Xylella fastidiosa, a bacterium that threatens tree fruit growing as a whole
Savvas Pastopoulos, Agronomist MSc — Pastopoulos Agricultural Ltd
Xylella fastidiosa is a pathogen that is still not widely known among Greek growers, yet it threatens Greek tree cropping as a whole. It is responsible for enormous destruction in the olive groves of southern Italy, and in other crops worldwide. In combination with a complex of wood-rotting fungi and with the leopard moth, it produces olive quick decline syndrome. Introduction of the pathogen into Greek territory could happen at any time.
in the Salento
affected
recorded
available
- The bacterium and its history
- How the plant falls ill — the biofilms
- The vectors — where the battle is decided
- Olive quick decline syndrome
- Symptoms in the other crops
- How large the problem is today
- Where Greece stands
- Management — why there is no cure
- What actually works
- What the grower does
- Sources
1. The bacterium and its history
The disease attacks mainly old olive groves, killing the trees. It had appeared earlier in California as well: as far back as 2010 the bacterium had been linked to the death of olive trees in urban plantings. However, the detection of the bacterium in symptomless trees as well caused confusion about the exact cause of the dieback.
The bacterium has been found throughout the Americas, in Italy, France, Spain and elsewhere. It was first recognised as a pathogen in 1987, when it was identified as the cause of Pierce's disease in the vineyards of California and of phony peach disease. It was subsequently reported as the principal cause of destructive diseases in almond, plum, sweet cherry and other species.
The bacterium belongs to the species Xylella fastidiosa and is divided into subspecies. Genetic work established that the Italian strain is different from the Californian one: it is Xylella fastidiosa subsp. pauca, most probably originating in Central America. That strain is known internationally as ST53 and is recognised as the causal agent of olive quick decline syndrome.
The host specificity of the strains has not been established with sufficient certainty. The Italian strain does not attack grapes or citrus, but it does attack olive, almond, sweet cherry and others. Two different subspecies have appeared on almond. In practical terms this means that it is not possible to predict which crops would be hit if the bacterium arrived — that depends on which strain arrives.
2. How the plant falls ill — the biofilms
The bacterium grows very slowly and establishes itself in the vascular tissue of the plant. It is transmitted mainly by insect vectors. Within the plant it can move through the vascular bundles both upwards and downwards, and during establishment there is also movement from one vessel to another — movement that is decisive for the progress of the disease.
Inside the xylem vessels the bacterium produces biofilms, which block the vessels. The resulting shortage of water and nutrients is what produces the foliar symptoms. The plant does not die from a toxin — it dies of thirst, with wet soil beneath it.
Biofilms are bacterial structures that protect the population from external agents such as antibiotics or antibodies, while also regulating the supply of food and water. The bacteria inside the film act collectively. The film consists mainly of proteins and water; its formation in the laboratory takes 20 days and proceeds in four phases, and it has been found to contain 144 different proteins.
The biofilm is the reason there is no cure. This is not a free-swimming bacterium that can be struck; it is an organised colony inside a protective shell, in a tissue that no contact product reaches.
Each plant species shows different symptoms when the disease is expressed: peach shows stunting, while on almond the leaves dry from the tip towards the petiole.
3. The vectors — where the battle is decided
The bacterium is transmitted mainly by insect vectors — spittlebugs and related xylem-feeding hemipterans. It can persist inside the insect's foregut and can multiply there. This persistence within the vector is a key element in the spread of the disease, since the bacterium can produce biofilms inside its insect hosts as well.
In Italy the spread of the bacterium was driven by Philaenus spumarius, a hemipteran of the family Aphrophoridae. The insect deposited the bacterium directly into the plant tissue, transmitting it from tree to tree. Eighty per cent of the insects collected from infected holdings were able to transmit the bacterium to healthy olive plants.
The most practically useful fact in the whole subject. The nymphs of Philaenus spumarius develop on the spontaneous herbaceous ground cover — not on the trees — and have very limited mobility. They do not contribute to the spread of the disease. They are, in other words, an immobile, concentrated and accessible target, unlike the adults, which fly.
And the figures are clear: in late spring, after the population peak, soil tillage reduced the density of P. spumarius by 60%, whereas frequent mowing of the cover reduced it by only 20% relative to the controls.
