Addressing Xylella fastidiosa in Europe: A pathogen par Priorité
In 2013, the appearance of Xylella fastidiosa (Xf) was followed by an outbreak in Apulia in Olive, in southern Italy. Currently, this bacterium, regarded as one of Europe’s most hazardous plant pathogens, is primarily associated with Olive Quick Decline Syndrome (OQDS) in Europe. The bacterium lives in the plant xylem tissue and is commonly spread by insect vectors feeding in the xylem. The bacterium leads to severe diseases in diverse crops, and ornamental and forest species, significantly impacting agriculture, gardens, and the environment, with substantial economic consequences.
Since the initial identification of Xylella fastidiosa subspecies pauca in Apulia, diverse subspecies have been detected in various European regions, including Corsica, mainland France, the Balearic Islands, mainland Spain, Portugal, central Italy (Tuscany) as well as in the Mediterranean countries, Lebanon and Israel.
Image: Olive Orchard in Apulia ©Shutterstock
A silent spreader
Deemed as a consequence of trade globalization and climate change, the pathogen was presumably introduced from the Americas. The ornamental plant trade has been proposed as the main pathway of Xf introduction and spread within Europe. A quiet and stealthy microorganism, it can be asymptomatic for years. Xf is categorized into five subspecies: fastidiosa, pauca, multiplex, sandyi, and morus. Subspecies fastidiosa and pauca have been extensively researched due to their economic impact on significant crops, particularly citrus and grapevine in South and North America, respectively.
Image: Olives in trees and infected Olive trees by the dreaded Xylella fastidiosa © Shutterstock
The categorization of subspecies is somewhat connected to the variety of hosts they target, even though specific strains possess the capability to infect multiple hosts. Primarily, the signs of infection induced by different Xf strains manifest as necrosis at the leaf margins or scorching, resembling the symptoms seen in grapevines affected by Xf subsp. fastidiosa. However, symptoms induced by X. fastidiosa subsp. pauca are characterized by foliar wilt and interveinal chlorosis (areas of the leaf located between the veins exhibit a yellowing or loss of green color), while those caused by X. fastidiosa subsp. multiplex in certain hosts may display dense canopies and reduced fruit size. Moreover, Xf does not have an independent, free-living phase in its life cycle and has exclusively been identified in connection with its plant and insect hosts. 696 plant species have been reported as either naturally or artificially infected hosts, belonging to 88 botanic families (EFSA, 2023).
After Apulia, in 2016, the first strain of Xf was detected in Mallorca and within one year, 400 cases of infection have been reported from the Balearic islands. In the same year, November, the Xf subspecies pauca ST80 was related to olive decline and mortality in Ibiza.
Mallorca in 2017, most of the 50 – 60 detected cases in olive trees in Mallorca belong to a subspecies of the Xf called multiplex, and to a strain that does not cause severe symptoms in this species. Thus, the efforts to eradicate it from this island
are focused on almond trees, which are worse hit by the bacterium.
In 2017, understanding the urgency and havoc this bacterium can create for the future of plant life, the Spanish “El Mundo” newspaper has described Xylella as the “Ebola for olive trees,”
Image: Almond fields in Mallorca © Shutterstock
Xylella fastidiosa sub species pauca in 2024
The Xf subsp. pauca, which is responsible for causing citrus variegated chlorosis (CVC), is a regulated biological agent and is listed under the Agricultural Bioterrorism Protection Act in the USA.
Xf subspecies pauca, which is found to be responsible for citrus variegated chlorosis in Brazil, has been reasonably well-characterized in coffee and citrus plants. Later it was found to infect not only olive trees but also ornamental hibiscus (Hibiscus rosa-sinensis), plum (Prunus domestica). After its appearance in Mallorca at the beginning of this year, Xf pauca now continues to create havoc in Apulia, although the more efficient implementation of containment measures has greatly decreased its speed of spread.
As the pauca strain continues to be identified, infecting more and more new species across the Mediterranean region, ongoing genetic research is revealing crucial characteristics of the pathogen. Considering the pathogen’s unexpected appearance in locations where it was previously undetected (potentially due to its asymptomatic nature), research endeavors are actively working to enhance our comprehension of Xf’s intra and inter-subspecies evolution shedding light on the bacterium’s relationships with host range and geographic distribution. Undoubtedly, it has been demonstrated that the bacterium exhibits adaptability to changes within and across hosts through the utilization of signaling systems.
“Living with the pathogen”
The detection of Xf subspecies pauca in Mallorca in 2023 did not come as a significant surprise to the community. Earlier research conducted in Corsica, France, had suggested that the Xf pathogen had likely been present for a longer duration than previously assumed. Similar observations were made in Mallorca, where a substantial mortality of almond trees was noted from around 2003 in the eastern part of the island, previously linked to fungal diseases. The probable introductions of subsp. fastidiosa and subsp. multiplex in Mallorca were traced back to approximately 1993, deduced from the reconstruction of the almond leaf scorch disease (ALSD) epidemic using evidence from epidemiology, dendrochronology, and molecular biology. Therefore, adopting preventive and phytosanitary measures is imperative, along with developing sound practices in soil management, fertilization, pruning, and irrigation. Simply uprooting trees would not eradicate the pathogen from the region. It is hypothesized that the risk of pathogen expansion to uninfected sites would escalate without effective control of all alternative host plants in OQDS-affected areas. Ultimately, an integrated strategy for controlling X. fastidiosa subsp. pauca should reflect the need for more inclusive governance and management of a quarantine plant pathogen affecting a vast area. Further research should focus on new strategies for managing X. fastidiosa subsp. pauca, including genetic research for resistance and vector control methods. However, the primary emphasis should be on enhancing treatment-focused studies to maintain plant productivity.
The BIOVEXO project is exploring innovative biopesticides targeting the Xylella bacterium on olives. As of 2024, the project activities have undertaken various trials of olive orchards in the study areas of Apulia (Italy), and Mallorca (Spain, Balearic Islands).
Fig: Predicted distribution or climate suitability map of Xylella fastidiosa depicted in green as areas with a simulated probability of occurrence > 50%. Climate suitability was predicted with an ensemble species distribution model (Schneider et al. 2019 PNAS). Red dots indicate the presence of Xylella fastidiosa according to the GBIF database. Grey areas represent currently unsuitable areas for Xylella fastidiosa. The bar charts indicate the number of trees tested within the BIOVEXO project from 2021 until 2024 in the Apulia and Mallorca trail areas.
In the trial areas, with the purpose of containment of the Xylella, application of the sample biopesticides is via foliar spray by endotherapy (injection in the trunks) and irrigation by drips .
Dr. Pasquale Saldarelli, senior scientist at CNR, Italy who is closely involved in these trials remarks “Indeed we are currently conducting large-scale field trials, which are now midway through, and have obtained promising results from the selected biopesticides. Let’s await the completion of these trials to achieve conclusive answers regarding the efficacy of the biopesticides in mitigating the diseases caused by this deadly Xylella bacterium.” His insights underscore the potential of biopesticides in combating this serious threat, offering hope for a future where sustainable agricultural practices can effectively manage and control plant diseases.
Stay tuned for more exciting updates on the groundbreaking BIOVEXO trials.
