Xylella fastidiosa in Europe: From the Introduction to the Current Status
Article information
Abstract
Xylella fastidiosa is xylem-limited bacterium capable of infecting a wide range of host plants, resulting in Pierce’s disease in grapevine, citrus variegated chlorosis, olive quick decline syndrome, peach phony disease, plum leaf scald, alfalfa dwarf, margin necrosis and leaf scorch affecting oleander, coffee, almond, pecan, mulberry, red maple, oak, and other types of cultivated and ornamental plants and forest trees. In the European Union, X. fastidiosa is listed as a quarantine organism. Since its first outbreak in the Apulia region of southern Italy in 2013 where it caused devastating disease on Olea europaea (called olive leaf scorch and quick decline), X. fastidiosa continued to spread and successfully established in some European countries (Corsica and PACA in France, Balearic Islands, Madrid and Comunitat Valenciana in Spain, and Porto in Portugal). The most recent data for Europe indicates that X. fastidiosa is present on 174 hosts, 25 of which were newly identified in 2021 (with further five hosts discovered in other parts of the world in the same year). From the six reported subspecies of X. fastidiosa worldwide, four have been recorded in European countries (fastidiosa, multiplex, pauca, and sandyi). Currently confirmed X. fastidiosa vector species are Philaenus spumarius, Neophilaenus campestris, and Philaenus italosignus, whereby only P. spumarius (which has been identified as the key vector in Apulia, Italy) is also present in Americas. X. fastidiosa control is currently based on pathogen-free propagation plant material, eradication, territory demarcation, and vector control, as well as use of resistant plant cultivars and bactericidal treatments.
Xylella fastidiosa Wells et al. is a gram-negative, slow-growing, fastidious bacterium of the Xanthomonadaceae family that colonizes the xylem vessels of its host plants and is transmitted by insect vectors that feed by sucking xylem sap (Uceda-Campos et al., 2022). It is extremely polyphagous and attacks a large number of taxonomically different host plant species on which it causes great economic damage. Although the symptoms caused by X. fastidiosa vary depending on the host plant, in general, because the bacteria block the transport of water and soluble minerals through the xylem, infected plants exhibit the leaf margin necrosis symptoms, along with the wilting and then drying of leaves, twigs and branches, as well as stunted growth and withering of certain plant parts. These processes often result in the death of diseased plants, thereby signifying the great economic importance of this bacterium.
X. fastidiosa causes a number of diseases of economic importance, namely Pierce’s disease (PD) in grapevine, olive quick decline syndrome (OQDS), citrus variegated chlorosis (CVC) or citrus X disease, pony peach disease and plum leaves scald, as well as diseases such as margin necrosis and leaf scorch affecting oleander, coffee, almonds, pecans, as well as other types of cultivated and ornamental plants and forest trees (De Lima et al., 1998; Saponari et al., 2017; Trkulja et al., 2014; Uceda-Campos et al., 2022). In addition, the bacterium has been found to behave as a commensal endophyte in a range of its plant hosts (Almeida and Nunney, 2015; Sicard et al., 2018). Thus far, at least 595 plant species from 275 genera and 85 families have been found to be infected with X. fastidiosa or X. taiwanensis (Su et al., 2016; Waliullah et al., 2022).
In many countries across the globe, X. fastidiosa is regulated as a quarantine pest. If its presence has been established, this strategy prevents the introduction of its new subspecies, while in countries where this bacterium has not been established the aim is to prevent its introduction into their territory. It is important to point out that, in countries where this pathogen has not been identified so far, X. fastidiosa vectors from the American continent are typically added to the list of harmful organisms. This bacterium has been on the EPPO A2 list since 2017, while according to Commission Implementing Regulation (EU) 2019/2072, X. fastidiosa is included in the List of Union quarantine pests and their respective codes, Annex II, Part B: Pests known to occur in the Union territory.
The phytopathogenic bacterium X. fastidiosa is currently one of the greatest phytosanitary threats to agricultural production in Europe (Trkulja et al., 2019b), where the presence of this pathogen was first identified in Italy in 2013, causing catastrophic economic damage, primarily to olive groves, some of which were hundreds of years old. As a result of this event, the centuries-long tradition of growing olives in the south of Italy has been called into question. After its first appearance in Italy, X. fastidiosa spread in a short time to several European countries.
Geographical Distribution in Europe
The liberalization of world trade and the use of different modes of transport has created a significant international threat from the increased spread of numerous types of harmful quarantine organisms to the most remote parts of the world through the transport of live plants, plant parts and plant products (Chapman et al., 2017; Trkulja et al., 2012). The plant pathogenic bacterium X. fastidiosa is an important example of this process (Frem et al., 2020).
Bacterium X. fastidiosa is a well-known pathogen on the American continent, from where it spread to Europe. The bacterium is particularly widespread in the Mediterranean region, where it causes significant economic damage to numerous cultivated and spontaneously growing plants (Godefroid et al., 2022). In Europe, it was officially confirmed for the first time in 2013 in Italy on olives (Saponari et al., 2013). Not long after its first detection, X. fastidiosa was detected in 2015 in France (EFSA Panel on Plant Health, 2015) and Switzerland (EPPO, 2015), followed by Germany (EPPO, 2016), and Spain in 2016 (Olmo et al., 2017), as well as Portugal in 2019 (EPPO, 2019a).