Tilling at the right moment is therefore three times as effective as mowing. In the affected areas of Italy, mechanical removal of the ground cover is now a compulsory measure. A second vector in the region is Neophilaenus campestris.
4. Olive quick decline syndrome
Once the vessels are blocked, the symptoms appear as dieback that starts at the tip and moves towards the petiole and closely resembles various nutrient deficiencies. The tops of the tree then dry out, and finally the whole plant. The end result looks very much like fire scorch.
The reason is simple and merciless: the bacterium is present in the entire vascular system, roots included. Cutting the canopy does not remove the pathogen — it merely forces the tree to spend its last reserves on growth that will also become blocked.
Olive quick decline syndrome was attributed to this bacterium in combination with fungi that attack olive wood and with the leopard moth — that is, it is a complex, not a single-factor disease.
5. Symptoms in the other crops
Beyond olive, the bacterium attacks almond, peach, sweet cherry, grapevine, plum, plane and oleander, among others.
Grapevine — Pierce's disease
The first symptoms are sudden death of large sections of the leaf blade. Around the necrotic patches the leaves turn red and yellow. The blades frequently drop from the petioles, leaving the petioles attached to the vine — a characteristic sign. The similarity of the symptoms to those of other diseases rules out diagnosis by eye.
Peach — phony peach disease
It appears as short shoots, short internodes and dark green leaves, with the shoots growing densely. The leaves on affected shoots remain on the tree well into the autumn. The disease progresses slowly, but whole trees are affected; they do not die, but they are less productive and more vulnerable to other diseases.
Almond — leaf scorch
The first symptoms appear in summer, on the leaves. The leaves show marginal scorch, similar to salt toxicity. A slow decline of the whole tree follows, with direct consequences for productivity.
6. How large the problem is today
The bacterium attacks so many plant species, cultivated and wild, that the scientific community is still counting — and in many of them it causes no symptoms at all.
- Hosts. The EFSA database, in the update covering the literature to the end of 2024, records 463 plant species confirmed by at least two different detection methods — and 727 species, 319 genera and 91 families if all methods are counted. The list is revised every six months.
- Apulia. In one major flare-up the infestation expanded from 8,000 to 23,000 hectares within two years. Today, in the Salento of southern Apulia, the disease has effectively destroyed the whole of the region's olive industry: more than 183,000 hectares and 21 million dead olive trees.
- Almond. In Spain the disease has spread over 878 square kilometres, across 47 infected orchards. Its appearance in the main almond-producing countries puts global supply at risk: the two European countries alone now carry about 653,000 hectares of almond.
The European and Mediterranean Plant Protection Organization (EPPO) classifies the bacterium as a serious risk to world agriculture.
7. Where Greece stands
On the basis of the surveys carried out in the member states, the territory of the European Union is considered free of Xylella fastidiosa, with the exception of the officially demarcated areas. The known outbreaks remain in Italy, France, Spain and Portugal. No outbreak has been confirmed in Greece.
The legal framework in force is now Implementing Regulation (EU) 2020/1201, which replaced the earlier 2015/789 and provides, on detection, for the establishment of a demarcated area comprising an infected zone and a surrounding buffer zone.
Olive is the most important crop in Greece, on the order of 1.7 million tonnes of fruit. Introduction of the pathogen into Greek territory could occur at any time, either through plant material or through infected insects carried by international transport.
And there is one further factor specific to the country: the physical form of Greek olive groves, with centuries-old trees on difficult terrain, will work against the control of any epidemic. These are not linear plantings with mechanical access, as in southern Italy.
The preparedness of the authorities, of agronomists and of growers has to be such that any action can be taken instantly, so that Greek agriculture is not led into a catastrophe.
8. Management — why there is no cure
The disease has no meaningful chemical cure. The measures that have to be taken are preventive.
The preventive measures in order of importance
- Keeping the bacterium out of the country and setting up a rapid-response mechanism at state level. This is the most important of all.
- Informing growers, so that they can recognise the disease at its early stages.
- Immediate notification: anyone who notices the slightest symptom must notify the local Directorate of Rural Development at once. The sample has to be examined by the Benaki Phytopathological Institute.
