to include 44 new species. The majority of the new species (70%) were identified in southern Italy (Apulia), Corsica and southern France (Provence-Alpes-Côte d'Azur region).
The following month EFSA published a report by the Italian National Research Council on the host range of X. fastidiosa CoDiRO, which confirmed that CoDiRO causes olive dieback.
In April, EFSA concluded that research being carried out in Apulia showed that certain treatments reduce the symptoms of disease caused by X. fastidiosa but do not eliminate the pathogen from infected plants.
EFSA’s experts concluded in August that there was no scientific evidence to indicate the presence of multiple types of X. fastidiosa in Apulia. They then reviewed new evidence regarding the categorisation of Vitis (grapevine), Citrus, Quercus ilex (holm oak) and Phoenix roebelenii (ornamental dwarf palm) as host plants of X. fastidiosa.
In January 2015 EFSA published a full pest risk assessment and evaluation of risk reduction options for X. fastidiosa in the EU. The scientific opinion included a list of host plants and European vectors of the bacterium. In March 2015, EFSA published a report categorising plants for planting, excluding seeds, according to the risk of introduction of X. fastidiosa.
In April EFSA responded to a claim by an Italian non-governmental organisation that a series of fungi – rather than X. fastidiosa – are the main causal agents of olive decline in Apulia. EFSA concluded that there was no scientific evidence that tracheomycotic fungi are the primary cause of olive die-back in Apulia.
A Scientific Opinion published by EFSA in September indicated that hot water treatment – whereby dormant plants and plant parts are submerged for 45 minutes in water heated to 50C – is a reliable method for controlling X. fastidiosa in dormant grapevine planting material.
In November 2015, more than 100 scientists from around the world attended a workshop hosted by EFSA to identify the main knowledge gaps and discuss research priorities for X. fastidiosa. In the same month, EFSA evaluated the results of ongoing studies and experiments carried out in Apulia, concluding that grapevine cannot be ruled out as a potential host of X. fastidiosa.
Following publication of EFSA’s advice, in February 2014 the European Commission put in place emergency measures to combat the spread and further introduction of the organism into the EU. These measures were further strengthened in May and October 2015.
In October 2013 Xylella fastidiosa was detected in olive trees in Lecce province in Apulia, Italy. It was the first outbreak of X. fastidiosa under field conditions to be reported in the European Union. In November of the same year EFSA provided the European Commission with urgent scientific advice and technical assistance on X. fastidiosa.
1. What is Xylella fastidiosa?
Xylella fastidiosa is a bacterial plant pathogen transmitted by insects that feed on xylem sap. It is associated with serious diseases in a wide range of plants around the world. For example, it causes Pierce’s disease in grapevine, citrus variegated chlorosis, phony peach disease, coffee leaf scorch, olive quick decline syndrome and other diseases that affect common trees such as plum, almond, oak and oleander.
2. Where is X. fastidiosa found?
Diseases caused by X. fastidiosa occur in tropical, sub-tropical and temperate areas. The bacterium is widespread in many areas of North, Central and South America and reached Europe in recent years. Official surveys carried out by EU Member States confirm so far that its presence is limited to Italy (southern Apulia), France (Provence-Alpes-Côte d'Azur and Corsica), Spain (Balearic Islands), and an isolated case in a greenhouse in Germany (Saxony).
3. How do plants become infected?
X. fastidiosa colonises the xylem network within plants. The main function of xylem vessels is to transport water from a plant’s roots to its leaves, so when the bacteria colonise the plant, the xylem vessels become blocked and the plant slowly dies. To reach a new host, X. fastidiosa has to be carried and transmitted by a vector – all xylem sap-feeding insects are potential vectors, until proven otherwise. The most common vectors worldwide are sharpshooter leafhopper (Cicadellinae), spittlebugs (Aphrophoridae) and froghoppers (Cercopidae). The meadow froghopper (Philaenus spumarius) is currently the only species confirmed to be a vector of the bacterium in Apulia.
4. What are the symptoms?
Symptoms range from leaf browning to plant death. The specific symptoms, sequence and pace of infection vary because of the wide range of potential host plants, the diversity of the pathogen and level of infection, as well as the different climatic conditions in areas where the bacterium is found. Many host plants infected with X. fastidiosa may not express any symptoms; other plants may show severe leaf scorching and wilting up to complete desiccation.
5. Which plants are vulnerable to attack by the bacterium?
EFSA maintains a database of known hosts, which currently lists 359 plant species – both naturally and experimentally infected hosts – from 204 genera and 75 botanical families. However, not all of the plants listed are susceptible to all “types” (subspecies or strains) of X. fastidiosa and, even when affected, plants will not necessarily become symptomatic. In addition, even plants belonging to the same species can express different levels of susceptibility depending on their variety and growing conditions. The identification of asymptomatic host species is crucial as they can act as symptomless carriers for this bacterium to new zones and more sensitive crops.
6. So is there more than one ‘type’ of Xylella fastidiosa?
Yes. There are five currently recognised sub-species of X. fastidiosa – fastidiosa, morus, multiplex, pauca and sandyi – which in turn have their own variants.
In Europe, the subspecies fastidiosa, associated with Pierce’s disease of grapevine and almond leaf scorch in the Americas, has been detected in the Balearic Islands in Spain, in isolated plants in a greenhouse in Germany (Saxony), and has been intercepted on some consignments of imported coffee plants. The subspecies multiplex has been reported in France (Corsica and Provence-Alpes-Côte d'Azur) and the Balearic Islands. The subspecies pauca, very common in South America, has been detected in Italy (Apulia), in the Balearic Islands, and in one isolated outbreak in France (Menton, Provence-Alpes-Côte d'Azur).
In addition, another species of Xylella, Xylella taiwanensis, occurs in Taiwan, where it causes pear leaf scorch. X. fastidiosa subspecies fastidiosa is also present in Taiwan.
7. What is being done to prevent the spread of X. fastidiosa in Europe?
8. What is EFSA’s role?
EFSA has published numerous reports and scientific opinions on X. fastidiosa since the beginning of the outbreak in southern Italy in 2013. In January 2015 EFSA published a full pest risk assessment and evaluation of risk reduction options for X. fastidiosa in the EU. The scientific opinion included a list of host plants and European vectors of the bacterium. Another significant output was the publication in March 2016 of a report by the Italian National Research Council on the host range of X. fastidiosa CoDiRO. This report was the result of an EFSA-funded pilot project aimed at assessing the host range of the Apulian strain of X. fastidiosa. The most crucial finding was confirmation that X. fastidiosa subsp. pauca (strain CoDiRO) is the causal agent of the olive dieback observed in Apulia. For full details of EFSA’s work in this area please see the “Completed work” section on this page.
 Complesso del Disseccamento Rapido dell’Olivo (Italian translation of olive quick decline syndrome)
9. What other activities has EFSA been involved in to improve our understanding of X. fastidiosa?
EFSA is currently funding research into the role of vectors in spreading the bacterium in Apulia, Italy. This two-year project includes field studies on the ecology and behaviour of the vectors, and the results are expected to be published at the beginning of 2018. EFSA is planning to support more research activity in the near future. EFSA is organising regular scientific events to identify the most urgent knowledge gaps, help coordinate the scientific community and inform and involve stakeholders in the research being carried out in Europe.
A major scientific conference on the latest findings related to X. fastidiosa will be held in Mallorca in November 2017. This year’s event follows the workshop “Xylella fastidiosa: knowledge gaps and research priorities for the EU” that was organised in 2015 by EFSA and the European Commission’s Directorates-General for Research and Innovation, Agriculture and Rural Development, and Health and Food Safety.