Preceding these developments, the occurrence of X. fastidiosa in Europe was reported on grapevines in Kosovo (Berisha et al., 1996, 1998); however, due to the lack of further research and doubts about the origin of the material, these reports remain unconfirmed (EPPO, 1998). Moreover, in 2011, a positive reaction to the presence of X. fastidiosa was recorded on one apricot sample in France using a serological test based on immunofluorescence, but these findings were not corroborated by further serological and molecular testing (Manceau et al., 2012). However, in 2012, in a greenhouse near Tours, also in France, X. fastidiosa was isolated from coffee plants (Coffea arabica and C. canephora), but these plants were soon eradicated (EPPO, 2012). In addition, since 2012, various European countries have reported the interception of coffee plants infected with X. fastidiosa from Latin America (Mexico, Ecuador, Costa Rica, and Honduras) (Bergsma-Vlami et al., 2015; EPPO, 2019b; Jacques et al., 2016; Legendre et al., 2014).
In Italy, X. fastidiosa was first discovered in 2013, in one of the main olive (Olea europaea L.) production areas—the Salento peninsula in the Apulia region—where it caused OQDS, resulting in the widespread drying of olive trees in this area (Fig. 1), and thus causing enormous socioeconomic and ecological damage (De Pascali et al., 2022; Girelli et al., 2022; Martelli et al., 2016; Saponari et al., 2013, 2019; Strona et al., 2017). The extent of the damage was exacerbated by water scarcity (De Pascali et al., 2022). In 2018, X. fastidiosa was also discovered on the Spartium junceum plants in the municipality of Monte Argentario (Grosseto) in Tuscany (Marchi et al., 2018), and in November 2021, the presence of this bacterium was also confirmed in the Canino municipality in the Lazio region (European Food Safety Authority et al., 2022b). When assessing the risk of the further spread of X. fastidiosa in Italy, forecast models of species distribution applied to the Italian territory showed a high probability of its occurrence in the Apulia, Calabria, Basilicata, Sicily, and Sardinia regions, as well as the coastal areas of Campania, Lazio, and southern Tuscany (Bosso et al., 2016b).
Symptoms caused by Xylella fastidiosa on olive in Apulia-South Italy: (A) initial symptoms on young trees, (B) leaves scorch (detail), (C) quick decline on olives, (D) dead olive tree (photo by Trkulja).
In Spain, X. fastidiosa was first discovered in 2016 in Majorca (Balearic Islands) on cherry (Prunus avium) and Polygala myrtifolia (Olmo et al., 2017), after which occurrences of this bacterium were recorded on numerous analyzed plants throughout the Balearic Islands (Majorca, Menorca and Ibiza) (EFSA Panel on Plant Health et al., 2018). In 2017, X. fastidiosa was also confirmed on grapevine in Mallorca (Moralejo et al., 2019), almonds in central Spain in the Alicante region (Giampetruzzi et al., 2019), as well as in Madrid in 2018 (European Food Safety Authority et al., 2021; Tihomirova-Hristova et al., 2019). In the same year, its presence was recorded on P. myrtifolia in the ornamental plant nursery located in Andalusia (Morelli et al., 2021).
In France, the first occurrence of the disease caused by X. fastidiosa was recorded in July 2015 on P. myrtifolia growing on the island of Corsica. At the beginning of November 2016, the total number of outbreaks in Corsica reached 289, with X. fastidiosa detected on 27 different plant species. In October 2015, the first outbreak of X. fastidiosa was also detected on P. myrtifolia along the southeastern Mediterranean coast of France. At the beginning of November 2016, 15 outbreaks were recorded in the French Riviera area of Provence and two new host plants (Spartium junceum and Lavandula angustifolia) were identified (Legendre et al., 2017). Moreover, X. fastidiosa was confirmed in 2019 in Provence (southwest), and then in Occitania (south) in 2020 on lavender hybrid plants, as well as on Afghan lavender and Jerusalem sage (European Food Safety Authority et al., 2022b; Morelli et al., 2021).
In Portugal, X. fastidiosa was first detected at the end of 2018 on asymptomatic lavender plants (Lavandula dentata) in the Porto region (EPPO, 2019a), and then in the surroundings of Lisbon and in the Algarve region in 2021 (EPPO, 2022).
In Switzerland, X. fastidiosa was detected on imported coffee plants in 2015, after which measures were taken to eradicate infected plants and ban the import of potential host plants (EPPO, 2015). Testing and monitoring in the following years confirmed that the bacterium did not infect any other host around the area where X. fastidiosa was initially found (EPPO, 2018a).
In Germany, X. fastidiosa was first identified in Saxony (EPPO, 2016) on privately grown oleander plants (Nerium oleander) as well as on individual Rosmarinus sp., Streptocarpus sp. and Erysimum sp. specimens (Julius Kühn-Institut, 2018). Rapid eradication of infected plants, followed by an administrative ban on the import of host plants, prevented further occurrences of this bacterium (EPPO, 2018b).
Due to the increased phytosanitary risk from the introduction and further spread of X. fastidiosa, special monitoring programs focusing on this quarantine phytopathogenic bacterium are being implemented across Europe, but its presence in other European countries has not been established to date (Cara et al., 2017; Gottsberger et al., 2019; Holeva et al., 2017; Jakovljevic, 2019; Jančar et al., 2019; Kornev et al., 2019; Mastin and Parnell, 2017; Mastin et al., 2019; Spasov et al., 2017; Trkulja et al., 2016, 2017a, 2019a).
Based on the aforementioned findings, it is evident that this dangerous quarantine phytopathogenic bacterium has spread very quickly in several European countries, posing the risk to other parts of Europe, especially the Mediterranean where olives and grapes are mostly produced. According to a maximum entropy (MaxEnt) model aimed at determining the current and forecasting the future distribution of X. fastidiosa in the Mediterranean region under climate change conditions, the potential distribution area of this bacterium presently includes Italy, Corsica, Spain, Portugal, Albania, Montenegro, Greece and Turkey, as well as all North African and Middle Eastern countries (Bosso et al., 2016a).
