A recent study (Ciervo et al. 2024) published in the Journal of Phytopathology analyzes a decade of monitoring data on Xylella fastidiosa, providing a detailed picture of the situation and suggesting management and control strategies.
According to the researchers, despite initial concerns, the incidence of the bacterium in the demarcated areas is surprisingly low, especially in the last three years of monitoring. This data suggests a possible review of disease management strategies, questioning the effectiveness and need for drastic measures such as the felling of monumental olive trees. (1)
Centuries-old olive trees in Puglia
Apulian olive growing, with a history dating back to the Bronze Age, contributes significantly to the region’s agricultural sector:
- represents 13% of the value of total agricultural production e
- affects 26% of the value of Italian olive production. (2)
Puglia is famous for the Plain of monumental olive trees, an area that extends between the provinces of Brindisi and Bari, where over 250.000 olive trees, many of which are centenary and some even millenary, dominate the landscape.
This area was included in the National Register of Historic Rural Landscapes of the Ministry of Agriculture and is a candidate for UNESCO recognition as a World Heritage Site. It plays a crucial role in conserving biodiversity and maintaining ecological balance. Traditional olive groves in fact constitute an agroecosystem that acts as a buffer to protect the surrounding area from the negative effects of intensive agriculture.
The threat of Xylella fastidiosa subsp. pauca
In the last ten years however, Puglia faced an unprecedented threat to the sustainability of this precious ecosystem: the onset of Xylella fastidiosa subsp. pauca (Xfp), a pathogenic bacterium native to the American continent, which causes the “olive rapid desiccation complex” (CoDiRO or OQDS, Olive Quick Decline Syndrome), leading to serious declines and death of the affected trees. (3,4) A wake-up call for the scientific and agricultural community, which has carried out a wide range of studies and monitoring to better understand the spread and impact of this pathogen. (5)
Identified for the first time in Puglia in 2013, Xylella fastidiosa has triggered a health alert at European level, given its ability to infect, in addition to the olive tree, around 500 plant species, including vines, almonds and oleanders.
The transmission of the pathogen occurs mainly through the feeding of insect vectors, such as Philaenus spumarius L., the ‘Meadow Spittle’, which feed on the sap of infected plants, spreading the disease.
The pathogenic bacterium colonizes the xylem vessels of the plant, causing a blockage in the flow of water and nutrients, leading to leaf burning, drying and other symptoms, up to the death of the plant. (6)
The diffusion of this bacterium (Figures 1 and 2) has put the survival of many ancient trees at risk, negatively influencing local biodiversity and threatening traditional agricultural practices that have supported rural communities for generations.
2.1) Management approach and strategies
La Xylella fastidiosa is included in the A2 list of quarantine phytopathogens of the European and Mediterranean Plant Protection Organization (EPPO). (3)
The authorities regional, in collaboration with researchers and farmers, have implemented a series of measures to contain and manage the spread of Xylella fastidiosa subsp. pauca (Xfp). (7) These include
– intensive monitoring of the affected areas,
– mathematical and epidemiological models on the expansion of Xfp in olive groves,
– selective culling and eradication of plants testing positive for Xfp, including all asymptomatic ones within a radius of 100 or 50 m,
– research on biological control methods and varietal resistance,
– application of pesticides
– implementation of inspections, quarantines and import restrictions to prevent the spread of the pathogen and control the vector population,
– promotion of sustainable agricultural practices among olive growers. (8,9)
Despite the efforts, control of Xfp has proven to be complex, given the pathogen’s high variability and wide host range.
The approach adopted in the management of of the protection of cultural heritage. (10,11)
2.2) Vector control
In the last few years there has been growing interest in the use of biological control methods against the meadow spittlebug, the insect vector of the pathogen X. annoying.
The release of natural enemies, as Zelus renardii Kolenati (Hemiptera: Reduviidae), has achieved significant results in containing populations of Philaenus spumarius L.
The life cycle of the meadow spittleweed involves the eggs hatching into nymphs, which then mutate into adults. The juvenile stages live on grasses protected by a foamy secretion, this has suggested the possibility of check the insect drying out this protective foam, thus causing the death of the juvenile stages. (12,13)
These measures, however, have thus far had limited success in controlling the disease.
Results of the study
The results of the monitoring investigations conducted in Puglia from 2013 to 2022 for the presence of the bacterium Xylella fastidiosa subsp. pauca (Xfp) in olive groves show a significant evolution in the number of hectares monitored and in the number of olive trees and other host plants sampled. (1)
In terms of relationships between the plants showing the symptoms of OQDS (Olive Quick Decline Syndrome) and those actually infected by Xfp, a significant decrease in correlations was noted:
– while in the 2014-2015 campaign 69,56% of symptomatic olive trees tested positive for Xfp,
– in subsequent periods this percentage decreased significantly, reaching a minimum of 3,21% in 2021-2022.
The details of the last two campaigns (Figure 3) highlights an extremely low incidence of Xfp in all monitored areas, with a percentage that does not exceed 0,23%.
Regarding eradication measures, the data indicates a more aggressive strategy in recent years (Figure 4). The number of trees uprooted in these areas, especially following the 50 meter rule around an infected tree, remains high. And it involves a significant number of secular and monumental olive trees.
The main criticism concerns the speed and lack of transparency with which drastic decisions were made, often without an adequate environmental impact assessment or without providing concrete evidence of the presence of the bacterium in the plants designated for culling.
While in the early years the plants uprooted were much fewer than those tested positive, starting from 2021, the number of plants removed has significantly exceeded the positive ones, with an increase exceeding 1050% and almost 1300% in the last two campaigns respectively.
