Xylella: Fakes, FAQs & Facts

A Xylella fastidiosa epidemic is threatening the olive oil production in Mediterranean countries.

In light of the severely damaging misinformation spread by science deniers and some journalists, I prepared a list of recurring questions, all related to frequently spread fake news.

Is it true that researchers are still debating about whether Xylella fastidiosa is correlated to the dieback of olive trees?

This is a recurring fake news inflated by activists denying science and by some journalists who endorse their views. One of the most common techniques used by pseudoscientists to cheat the public is to claim that there is a division in the scientific community and to give the same weight to the unsupported views of some few researchers in contrast with those largely prevalent in the scientific community opposite views (false balance). While science, by definition, can never be settled on a defined topic, the debate on its results can, until there is new evidence.

That said, there are  now several research papers from different research groups all over the world (published beginning in 2014, just a few months after the discovery of the bacterium in Apulia), reporting a robust correlation between the infection of Xylella fastidiosa subsp. pauca and the development of Olive Quick Decline Syndrome (OQDS) in at least three different countries (Italy, Argentina, and Brazil). Moreover, experiments showing that inoculation of olive trees with the Xylella strain found in Apulia leads to OQDS and plant death have also been published. Here is a partial list of papers, including original data in support of the role of Xylella in OQDS.

The number of infected olive trees found during all the yearly monitoring campaigns in Apulia is always small, in comparison to the overall number of plants screened (around 2%). Why do experts and officers talk of an epidemic if there are only a few plants infected?

To infer information on the overall prevalence of infected plants in Apulia using the percentage of infected trees identified in a monitoring campaign, as well as the overall number of plants tested and found infected during the monitoring campaigns, is misleading, as the scientific community and the Apulian phytosanitary service have repeatedly confirmed in response to the false claims made by some journalists and Science denialists.

Yearly monitoring campaigns in Apulia are devoted to mapping the area where the bacterium is currently identifiable. This, in turns, implies that the monitoring activity is concentrated in areas considered still free of the bacterium or where the bacterium has arrived very recently. In these areas, most of the sampled plants are expected to be free of the bacterium, and in fact they are. Unfortunately, each year a few infected plants are found in areas, where the bacterium was previously unknown, and/or newly infected trees are discovered in areas, where only very few plants were known to be infected previously; this testifies to the advancement of the epidemic. To understand how the infected trees can pass from 2-3% of the plants in an orchard to 20 times more in a single year, please refer to:

Far from the areas monitored (where, as noted, the percentage of infected plants is low), if you sample at the heart of the epidemic, in places where Xylella has long gained a foothold, the devastation is clearly apparent, in the form of uncountable dead or dying olive trees. There, by sampling symptomatic (but not yet dead) trees, researchers invariably found the bacterium. See, for example:

There are some olive trees, declared infected several years ago but still standing, which do not show any symptoms and are healthy. One example is an olive which was said affected by EFSA in 2015, which is however still perfectly intact and healthy. Why do researchers claim that Xylella is such a severe threat, and there is no hope for infected plants?

When compiling its 2015 database of Xylella host plants (not intended to map infected olive trees), EFSA produced a supplementary Excel table containing a list of literature-derived examples of individual infected plants for the various possible host species. Among them, EFSA included several olive trees, documented at the time in the literature. When, in the original paper used as a source by EFSA, the geographical coordinates were missing, EFSA inserted the generic (Centroid) coordinates for the approximate localisation of the corresponding plants. For example, if in the original paper the actual coordinates were not given, but the infected plants were said, e.g., to be in Salento, EFSA inserted the geographical coordinate of the Salento centroid (according to some of the multiple calculations available for it). Therefore, you could find in the Excel files several plants with exactly the same latitude and longitude or coordinates corresponding to some commercial activity in the centre of a town. This is also the origin of the coordinates provided for an infected tree, which was labelled with the Apulia centroid coordinates (taken from Wikipedia). Those coordinates, however, happened to correspond to a single olive plant in the Corato municipality. Science denialists, eager to find a plant “officially infected,” used that plant as an example of “officially infected,” but asymptomatic, tree. EFSA has now clarified how the plants in its databases were georeferenced, has inserted more accurate coordinates if available and has specified the intended usage of its “host plants” list.