Host Plants in Europe
Bacterium X. fastidiosa is a polyphagous pathogen that attacks a wide range of host plants, from weeds and ornamental plants and shrubs to economically very important fruit-growing, woody and forest plants (Girelli et al., 2022; Godefroid et al., 2022). Currently, the list of host plants maintained by EFSA includes 694 plant species from 299 genera and 88 botanical families that have been confirmed to be infected with X. fastidiosa regardless of the detection method used, with 412 plant species from 190 genera and 68 botanical families of infections confirmed by at least two different detection tests (European Food Safety Authority et al., 2022a). Moreover, 174 host plant species from 91 genera and 44 botanical families are currently identified in Europe (European Food Safety Authority et al., 2022a, 2022b), as shown in Table 1.
List of host plants on which Xylella fastidiosa has been confirmed in European countries
The number of plant species positive for X. fastidiosa increases every year because the list of host plants for this bacterium in Europe is regularly updated. Thus, as can be seen from Table 2, the number of host plants in Europe has consistently increased during the 2013–2021 period, and a similar upward trend was noted in the number of newly discovered host plants in the world as well as in Europe (EFSA Panel on Plant Health, 2015; EFSA Panel on Plant Health et al., 2018; European Food Safety Authority, 2016, 2018, 2020; European Food Safety Authority et al., 2021, 2022a, 2022b).
Number of Xylella fastidiosa hosts present in Europe during the 2013–2021 period
For example, in 2020, 28 new host plant species in the world susceptible to X. fastidiosa were reported in France, Spain, Italy and Portugal, while further 25 in the same four countries were identified in 2021. In addition, 12 new genera of host plants naturally infected with X. fastidiosa that were newly identified in the world were recorded in France, Spain and Portugal in 2020, while in 2021, 13 new genera of host plants were identified in France, Portugal and Italy. Moreover, two host plant families (Woodsiaceae and Dennstaedtiaceae) that had not been previously discovered at the global level were also confirmed in Portugal in 2020, and further two (Elaeagnaceae and Hypericaceae) were recorded in 2021 (European Food Safety Authority et al., 2021, 2022a, 2022b).
In Europe, the most important and economically significant X. fastidiosa hosts are olive, stone fruit species, grapevine, citrus and forest trees (European Food Safety Authority et al., 2021; Frem et al., 2021). According to Markheiser et al. (2020), cherry and grapevine may be seriously threatened by the further spread of this pathogen in Europe. Among woody and forest species in Europe, natural infections have been recorded on plants from the genera Acer (A. pseudoplatanus), Ficus (F. carica), Fraxinus (F. angustifolia), Juglans (J. regia), Olea (O. europaea), Prunus (P. avium, P. cerasifera), and Quercus (Q. ilex, Q. suber) (Desprez-Loustau et al., 2021). Symptoms caused on the economically important plant species such as grapevine, citrus, peach, coffee, olive, plum, and sweet orange were described by Baldi and La Porta (2017).
In Italy, after the first discovery of X. fastidiosa in olives, and subsequently on the surrounding oleander and almond plants in the Apulia region, this bacterium was also confirmed on other plants, such as ornamental plants, endemic species of the Mediterranean flora, cherry, as well as milkweed, rosemary and acacia (Cavalieri et al., 2019; Cornara et al., 2017a; Saponari et al., 2019). Apart from olive, EFSA Panel on Plant Health et al. (2018) states that the species most commonly and most severely affected by this pathogen are N. oleander, Acacia saligna, and P. myrtifolia, which tend to exhibit more intense drying and withering symptoms, while Rhamnus alaternus, Myoporum insulare and Westringia glabra serve as hosts without developing marked disease symptoms. According to Saponari et al. (2019), infected P. myrtifolia, Laurus nobilis, Myrtus communis, M. insulare, and Dodonaea viscosa purpurea specimens were discovered on the Salento peninsula. Aimed to determining the possibility of vector transmission of the bacterium to different host plants, Cornara et al. (2017a) noted that the bacterium was confirmed in olive, oleander, grapevine, sweet orange (Citrus sinensis), stone fruit rootstock GF677 (Prunus persica × Prunus dulcis hybrid) and flowers (Catharanthus roseus). In Italy, 62 X. fastidiosa host plants are currently identified—including olive, S. junceum, P. myrtifolia, almond, rosemary, lavender, common myrtle and fig—which are typical for the Italian region (European Food Safety Authority et al., 2022b).
In France, after the first discovery of X. fastidiosa in Corsica on P. myrtifolia plants, the bacterium was also found in mainland regions. Analyses involving 45,000 samples of different plant species revealed presence of this bacterium in approximately 3% of plant material taken from 49 plant species, including mainly ornamental plants and shrubs, but also several forest species, such as Acer pseudoplatanus, P. avium, Quercus suber, and Q. ilex (Desprez-Loustau et al., 2021). In France, 72 X. fastidiosa host plants are currently identified—including P. myrtifolia, rosemary, lavender, as well as olive and almond—which are typical for this region, but also Afghan lavender and Jerusalem sage (European Food Safety Authority et al., 2022b).
In Spain, the pathogen was first detected on cherry (P. avium) and P. myrtifolia (Olmo et al., 2017). In the Balearic Islands, 18 host plant species have been identified, including J. regia, P. avium and some forest tree species such as Fraxinus angustifolia (Desprez-Loustau et al., 2021). In Spain, 46 X. fastidiosa host plants are currently identified—including olive, grapevine and almond—which are typical for this region, as well as rosemary, Helichrysum italicum, and several other spontaneous flora species (European Food Safety Authority et al., 2022a, 2022b; Moralejo et al., 2019).