- Control of propagation material. Introduction of the bacterium is most likely to occur through propagation material from Italy or another infected region. The European Union has banned the movement of propagation material from the province of Lecce in Apulia.
In Italy, copper–zinc products were tested as foliar fertilisers and produced a revival of diseased plants. However, those plants remain infected and continue to act as sources of inoculum for the healthy ones.
According to the assessment by the European Food Safety Authority (EFSA), there is NO cure, and any application of either bactericides or copper and zinc fertilisers — although it may improve the appearance of the plants — is ineffective at eliminating the disease, because infected plants remain infected.
To date, destruction of the affected trees on site appears to be the only effective method of preventing the disease from spreading.
9. What actually works
The most significant progress since the epidemic broke out. Two olive cultivars have proved resistant to X. fastidiosa subsp. pauca:
- Leccino
- FS-17, also known as Favolosa
The mechanism: the resistance is due to the low presence of the bacterium within the xylem. Infected trees of these cultivars show limited dieback in the canopy, with low and localised bacterial populations. This is not immunity — it is tolerance that keeps the tree alive and productive.
The practical consequence is already institutional: in the infected areas of Italy, the legislation in force permits olive groves to be replanted only with Leccino and FS-17. Replanting with resistant genotypes is today regarded as the most feasible and most promising strategy for managing the bacterium.
10. What the grower does
In a region with no outbreak — that is, in all of Greece today
- Propagation material only with a plant passport and documented origin. No import of plants from demarcated areas, on any pretext — ornamentals included.
- Attention to ornamentals. Oleander, lavender and rosemary are among the plants under surveillance. An ornamental plant in a yard is an entry point every bit as much as an olive sapling.
- Observation. Any dieback of olive tops, any marginal leaf scorch on almond that is not explained by salinity, drought or nutrient deficiency, deserves a second look.
- A sample, not a spray. On any suspicion, notification of the Directorate of Rural Development and dispatch of a sample. No product is going to help, and delayed diagnosis is exactly what destroyed Apulia.
Note: this article is informative in character. The phytosanitary situation changes; for the official and current picture of the demarcated areas, the announcements of the European Commission and of the Greek Ministry of Rural Development and Food apply.
11. Sources
- EFSA (2016). Update of a database of host plants of Xylella fastidiosa. EFSA Journal 14(2):4378.
- EFSA (2016). Treatment solutions to cure Xylella fastidiosa diseased plants. EFSA Journal 14(4):4456. The assessment on which the statement that there is no cure rests.
- Saponari, M. and co-workers — papers on the identification of the strain and on transmission by Philaenus spumarius in Apulia.
- Martelli, G.P. and co-workers — reviews of olive quick decline syndrome.
- Xylella fastidiosa — European Commission, Plant Health. The official page with the demarcated areas and the measures in force.
- Implementing Regulation (EU) 2020/1201 on measures to prevent the introduction and spread of Xylella fastidiosa (consolidated text).
- Database of susceptible host plants — European Commission.
- Update of the Xylella spp. host plant database — systematic literature search up to 31 December 2024. EFSA Journal. Source for the 463 and 727 host species.
- Ground cover management in olive groves reduces populations of Philaenus spumarius, vector of Xylella fastidiosa. Journal of Economic Entomology 114(4):1716. Source for the 60% reduction with tillage against 20% with mowing.
- Habitat manipulation for sustainable management of Philaenus spumarius, the main vector of Xylella fastidiosa in Europe.
- Trade-offs between Xylella fastidiosa vector control and conservation of beneficial arthropods in Mediterranean olive groves. Journal of Economic Entomology.
- The Xylella fastidiosa-resistant olive cultivar “Leccino” has stable endophytic microbiota during the olive quick decline syndrome (OQDS).
- Screening of olive biodiversity defines genotypes potentially resistant to Xylella fastidiosa. Frontiers in Plant Science. Source for Leccino and FS-17 and for the replanting rules.
- Susceptible and resistant olive cultivars show differential physiological response to Xylella fastidiosa infections.
- Xylella fastidiosa — European Food Safety Authority (EFSA).
- Host plants of Xylella fastidiosa — EPPO Global Database.
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