Researchers shed light on the mechanism of resistance to Xylella fastidiosa

Researchers from all over the world are carrying out a comprehensive investigation under the EU BeXyl project, to identify new resistant genotypes and give plant breeders new tools to obtain cultivars less vulnerable to Xylella fastidiosa infection. The ultimate goal is to integrate phenotypic, physiological, and genetic data to identify key genes and develop reliable markers associated to resistance. These tools will accelerate the selection of resistant cultivars and strengthen the resilience of European agriculture against Xylella fastidiosa.

The Context

Host plant resistance is the main pillar of a successful management of Xylella fastidiosa – one of the most threatening plant pathogens for European agriculture and biodiversity. But identifying new resistant genotypes and understanding if and how the resistance traits can be transferred to other cultivars requires years of research. Thanks to the BeXyl Project, an international team is carrying out one of the most comprehensive investigations on this matter, combining experiments in controlled environment, field observations, and in-depth genomic analyses, with the aim of decoding resistance traits to the pathogen, and laying solid foundations for genetic improvement.

The Research

Researchers from institutes and producers’ organisations involved in the project (ARO-VOL, IFAPA, CSIC, IVIA, UDG, CNR, Auburn University, University of Munich, CRSFA, UNAPROL, IRFAP, INTA, and University of Costa Rica) studied the response of olive, almond, coffee, persimmon, carob, and other species to infection by Xylella fastidiosa in greenhouses experiments. The plants were deliberately exposed to different bacterial strains, primarily of the multiplex and pauca subspecies, with a particular focus on almond and olives, the most valuable affected crops in Europe.

The results showed high variability:

·         In olives, cultivars such as ‘Arbequina’ and ‘Arbosana’ showed high infection rates, especially with the CoDiRo strain of subspecies pauca – the most virulent to this crop. Others, like ‘Leccino’, ‘Frantoio’, and ‘Gordal’, had lower bacterial loads and fewer symptoms, confirming signs of tolerance.

·         In almonds, ‘Belona’ and ‘Marcona’ cultivars were highly susceptible, while ‘Antoñeta’ and ‘Avijor’ showed lower susceptibility.

·         Some species — persimmon, carob, pitaya — did not become infected, suggesting they are not natural hosts of the subspecies of the bacterium evaluated.

The living laboratory: Studying resistance in infected areas

Alongside greenhouse trials, it is essential to observe plant behavior in areas where the disease and its vectors are present. For this purpose, in 2025 a large international olive collection was established in Sencelles (Mallorca, Spain), in a unique area of the world where the three main Xylella fastidiosa subspecies (fastidiosa, multiplex and pauca) coexists within a very small geographical area.

The collection includes 372 plants from 26 cultivars and 12 countries of origin. This collection will be maintained beyond the project duration and is expected to provide valuable information on varietal susceptibility. This field represents a unique platform to understand how cultivars respond over time to natural infection and to identify the most tolerant ones.

Inside the plant: Molecular and physiological analyses

To better understand the mechanisms that allow some cultivars to tolerate Xylella fastidiosa infection, several research groups conducted a series of molecular and physiological analyses, identifying over one million genetic differences in olive varieties to pinpoint genes potentially involved in disease tolerance. Transcriptomes — the set of active genes — were studied in healthy and infected tissues of tolerant and susceptible cultivars to determine which genes are “turned on” or “off” during infection. Scientists also analysed the composition of nutrients in the plants, to understand whether and how infection alters the balance of essential mineral elements. Measurements of responses to water stress were conducted, as water availability strongly influences disease progression and can modulate a plant’s ability to resist infection.

In parallel, the University of Munich (LMU) group studied a specific family of immune receptors, called LRR-RLK class XII, to identify molecular differences between susceptible and resistant cultivars, seeking possible innate defence mechanisms.

Towards new resistant varieties

Based on the acquired knowledge, the breeding groups produced 300–500 new seedlings each year, evaluating both agronomic traits and potential resistance to Xylella fastidiosa. New crosses – including ‘Arbosana’ × ‘Leccino’ – are now under field evaluation.

Outlook

In the fourth and final year of the project, researchers will expand the experimental platform, assess almond collections and crosses, and complete the analyses on 900 olive plants, as all as perform the propagation and planting of selected new genotypes.

At the same time, studies on LRR-RLK class XII immune receptors will be deepened through functional laboratory analyses to identify molecular mechanisms underlying resistance to Xylella fastidiosa. These activities will ensure the continuity of data collection and the valorisation of experimental materials beyond the project’s duration.

Xylella fastidiosa

Xylella fastidiosa is one of the world's most threatening plant pathogens, able to colonise over 700 plant species. It is responsible for diseases causing millions of dollars' worth of agricultural and biodiversity losses in the US and Brazil. In October 2013, the bacterium was detected in the EU for the first time, on olive trees in the Apulia region of southern Italy. Since then, outbreaks have also been detected in regions of France, Spain and Portugal.

Further information can be found at: https://www.efsa.europa.eu/en/topics/topic/xylella-fastidiosa

BeXyl – Beyond Xylella

The EU-funded project BeXyl-Beyond Xylella brings together over 30 partners from science, industry, and public authorities. The project aims to conduct research and implement the most sophisticated management strategies against current Xylella fastidiosa outbreaks, as well as preventing new introductions to Europe.