In Portugal, 62 X. fastidiosa host plants are currently identified—including olive, lavender, asparagus, Q. suber, rosemary, Artemisia arborescens, Coprosma repens, Vinca major, M. communis, and Ulex minor—which are typical for this region (European Food Safety Authority et al., 2022a, 2022b).
X. fastidiosa Subspecies Present in Europe
X. fastidiosa populations are highly heterogeneous, and based on DNA-DNA hybridization and 16S-23S rRNA internal transcribed spacer (ITS) sequencing, Schaad et al. (2004a) described and formally proposed three subspecies within this species, namely X. fastidiosa subsp. piercei, which was later modified to subsp. fastidiosa (Schaad et al., 2004b), X. fastidiosa subsp. multiplex and X. fastidiosa subsp. pauca. Three further subspecies within this species were subsequently described, resulting in the following six X. fastidiosa subspecies: (1) X. fastidiosa subsp. fastidiosa; (2) X. fastidiosa subsp. multiplex; (3) X. fastidiosa subsp. pauca; (4) X. fastidiosa subsp. sandyi; (5) X. fastidiosa subsp. tashke; and (6) X. fastidiosa subsp. morus (Almeida and Nunney, 2015; EFSA Panel on Plant Health et al., 2018). Currently, more than 80 sequenced whole genomes of all six X. fastidiosa subspecies are available in the National Center for Biotechnology Information (NCBI) database with genome sizes ranging from 2.4 to 2.7 Mb, with a G + C content of 51–52 mol%, with contig numbers ranging from 1 to over 400, ambiguous base counts ranging from 0 to over 300, and completeness generally exceeding 99% (Johnson et al., 2022).
Among all listed subspecies, the most economically harmful are fastidiosa, pauca, multiplex and sandyi, which have been confirmed in numerous countries around the world (Denancé et al., 2019; Sertedakis et al., 2022). In addition, different X. fastidiosa subspecies cause different disease symptoms on different host plants, with the fastidiosa subspecies causing PD in grapevine (Vitis vinifera), the multiplex subspecies causing almond leaf scorch and similar symptoms on other nut and woody plants, and subsp. pauca causing CVC (Citrus spp.), as well as coffee leaf scorch and OQDS, while subsp. sandyi is responsible for oleander leaf scorch (OLS) (Schaad et al., 2004a; Trkulja, 2014; Baldi and La Porta, 2017; Rapicavoli et al., 2018).
In several European countries (Italy, France, Spain and Portugal), the presence of multiplex, pauca and fastidiosa subspecies characterized by different sequence types was established (Denancé et al., 2017; EFSA Panel on Plant Health et al., 2019b; Giampetruzzi et al., 2016; Marcelletti and Scortichini, 2016a) reflecting the diversity of subspecies currently present in Europe (Table 3). Although the number of hosts that these subspecies infect in Europe varies, it is gradually increasing. According to European Food Safety Authority et al. (2022b), in nature, the greatest number of plants are infected by the multiplex subspecies, followed by pauca and fastidiosa (202, 56, and 53, respectively), while artificial inoculation by fastidiosa is most effective, followed by pauca and multiplex subspecies (with 74, 27, and 25 infected plants, respectively).
List of Xylella fastidiosa subspecies present in European countries
The name X. fastidiosa subsp. fastidiosa derives from the Latin term fastidiosus, meaning highly critical, and referring to the nutritional fastidiousness of the organism. Strains of this subspecies grow faster on PD2, PW, BCYE, and CS20 media, and are more resistant to penicillin and less resistant to carbenicillin than subsp. multiplex and pauca. This subspecies causes PD in grapevine (Vitis vinifera), almond (Prunus amygdalus L.), cherry (Prunus avium), alfalfa (Medicago sativa), coffee (Coffea arabica and C. canephora), oleander (Nerium oleander), maples (Acer spp.), and American black elderberry (Sambucus canadensis) (Marcelletti and Scortichini, 2016a). According to the EFSA Panel on Plant Health et al. (2018) report, the fastidiosa subspecies is found on a larger number of perennial plants, shrubs and trees. In Europe, it was detected for the first time on P. avium and P. myrtifolia plants grown in Spain (Olmo et al., 2017), where it was later established on grapevine (Vitis vinifera) (Moralejo et al., 2019). This event was followed by its discovery in the Saxony region in central-western Germany, on a privately grown oleander that was infected with the X. fastidiosa subsp. fastidiosa ST1, after which this subspecies was soon discovered on Rosmarinus sp., Streptocarpus sp. and Erysimum sp. (Julius Kühn-Institut, 2018; Markheiser et al., 2020). The first record of X. fastidiosa subsp. fastidiosa in Europe on new hosts involved one naturally infected (Ruta chalepensis) and three artificially inoculated plant species from the genus Vitis (Vitis × doaniana, Vitis treleasei, and V. vinifera hybrid) in Spain (European Food Safety Authority et al., 2021).