One olive tree was found infected in the municipality of Monopoli and destined for uprooting. However, when the tests were repeated, the very same plant was found free of the bacterium. How can we trust the laboratory analyses?

Errors in a procedure involving the sampling and testing of several hundreds of thousands of plants are always a risk. To minimise it, samples are checked independently, with different methods, by two laboratories, who are blind concerning the provenance of the samples. However, the correct attribution of a sample to a given plant relies on the accuracy of the sample labelling on-site, which is not controlled independently. Samples from the tree in question might have been erroneously labelled; resampling and blind repetition of the laboratory tests subsequently corrected the error.

In facts, PCR testing of trees for the presence of Xylella is accurate and reproducible. When independent research groups re-tested some trees, which were previously found infected by others, results have always been consistent. For example, consider the following papers:

Both those papers report the retesting of olive trees previously found infected, which show the consistency of the molecular tests available for Xylella.

More evidence for the reproducibility of monitoring results are provided by some double-checking of selected olive trees sampled twice by independent field operators. One example – an infected olive tree, tested twice and consistently found to host the bacterium – is contained in the SELGE prot. 192/2018 and SELGE prot. 53/2019, as can be seen comparing the coordinates for the only plant of the Fasano municipality in both the SELGE documents.

Several different approaches, both empirical and scientific, proved that there are remedies for treating infected trees. Why do researchers and officers claim that there is still no cure for the disease?

So far, there are no remedies for the infection by Xylella, irrespective of some publications on a small number of olive trees claiming the effectiveness of a zinc-copper citrate complex (Dentamet®) or a publication in a predatory journal reporting the effectiveness of a treatment called Mycosat®. This is also reflected in the following EFSA documents:

On empirical grounds, grafting of resistant cultivars to save infected trees seems a promising approach, but the scientific proof of the approach’s effectiveness is still lacking.

Other proposed “empirical” remedies, including soap, “informed water”, planting of “Apulian broccoli” in infected or susceptible orchards and other oddities lack evidence of any effectiveness and/or a mechanism of action.

Why are factors others than Xylella, such as fungal infections or environmental pollution, not taken seriously into accounts by the scientific community?

This is not true.

First, when the Xylella was initially detected on olive trees, the publication reporting its finding was flanked by another reporting the complete list of other known olive parasitic threats. At the time (2013), scientists were still investigating whether the quarantine pest Xylella was the one and only responsible for the olive quick decline syndrome, as it later turned out to be the case. This proves that scientists were at the time open to considering many possibilities, but the evidence quickly turned in favour of Xylella as the “necessary and sufficient” pathogen.

Second, in June 2018 the results of the research project “Epizyxy”, dedicated to evaluating a possible role of fungal infections in the olive quick decline syndrome, were presented to the public in Lecce by prof. F.Nigro. None of the several fungal pathogens was connected to the olive dieback; there were no differences in the abundance of fungal species in olive declining for the OQDS with respect to control plants.

Third, environmental pollution, usage of chemicals, organic regimen and other variables were never found to be connected at all to the OQDS in a consistent and conserved way. On the contrary, even when testing orchards very different with respect to environmental and management parameters, those variables did not emerge as driving the disease.

Is it possible that the entire Xylella epidemic story is made up, by those who want to transform the traditional landscape in favour of more lucrative economic activities, or that the bacterium was introduced on purpose to force landscape transformation?

To think that there was a deliberate fraud like that, one should imagine a world plot involving researchers and authorities to spread Xylella fastidiosa subsp. pauca among olive orchards located in:

  1. Italy
  2. Brazil;
  3. Argentina;
  4. Spain;
  5. France.

Xylella is by now a global threat to olive trees, and childish plot theories should be abandoned.


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