Drought and Wild Shrubs Amplify Xylella Damage in Almond Orchards, Study Finds

A devastating bacterial pathogen that has reshaped the Mediterranean countryside is not acting alone. New research from the University of Córdoba suggests that the damage inflicted on almond trees by Xylella fastidiosa, one of the most feared plant pathogens in the world, depends heavily on two forces that farmers cannot see under a microscope: the climate around the orchard and the wild vegetation at its edges. The findings, published in the journal NeoBiota, challenge the long-standing assumption that the natural environment buffers crops against disease, and they point toward a new generation of landscape-scale strategies for managing an epidemic that has already caused severe losses in Spain.

Xylella fastidiosa is an insect-borne bacterium that colonizes the water-conducting vessels of plants, eventually blocking the flow of water and nutrients until trees dry out and die. First detected in Spain in 2016, its arrival alarmed the farming community, since the pathogen is more commonly associated with olive trees but also inflicts heavy losses on almond crops. Since 2017, the Alicante region has experienced a high incidence of the disease in almond orchards, where infections reduce harvests and kill trees, translating into major economic losses for growers in one of Spain’s most important almond-producing areas.

Until now, most research on the pathogen had concentrated on detecting infections and exploring biological control options, and the principal management response has been the eradication of infected trees. Crucially, those earlier studies focused solely on the crop itself, in this case almond trees, without considering the surrounding environment. A team led by the ERSAF research group at the University of Córdoba set out to change that perspective, widening the lens to the landscape scale and examining the ecological interactions that unfold across entire ecosystems rather than within single orchards.

The aim of the work, according to Ricardo Hernández, lead author of the study, was to assess how infection by Xylella fastidiosa, environmental factors, and adjacent vegetation jointly influenced the impact of the pathogen on almond crops. Rather than treating the bacterium as the sole driver of disease, the researchers designed their investigation to capture the combined influence of climate, water availability, and the composition of plant communities surrounding affected farms, an approach that reflects how epidemics actually unfold in real agricultural landscapes.

To do this, the team gathered information on the plant health status of almond trees across 19 plots located in an area of Alicante at the center of a major Xylella outbreak. Beyond the orchards themselves, the researchers sampled the natural surroundings, recording the type and condition of the surrounding shrubs as well as the climatic factors prevailing in the area. This landscape-scale design made it possible to identify environmental and biological relationships between the different elements of the landscape and to determine how those relationships shaped the impact of the disease.

The results revealed that the disease is driven by a combination of factors rather than by the effect of the pathogen alone. Among the most striking discoveries was the role of wild shrub species that act as reservoirs of the bacterium. The researchers observed that a greater presence of rockrose, gorse, and buckthorn, shrub species carrying high pathogen loads, was linked to a greater impact of infection on the almond trees. This finding directly contradicts the common belief that the natural environment acts as a buffer against the effects of pathogens on crops. In these landscapes, adjacent wild vegetation appears to serve instead as a source of inoculum, sustaining the bacterium and intensifying pressure on nearby orchards.

Even more consequential was the role of water. According to the study, the single most important factor aggravating the impact of the disease was water stress. Temperature and the lack of water influenced the severity of infection in both the almond trees and the adjacent shrubland, demonstrating the decisive importance of climate in mediating how badly the pathogen strikes. The connection is biologically plausible: Xylella fastidiosa occupies the very vessels that carry water through the plant, so trees already strained by drought have less capacity to tolerate the additional hydraulic disruption the bacterium causes. In drought-prone Mediterranean settings, that interaction can turn a manageable infection into a lethal one.

The study was the result of a collaboration between the University of Córdoba’s ERSAF and Agricultural Entomology groups, the Institute for Sustainable Agriculture (IAS-CSIC), and the Doñana Biological Station, bringing together expertise in landscape ecology, plant pathology, and entomology. By integrating these perspectives, the team was able to connect patterns of plant health decline across an active invasion with the environmental drivers that shape them, producing one of the most comprehensive field-based assessments of how landscape context modulates Xylella impacts in Mediterranean agroecosystems.

The practical implications are significant. In light of these findings, the researchers argue that management and surveillance efforts should prioritize orchards located in high-risk settings, particularly those in fragmented, drought-prone landscapes featuring extensive cover of pathogen-hosting shrubs in the adjacent vegetation. In other words, not all orchards face the same risk, and resources for monitoring and control could be targeted far more efficiently by mapping where susceptible crops, reservoir shrubs, and stressful climates overlap. Selective control of the reservoir shrubland and supplementary irrigation to counter drought are identified as possible control strategies warranting further study, offering growers potential tools that go beyond the costly and often controversial eradication of infected trees.

For a pathogen with no known cure once a tree is infected, prevention and landscape management may prove to be the most powerful weapons available. By showing that drought and reservoir vegetation jointly amplify the toll of Xylella fastidiosa on almond trees, the Córdoba team has reframed the fight against the bacterium as an ecological problem as much as an agricultural one. As climate change brings hotter, drier conditions to the Mediterranean, the interaction between water stress and pathogen reservoirs is likely to become even more pronounced, making landscape-scale planning an increasingly urgent priority for protecting almond orchards and the farming communities that depend on them.

Subject of Research: Landscape and environmental drivers of Xylella fastidiosa impact on almond trees in Mediterranean agroecosystems

Article Title: Drought shapes the impact of Xylella fastidiosa on almond trees

Article References: Drought shapes the impact of Xylella fastidiosa on almond trees. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Xylella fastidiosa, almond trees, drought, water stress, plant pathology, landscape ecology, Mediterranean agriculture, Alicante, reservoir shrubs, crop disease management, University of Córdoba, NeoBiota