X. fastidiosa subsp. multiplex derives its name from the adjective multiplex, meaning numerous, referring to the large number of host plants in which the bacterium causes disease, the most important of which are peach (Prunus persica), plum (Prunus domestica), almond (P. dulcis), elm (Ulmus spp.), pigeon grape (Vitis aestivalis), sycamore (Platanus spp.), and other forest trees. Strains of the multiplex subspecies grow much faster on PW medium than on PD2, BCYE or CS20, and are more sensitive to penicillin and more resistant to carbenicillin than the fastidiosa subspecies (Marcelletti and Scortichini, 2016a). The subspecies multiplex was established in Spain on P. myrtifolia (Olmo et al., 2017). In France, ST6 and ST7 were found in Corsica and Provence-Alpes-Côte d’Azur (PACA) regions on P. myrtifolia, S. junceum and in almost 70 other plant species (Cunty et al., 2022; Denancé et al., 2017), ST6 in new region Occitanie (Aude area) from natural and urban settings and from a nursery (Cunty et al., 2022) and recently discovered two new variants genetically related to the subsp. multiplex and assigned to ST88 found on Polygala myrtifolia, Hebe sp. Osteospermum ecklonis, Lavandula x intermedia, Coronilla glauca, Euryops chrysanthemoides, and ST89 on Myoporum sp., and Viburnum tinus in two areas of PACA region (Cunty et al., 2022). In Italy, the multiplex subspecies was first discovered in Tuscany (Monte Argentario site) in S. junceum, P. myrtifolia, and R. alaternus plant samples (Marchi et al., 2018), whereas in Portugal, it first emerged on lavender (L. dentata) in 2019 (EPPO, 2019a). In 2021, this subspecies was identified in Spain, Italy, Portugal and France in 24 new plant species naturally infected in the EU (European Food Safety Authority et al., 2021). The following year, 19 new host plants were identified in Portugal, Spain and France, and the multiplex subspecies was determined in 15 of these cases, while the exact subspecies was not specified for the remaining four host plants (European Food Safety Authority et al., 2022b).
X. fastidiosa subsp. pauca derives its name from the Latin term pauca, meaning few, reflecting its narrow host range. The pauca and multiplex subspecies grow more slowly on PD2, PW, BCYE, and CS20 media, and are more sensitive to penicillin and more resistant to carbenicillin than the fastidiosa subspecies (Marcelletti and Scortichini, 2016a). This subspecies causes disease to citrus (Citrus spp.) and coffee (C. arabica), and have been found associated with oleander, almond, cherry and with olive (O. europaea) trees showing extensive leaf scorching/wilting and twig die-back in Apulia (Southern Italy), Argentina and Brazil. The symptoms caused by this subspecies vary from host to host, but usually manifest as leaf margin burns, leaf nerve chlorosis, wilting and stunting (Marcelletti and Scortichini, 2016a). The pauca subspecies causes nerve chlorosis in citrus and develops on fewer hosts. In Italy, it was found in coffee as well as olive plants (EFSA Panel on Plant Health et al., 2018; Luvisi et al., 2017; Mang et al., 2016; Marcelletti and Scortichini, 2016b), representing the first confirmed record of this bacterium in Europe (De Pascali et al., 2022). This subspecies was introduced from Central America where it is an endemic pathogen (Girelli et al., 2022). Its identification on Italian olive grown in Apulia commonly known as “CoDiRO” or “ST53” raised concern for other plant species, Citrus spp. in particular, where this subspecies causes extensive damage. Although Italian strains failed to infect citrus (Elbeaino et al., 2014a; Guan et al., 2015), infections were noted in 30 other host plants surrounding olive groves in Apulia (Saponari et al., 2019). Millions of olive trees have perished due to the damage caused by this subspecies, with devastating socioeconomic consequences for the local communities (Godefroid et al., 2019). In France, ST53 of the pauca subspecies was found in Corsica on P. myrtifolia and Q. ilex plants (Denancé et al., 2017) and in a unique area in PACA region (Cunty et al., 2022). Its presence on two new hosts in Europe was also confirmed, one of which was naturally infected (Ulex parviflorus) in Spain while the other (basil, Ocimum basilicum) was artificially inoculated under experimental conditions (European Food Safety Authority et al., 2021).
X. fastidiosa subsp. sandy was described and formally proposed by Schuenzel et al. (2005) when the isolates that cause OLS were determined on the American continent. Subsequently, based on the findings yielded by multiple molecular analyses (multiplex polymerase chain reaction [PCR], incurred sample reanalysis, and random amplified polymorphic DNA), Hernandez-Martinez et al. (2007) established that the strains isolated from Hemerocallis spp., Jacaranda mimosifolia and Magnolia grandiflora belong to the subspecies sandyi. In Europe, the presence of ST76 of this subspecies was determined on P. myrtifolia in Corsica, but since eradication measures were immediately undertaken, no further instances of this strain were detected (Denancé et al., 2017) (Table 3).
X. fastidiosa subsp. tashke was proposed by Randall et al. (2009) based on the sequence analysis of the 16S–23S ribosomal ITS region of isolates obtained from the plant Chitalpa tashkentensis and other sequences sourced from gene banks. Using these findings, the phylogenetic tree, and the previously described four subspecies, these authors grouped the isolates obtained from C. tashkentensis into a separate cluster as a new subspecies for which the authors proposed the name X. fastidiosa subsp. tashke. Thus far, its presence has not been established in Europe.
X. fastidiosa subsp. morus was isolated by Guan et al. (2014) from white mulberry (Morus alba) in 2011 in Beltsville, USA. In Europe, its presence has not been established thus far.
