Italian, Spanish and Greek farmers set to lose billions unless drastic action is taken to combat bacterium that has killed millions of trees

One of southern Europe’s most important staples, olive oil, is under pressure from a potentially deadly disease that new research shows could infect nearly all of the productive areas of Italy, Greece and Spain.

Economic losses could be as high as €5bn over the next 50 years for Italy, where at least 1m trees have already died, if nothing is done to halt the spread of the disease and olive groves are not replanted, the study found. In Spain, losses could total €17bn over the same period, and Greece could suffer to the tune of about €2bn.

Drastic action would be required to halt the spread, including felling seemingly healthy olive trees. However, if preventive measures are taken, and if orchards can be replanted with resistant strains of olive, the economic impact could be cut to as little as €10m a year in Italy, with proportionally similar benefits in Spain and Greece. The three countries represent 95% of European production.

About 17% of Italy’s olive producing areas are currently estimated to be infected with the disease, olive quick decline syndrome, which has become colloquially known as olive leprosy and has also struck in France, Spain and Portugal. There is no practical cure for the disease, though work to develop resistant varieties is under way.

The researchers analysed to what extent the climatic conditions in each of the three countries was suitable for the spread of Xylella fastidiosa, the bacterium that causes olive quick decline syndrome. The first cases of the disease, which is transmitted by insects and causes delayed growth, withering leaves, declining fruit and the eventual death of the tree, were found in Italy in 2013.

They found that most current olive production in all three countries falls within the range of climates in which the bacteria can be expected to flourish, with between 85% and 99% of all producing areas susceptible. The spread of the disease is currently 5km a year, but could be reduced to a little over 1km a year with appropriate measures.

Facebook Twitter Pinterest A stricken olive tree in a field belonging to the Mediterranean Agronomic Institute (CNR) in Apulia, Italy, used for research into the disease. Photograph: Charles Onians/AFP/Getty Images

Some of the measures likely to be needed to slow the spread of the disease, however, will cause hardship and will not be popular. They involve tree felling around areas where the disease has gained a foothold to create a cordon sanitaire, needed because many trees can be infected without showing outward symptoms of the disease. But the scientists noted that when these control measures were tried, they “resulted in great societal unrest in the affected region”.

Preparing for plant diseases, even of vital crops, is “a notoriously difficult problem”, said Kevin Schneider, of Wageningen University in the Netherlands, lead author of the paper published in the journal Proceedings of the National Academy of Sciences. “There are clear guidelines and protocols enacted to prepare as much as possible against the introduction of harmful plant diseases. [But] some species can change ecosystems, some have economic impact, and some do not establish successfully at all and in turn do not cause impact.”

He told the Guardian that farmers and governments in the affected regions should heed scientific advice. “Farmers need to stay vigilant and adhere to the imposed mitigation measures. Government interventions are warranted with regard to economic considerations, and government support for adaptation strategies, such as [developing and propagating] resistant varieties, is important.”

The losses are not just economic, he added: “We did not account for the cultural heritage value associated with these trees. Some of the European olive trees are hundreds of years old and there is currently no practical cure under field conditions for the disease. I think aiming at containing the pathogen and finding ways to adapt to the situation is certainly a good idea.”

It is unlikely to be possible to eradicate the disease altogether, the researchers concluded, owing to the size of the area infected, the problem of symptomless but infectious host plants, and the fact that the tight network of olive orchards in Apulia in Italy will serve as a reservoir.

In 2017, the production value of olives in Europe was about €2.4bn, and for olive oil €6.7bn.