Xylella Fastidiosa Disease Threatening the Olive Trees, the Remedies… and the Dogs

Anyone who loves the olive tree will have heard of the disease caused by the bacterium Xylella fastidiosa in recent years.

The bacterium Xylella fastidiosa (Xf), is one of the most dangerous diseases in the olive tree, hence its name, Xylella ‘the fastidious’, as it leads to Olive Quick Decline Syndrome (OQDS).

It is a plant pathogenic bacterium with over 300 species of hosts (organisms that carry a disease to other organisms). In this case, the other organisms are mainly olive, grapevine, plane tree, oleander, lavender, rosemary and citrus, while the hosts are mycorrhizal insects of the families Cercopidae and Cicadellidae, such as cicadas, where they grow in the woody vessels of plants and move both upwards and downwards, interfering with the movement of their sap, i.e. water and nutrients, from the root to other parts of the plant.

But let’s see, where did the bacterium of Xylella began and how is it transmitted?

Xylella is not a new disease. It was first reported in California in the 1930s in citrus and stone fruits, and there have been sporadic reports in Asia.  The first symptoms in Europe were detected in September 2013 in southern Italy, in the Lecce region of Apulia. Subsequently, the disease quickly spread throughout Italy, Spain, France, France, Portugal, Belgium, Germany and Cyprus. Fortunately, no presence of the bacterium has been found in our country to date.

The Mediterranean climate and high temperatures (optimal 26-28 oC), combined with the health of the plants, favour the development and spread of the disease. Xylella, has the ability to be transported over long distances through infected propagating material, but also by the insects that carry it, as mentioned above. However, negligible transmission has been observed through fruits and plant parts, and low transmission through seeds. At short physical distances, it is transmitted through host insects.

xylella fastidiosa bacterium
Πηγή: Shutterstock

But how can we tell if a plant carries the Xf bacterium?

The symptoms of the disease are stunting, chlorosis, then wilting and finally leaf burning. At the same time, the tips of the branches become dead, with the final stage being the collapse of the whole tree. So it is not surprising why the symptoms can often be confused with water shortage, high salinity, sunburn and also verticillosis (Verticillium dahlia fungus).

There is of course the possibility that a tree, although infected, may not show any symptoms, which makes it difficult to identify the bacterium. In general, once an olive tree is infected it will show visible symptoms for up to 390 days, and it is important to note that trees in poor health will show symptoms more quickly than others.

Is there a way to fight the disease?

Unfortunately, to date, no official cure has been found. Therefore, the only weapon is prevention and early notification of the competent authorities in case of symptoms, so that the bacterium does not have time to establish itself on a large number of plants.

In the event of an infestation, it is proposed to grub up the affected trees, which has provoked strong reactions, and to spray with insecticides in order to reduce the host populations. Of course, the intensive application of such preparations has a significant impact on the safety of the food produced, but also on the ecosystem. In the long term, therefore, practices that do not contribute to biodiversity, soil health and ecosystem sustainability provide fertile ground for new diseases.

Given the above and in combination with the urgency of the situation, the International Network for Xylella ‘Lubixyl innovations’, in which our country participates, advocates more sustainable practices. Specifically, through laboratory research, it was found that the enzyme of galactose, lactoperoxidase (which helps protect infants from the bacteria), if sprayed on affected trees, kills the bacterium and the plants survive. Thus, in large-scale applications, it may be a mitigating factor in the problem.

An important finding is that varieties that show partial resistance, although this does not mean that they do not become diseased. In particular, Fs-17 shows fewer symptoms, while the Italian Leccino shows signs of infection after 2 to 3 years. However, as for the more famous Greek varieties, Kalamon and Koroneiki were found to be particularly susceptible to the bacterium.

What measures must be respected in Greece?

According to the European Union, Xylella is included in the list of the 20 most harmful organisms. Its consequences translate not only ecologically, but economically, socially and culturally. 

In Greece it was found that over a period of 50 years the economic damage due to production losses can reach 2 billion. In addition, at the cultural level, the bacterium has killed over-aged olive trees, which were part of cultural heritage.

Xylella can enter our country through contaminated plant material imported from other countries, as happened during the Olympic Games with the palm weevil. Thus, certain measures need to be taken into account. Some of them are:

  • Trees transported to Greece must carry phytosanitary passports, which have been presented to the company by the competent authorities of each country. Any import operation not accompanied by the necessary documents is considered illegal.
  • Trees still being transported from Greece within or outside the country must also comply with the phytosanitary legislation.
  • Greek nurserymen should not choose areas with high incidences of Xf, for the import of plant material or in case of such necessity the control should be too strict.
  • At the same time, nurseries involved in production should carry purchase invoices in order to facilitate the traceability of the disease.
  • There should be effective coordination and integration of public agencies to facilitate detection and control of the disease.
  • For any suspicious symptoms in plant species (such as olive, oleander, vine, etc.), immediately contact the respective DAOK of the area for further investigation.
Xylella detector dogs delta trees
Credits: Agostino Petroni

Fact! Specially trained dogs against Xylella fastidiosa (Xylella Detector Dogs)

And yet scientists are enlisting dogs to fight the bacteria! Specifically, specially trained dogs, known to have a highly developed sense of smell (20 times more olfactory receptors than humans), have the ability to sniff out the bacterium. 

So in 2020 Nicola di Noia, general manager of Italy’s largest olive oil producer consortium and an agronomist, having previously worked with dogs to detect drugs and explosives as a gendarmerie officer, thought of the case of detecting the bacterium.

However, the most suitable dog breeds for detecting the bacterium are the Labrador, Coker Spaniel, Springer Spaniel and German Shepherd. Of course, this training is not easy and takes time, but experts hope that it will be another through for the treatment of the disease.

The dog is not only man’s best friend, but also the olive trees’!