The climate of a large area of Europe (including Spain, France, the British Isles, Italy, the Adriatic coast, Greece, Turkey and some coastal areas of the Black Sea) appears to be very suitable for the potential distribution of the multiplex subspecies. According to Godefroid et al. (2019), the climate in Spain, France, Italy, Croatia, Greece, and Turkey, as well as the coastal regions of North Africa, seems very favorable for the potential distribution of the fastidiosa subspecies, whereas the climatic conditions prevailing in the coastal regions of the Mediterranean (with the exception of southern Portugal and the Spanish Atlantic coast) is conducive for the spread of the pauca subspecies. The same authors state that the climatic conditions in the northern and eastern regions of Europe (northeastern France, Belgium, the Netherlands, Germany, etc.) are less favorable for the spread and distribution of the pathogen X. fastidiosa.
X. fastidiosa Vectors in Europe
X. fastidiosa is a phytopathogenic bacterium that develops in the xylem of host plants. A large number of potential vectors of this bacterium are presently known, all of which are insects that feed on xylem sap. While the X. fastidiosa vectors in North and South America are relatively well known, since the first appearance of this quarantine bacterium in Italy in 2013, only a few vectors have been identified in Europe. Thus, expanding the current knowledge about its vectors is essential for better understanding the cycle of disease development, as well as adopting adequate vector control measures with the aim of mitigating and preventing attacks and disease occurrence on new host plants and in new localities (EFSA Panel on Plant Health et al., 2019c; Elbeaino et al., 2014b; Trkulja et al., 2017b).
According to the EFSA Panel on Plant Health et al.’s (2019c) report, the vectors of this phytopathogenic bacterium are species belonging to the cicada families Aphrophoridae, Cicadellidae, and Membracidae, but only members of Aphrophoridae family serve as X. fastidiosa vectors in Europe. According to Di Serio et al. (2019), 99 potential European vectors feed on xylem sap, while Cornara et al. (2019) recognize 78 potential vectors from Cercopoidea and Cicadoidea families and Cicadellidae: subfamily Cicadellinae.
Three cicada species from the Aphrophoridae family—Philaenus spumarius L., Neophilaenus campestris Fallén, and Philaenus italosignus Drosopoulos & Remane (Elbeaino et al., 2014b; European Food Safety Authority et al., 2019; López-Mercadal et al., 2021)—are the most important and fully competent vectors of X. fastidiosa that have been confirmed in Europe to date, with P. spumarius having particular significance for the spread of this pathogen across the continent (Avosani et al., 2022).
P. spumarius (meadow spittlebug) is one of the most numerous, widespread and important, and thus the most studied vectors of this bacterium in Europe (Cruaud et al., 2018; EFSA Panel on Plant Health et al., 2018; European Food Safety Authority et al., 2019; Godefroid et al., 2022; Morente et al., 2022), even though it is present worldwide, including Africa, Asia and North America. In Europe, the widespread distribution of this vector—from Lapland (Finland) to the Mediterranean (Cornara et al., 2018)—causes concern about the mass transmission and spread of X. fastidiosa, especially in the Mediterranean countries, where the plant species susceptible to its attacks (olive in particular) are mostly produced. Using molecular PCR tests, Cunty et al. (2020) determined the enormous potential of bacterial transmission by this insect, which is not surprising given that P. spumarius is a highly polyphagous species and can survive on different hosts from several plant families. This assertion was confirmed by investigations in Italy (Liguria and Apulia) (Di Serio et al., 2019), where P. spumarius individuals developed and lived on the members of all tested plant families. However, P. spumarius specimens favored plants from the families Fabaceae and Asteraceae, while a significantly lower number was recorded on plants from the Poaceae, Apiaceae and Rubiaceae families (Di Serio et al., 2019). In Spain (Majorca), although P. spumarius nymphs were observed on numerous hosts, they showed the greatest preference for plants from the Asteraceae and Fabaceae families (López-Mercadal et al., 2021). In the central and southern regions of Spain, as well as in the northwest of Portugal, P. spumarius was present in olive groves on herbaceous vegetation, mainly on plants from the Asteraceae, Apiaceae and Geraniaceae families (Morente et al., 2018). In Germany, P. spumarius is widespread, and is found on ornamental, autochthonous and economically important cultivated plants such as grapevine and cherry, which are important X. fastidiosa hosts (Markheiser et al., 2020). In Spain, on the Balearic Islands, P. spumarius is the main and most widespread vector of X. fastidiosa (López-Mercadal et al., 2021), and managed to transfer this bacterium from almond to almond and from grapevine to grapevine under experimental conditions (Olmo et al., 2021). Successful transmission of X. fastidiosa by P. spumarius to different hosts was also confirmed by Cornara et al. (2017a, 2017b), while Cunty et al. (2020) identified 1,000 hosts (mostly dicotyledonous plants) of this insect.
Cornara et al. (2018) provided a detailed description of the biology of P. spumarius, stating that this species overwinters in the egg stage and has one generation per year. Females mate with several males and lay eggs in autumn, in groups of up to 30 specimens, producing 350–400 eggs per mating season. Eggs are usually deposited along the edges of orchards, as well as on agricultural land after harvest, herbs, dead plant parts, plant debris, cracks and trunk bark, typically close to the soil. Egg development takes about 5–7 weeks, and the larval period lasts 35–100 days, depending on the environmental conditions. After the larvae feed on the host plants, they produce saliva, which gradually dries and hardens, allowing the formation of a cocoon in which they hibernate. Larvae hatch in spring (usually in April or May, depending on the temperatures), and adult insects (imaga) appear in June, and reach maturity in July and August (Kereši et al., 2019). Adult insects live until autumn, when they are killed by low temperatures (frost), although some individuals can overwinter until spring (Cunty et al., 2020; Kereši et al., 2019). The second generation of this pest has been identified in Greece (Di Serio et al., 2019).