4) Future prospects and conclusions
The battle against Xylella annoying subsp. pauca in the olive groves of Puglia is far from being won, but the decade of monitoring has provided valuable data that can guide future management strategies. Among other things recently, a new strain of Xylella fastidiosa, which affects vineyards, almond and cherry orchards but not olive trees, was discovered in Puglia, raising concerns for local crops and requiring immediate containment measures. (14)
The protection of Apulian olive growing it requires collective commitment and innovative solutions, based on science and respect for the environment and biodiversity. Furthermore, raising awareness and training farmers play a key role in mitigating the impact of the disease.
The authors of the study in question question the effectiveness and necessity of drastic measures such as the felling of monumental olive trees and underline the importance of a multifocal approach to address the Xfp crisis, combining phytosanitary surveillance, scientific research and cooperation among stakeholders.
It is necessary continue to explore new strategies for managing xylella, including
– the genetic improvement of olive trees for resistance to Xfp,
– the use of biofertilizers e
– the development of vector biological control methods. (15)
Gabriele Sapienza
Footnotes
(1) Ciervo, M., & Scortichini, M. (2024).A decade of monitoring surveys for Xylella fastidiosa subsp. pauca in olive groves in Apulia (Italy) reveals a low incidence of the bacterium in the demarcated areas. Journal of Phytopathology, 00, e13272. https://doi.org/10.1111/jph.13272
(2) Scortichini, M. The Multi-Millennial Olive Agroecosystem of Salento (Apulia, Italy) Threatened by Xylella Fastidiosa Subsp. Pauca: A Working Possibility of Restoration. Sustainability 2020, 12, 6700. https://doi.org/10.3390/su12176700
(3) EFSA Panel on plant health (PLH). (2019). Update of the scientific opinion on the risks to plant health posed by Xylella fastidiosa in the EU territory. EFSAJournal, 17, 5665. https://doi.org/10.2903/j.efsa.2019.5665
(4) European Plant Protection Organization (EPPO). (2019) PM 7/24 (4) Xylella fastidiosa. EPPO Bulletin, 49: 175–227. https://doi.org/10.1111/epp.12575
(5) Paragraph 6 of the article, Dario Dongo. Invasive alien species, an overlooked threat. IPBES report. GIFT (Great Italian Food Trade). 10.1.24
(6) Fierro, A., Liccardo, A. & Porcelli, F. A lattice model to manage the vector and the infection of the Xylella fastidiosa on olive trees. Sci Rep 9, 8723 (2019). https://doi.org/10.1038/s41598-019-44997-4
(7) Puglia Region. (2013). DgR n. 2023: Emergency measures for prevention, control and eradication of Xf associated with “CoDiRO” https://doczz.it/doc/1397325/delibera—1999-2016—documento-1 and the National Emergency Plan for the management of Xylella fastidiosa in Italy. https://www.gazzettaufficiale.it/do/atto/serie_generale/caricaPdf?cdimg=18A0239600400010110001&dgu=2018-04-06&art.dataPubblicazioneGazzetta=2018-04-06&art.codiceRedazionale=18A02396&art.num=1&art.tiposerie=SG
(8) Kottelenberg D, Hemerik L, Saponari M, van der Werf W. Shape and rate of movement of the invasion front of Xylella fastidiosa spp. pauca in Puglia. Sci Rep. 2021 Jan 13;11(1):1061. doi: 10.1038/s41598-020-79279-x. PMID: 33441697; PMCID: PMC7806996. https://doi.org/10.1038/s41598-020-79279-x
(9) White SM, Navas-Cortés JA, Bullock JM, Boscia D, Chapman DS. Estimating the epidemiology of emerging Xylella fastidiosa outbreaks in olives. Plant Pathol. 2020; 69:1403–1413. https://doi.org/10.1111/ppa.13238
(10) Dario Dongo, Giulia Torre. Xylella and emergency decree, debate and petition. GIFT (Great Italian Food Trade). 19.4.19
(11) Dario Dongo, Marina De Nobili, Guido Cortese. Xylella Fastidiosa, the solution at your fingertips. GIFT (Great Italian Food Trade). 23.2.19
(12) Cornara Daniele, Porcelli Francesco, Boscia D., De Stradis Angelo. (2014). Survey of the Auchenorryncha in the Salento peninsula searching for vectors of Xylella fastidiosa CoDiRO strain, and Biology and ethology of Philaenus spumarius. https://www.researchgate.net/publication/271443963_Survey_of_the_Auchenorryncha_in_the_salento_peninsula_searching_for_vectors_of_Xylella_fastidiosa_codiro_strain_and_Biology_and_etology_of_Philaenus_spumarius
(13)Cornara, D., Bosco, D. & Fereres, A. Philaenus spumarius: when an old acquaintance becomes a new threat to European agriculture. J Pest Sci 91, 957–972 (2018). https://doi.org/10.1007/s10340-018-0966-0
(14) New strain of Xylella found in Puglia, a plant pathologist agronomist speaks. fresh plaza. https://www.freshplaza.it/article/9603054/nuovo-ceppo-di-xylella-ritrovato-in-puglia-parla-un-agronomo-fitopatologo/
(15) Dario Dongo, Andrea Adelmo Della Penna. Biostimulants in olive growing, organic revolution. Scientific review. GIFT (Great Italian Food Trade). 14.5.21
Graduated in Agronomy, with experience in sustainable agriculture and permaculture, laboratory and ecological monitoring.