N. campestris is the second most important X. fastidiosa vector and is widely distributed in Europe, especially in the Mediterranean region (Cavalieri et al., 2019). This insect is widespread in Spain and Portugal where it poses a significant threat, especially to commercial olive, grapevine and almond plantations (Lago et al., 2021; Morente et al., 2018). In these countries, it is mainly present on Avena sp. and Bromus sp., as well as other plants from the Poaceae family (Cornara et al., 2019; Morente et al., 2018). The ability of N. campestris to adopt and transfer the bacterium X. fastidiosa under experimental conditions from infected to different host plants was reported by Cavalieri et al. (2018, 2019).
P. italosignus as a vector of the X. fastidiosa strain originating from the Apulia region was found to have limited presence in the southern parts of Italy and Sicily, while its greater distribution was recorded in the northern and central regions of this country, where it has a large number of different agricultural plants as potential hosts (Cavalieri et al., 2019; Panzavolta et al., 2019). Although little is known about the distribution of this pest and the plants it prefers, it has been established that the nymphs develop and females lay eggs on Asphodelus spp. plants. In parts of Italy where olive is the dominant plant species, the presence of this insect is rarely recorded (Cavalieri et al., 2019), while Panzavolta et al. (2019) noted a significant population of this insect in Tuscany on Asphodelus ramosus as well as on olives. Transmission of the bacterium by this insect to P. myrtifolia, olive and cherry plants under experimental conditions was also confirmed by Cavalieri et al. (2019), indicating a high risk of transmission of X. fastidiosa to various economically important cultivated plant species through this vector in localities where it is present.
The aforementioned finding indicates that these three cicada species are capable and present X. fastidiosa vectors in Europe. However, numerous studies indicate that other cicada species can be potential vectors of this dangerous phytopathogenic bacterium in Europe, such as Cicada orni L., Latilica tunetana Matsumura, Aphrophora alni Fallen, Cercopis vulnerata Rossi, Evacanthus interruptus L., and E. acuminatus Fabricius, Cicadella viridis L. (Cornara et al., 2019, 2020), as well as Euscelis lineolatus Brulle (Elbeaino et al., 2014b) and others, highlighting the importance of further research, continuous monitoring and testing of potential, new and hitherto unknown species of insects as potential X. fastidiosa vectors on different host plants.
X. fastidiosa Management in Europe
Current X. fastidiosa control strategies are mainly focused on pathogen-free propagation plant material, phytoquarantine, eradication, vector control, growth of resistant or tolerant plant cultivars, and bactericidal treatments (Commission Implementing Regulation (EU) 2020/1201; EFSA Panel on Plant Health et al., 2019a; European Food Safety Authority, 2020; Kyrkou et al., 2018; Pavan et al., 2021; Scortichini et al., 2021). For example, EU phytoquarantine regulations mandate that, prior to export or import into the EU, any plant recognized as X. fastidiosa host must be tested for the presence of bacterium (European Food Safety Authority et al., 2020). Likewise, eradication proposed by Commission Implementing Regulation (EU) 2020/1201 should be applied to any new outbreak of X. fastidiosa, with the exception of infected zones where containment measures are authorized (e.g., South of Apulia, Corsica, and Baleares). In such cases, areas in which X. fastidiosa infection is confirmed are demarcated, and the planting of host plants is prohibited in the infected zone (Commission Implementing Regulation (EU) 2020/1201; Pavan et al., 2021).
In the Italian legislation, further provisions are provided, whereby (1) distinction is made between “host” (species susceptible to all X. fastidiosa subspecies worldwide) and “specified” (species susceptible to the local X. fastidiosa genotype) plants; (2) “containment” rather than “eradication” is adopted as the control measure in Apulia; and (3) both infected and buffer zones are clearly demarcated (Morelli et al., 2021). The legislation further prohibits planting of susceptible hosts in infected areas (except for species or cultivars found to be resistant or tolerant), as well as movement of “host” or “specified” plants out of the demarcated areas.
Chemical control currently relies on copper compounds and several microbial biopesticides, as well as antibiotics where permitted. According to several reports, application of N-acetylcysteine (NAC) (Saponari and Boscia, 2019), copper (II) sulfate, menadione, benzethonium chloride, and abscisic acid (Ge et al., 2020; Muranaka et al., 2013; Zhang et al., 2019) as well as antibiotic (oxytetracycline) along with NAC, Zn, or Cu, and citric acid fertilizer (Scortichini et al., 2018) can achieve effective X. fastidiosa control under greenhouse conditions. Biocomplex Dentamet—a compound containing zinc, copper, and citric acid—can reduce X. fastidiosa subsp. pauca multiplication rate in olive trees in Salento (Apulia, Italy) by application to the canopy or injection into the trunk (Scortichini et al., 2018, 2021, 2022).
Owing to its endophytic nature, full control of X. fastidiosa remains challenging, as the ability of available bactericides and mineral-based compounds to access xylem vessels where the pathogen establishes is limited (EFSA Panel on Plant Health et al., 2019a; Kyrkou et al., 2018; Montesinos et al., 2022; Morelli et al., 2021; Tatulli et al., 2022). Thus, new chemical compounds are required to achieve effective disease suppression. With this aim, Baldassarre et al. (2020) studied interactions between calcium carbonate nanocrystals and bacteria cells, as well as their application in olive to verify uptake. The authors demonstrated that these nanocrystals can be adsorbed by the roots as well as effectively translocated into the plant vessels and other tissues. These findings indicate that different compounds (phytodrugs, fertilizers) can be delivered and released into the plants using these nanocarriers. Although the authors indicated that nanoCaCO3-based phytodrugs could potentially provide a cure for X. fastidiosa infections, more extensive testing needs to be carried out in the future to confirm this assertion and other potential anti-X. fastidiosa agents should also be investigated.
Besides chemical control, cultural practices could potentially help reduce vector activity and population density (Sanna et al., 2021). For example, for cicadas P. spumarius, N. campestris and P. italosignus, this may include compulsory mechanical weed control (tillage) in spring, along with the use of insecticide sprays to control adults (Cavalieri et al., 2018; Cornara et al., 2018; Pavan et al., 2021). Management of ground vegetation, as well as correct timing of soil tilling to disrupt nymph development and reduce adult emergence, have been shown by Sanna et al. (2021) to effectively control P. spumarius population size. However, control of adults on olives requires several insecticide applications throughout their entire feeding period. According to Dongiovanni et al. (2018), pyrethroids (deltamethrin) and neonicotinoids (acetamiprid) are the most efficacious. Soil tillage, combined with the application of pyroherbicides, herbicides, neonicotinoids and pyretroids in spring, has also been shown by Saponari and Boscia (2019) to almost eliminate the presence of juvenile P. spumarius. Liccardo et al. (2020) similarly reported that an inundation strategy with Zelus renardii—a known P. spumarius predator—can be an effective “green” solution to X. fastidiosa invasion, as it results in a pathogen incidence reduction to below 10%. According to EPPO (2020), in Apulia, elimination of weeds within and around olive groves in winter and spring reduced the abundance of both P. spumarius and N. campestris on olive trees and ground vegetation to almost zero. Moralejo et al. (2019) similarly found that weed control and soil tillage in spring may contribute to vector population reduction, which would in turn hinder X. fastidiosa spread.
Olive cultivar ‘Leccino’ and selection ‘FS17’ were found to be resistant to X. fastidiosa by several authors (Boscia et al., 2017; Saldarelli et al., 2022; Saponari and Boscia, 2019). According to Saldarelli et al. (2022), in ‘Leccino’, resistance appears to develop via a complex of mechanisms involving both genomic and physiological factors, which limit the bacterial population size. These authors also found that ‘Leccino’ is more resilient to the X. fastidiosa infection as its physiological response to the water stress is not as extreme as in susceptible cultivars such as Cellina di Nardò and Ogliarola Salentina. As X. fastidiosa infection limits the water supply to leaves (Surano et al., 2022), physiological parameters can be used in breeding programs to ensure that olive genotypes resistant to the bacterium are chosen. For example, using simple sequence repeat marker analysis, Pavan et al. (2021) demonstrated that the olive genotypes ‘Frantoio’ and ‘Nocellara Messinese’ exhibited partial resistance. Considering that genetic resistance presents the most promising long-term X. fastidiosa management strategy, an intense screening program for resistance was started for more than 100 olive selections, combining natural pressure and artificial inoculation with the Apulian X. fastidiosa strain (Saponari and Boscia, 2019). Similarly, as a part of their study focusing on grapevine, Kyrkou et al. (2018) found that V. vinifera cultivars are susceptible to Pierce’s disease, as well as demonstrated resistance in other species that are not of sufficiently high quality for wine production.
Integrated pest management also includes modeling for X. fastidiosa spread. For example, White et al. (2017) developed a spatially-explicit simulation model for X. fastidiosa to provide guidance for predicting spread in the early stages of invasion and inform management strategies. Based on these simulations, the authors identified optimal control scenarios that minimize control effort while resulting in the greatest reduction in X. fastidiosa spread, suggesting that buffer zone width increase should be favored over surveillance efforts as control budgets increase. Their findings further highlighted the importance of non-olive hosts in increasing the disease spread rate. More recently, Brunetti et al. (2020) proposed a mathematical model ODE system for X. fastidiosa epidemics in the Mediterranean regions. Guided by numerical simulations, they identified the key components of this plant−insect−bacterium epidemic system that should be considered as potential long-term biocontrol strategy targets. Furthermore, Kyrkou et al. (2018) developed a X. fastidiosa epidemiological model denoted as Xff to describe the PD dynamics in vineyards. The Xff identifies key parameters in the disease spread that should be targeted by current and future intervention strategies.
Fierro et al. (2019) developed “lattice model” while Liccardo et al. (2020) proposed a “biological control model” for managing X. fastidiosa vectors and infection in olive trees. Both models include three control steps, each involving one or several actions aimed at reducing vector population size and the number of adults infecting Xylella-free plants (Picciotti et al., 2021). More recently, Godefroid et al. (2022) fitted bioclimatic species distribution models to empirical data to depict the macroclimatic preferences of P. spumarius as the major epidemiologically relevant vector currently responsible for X. fastidiosa spread in Europe. Finally, X. fastidiosa control measures also rely on minimizing other sources of stress to the host plant, such as drought, overproduction, and other diseases (EPPO, 2020).
Conclusion
This review provides a comprehensive and up-to-date assessment of the X. fastidiosa presence in Europe. The emergence of the bacterium throughout Italy, Spain, France and Portugal—with 174 hosts identified thus far—confirms that X. fastidiosa is spreading across the continent at an alarming rate. In particular, available evidence indicates that the conditions in central and southern Europe are suitable for the introduction and further spread of X. fastidiosa. European outbreaks of X. fastidiosa-related diseases present an undeletable trace in the history of plant pathology.
Notes
Conflicts of Interest
No potential conflict of interest relevant to this article was reported.
References
Commission Implementing Regulation (EU) 2019/2072. 10 December 2019. Official Journal of the European Union L 319/1–L 319/279
Commission Implementing Regulation (EU) 2020/1201. 17 August 2020. Official Journal of the European Union L 269/2–L 269/39