"Xylella cannot be eradicated, we have to learn to live with it"

How did you get involved in the study of Xylella in the Balearic Islands?

I started through an internship at the Plant Health Laboratory of the Balearic Islands, then I was offered to continue on this project subsidised by sustainable tourism tax. Specifically, I worked on Lot 6, which focused on studying the seasonal dynamics of symptoms and infection levels in three crops: olives, almonds and grapevines.

For those who are not familiar with it, what exactly is Xylella fastidiosa?

It’s a bacterium that lives in the xylem of plants. It can’t be spotted directly, but causes symptoms similar to drought or nutritional deficiencies. This is why it’s difficult to detect with the naked eye.

Why’s it so dangerous for Balearic crops?

Because it clogs the xylem, preventing water from passing through. And here lots of plantations aren’t watered and don’t receive regular care. In such cases, the bacterium easily settles and spreads, gradually causing the tree to die.

How can farmers detect whether their crops are affected?

In almond trees, the leaves dry out starting from the tips, with a yellowish halo. In grapevines, this halo can be reddish or yellowish depending on the variety. There are also symptoms such as dry branches, fallen leaves that make the stem dry (like a stick), and isolated green areas between dry tissue. In olive trees, drying is similar.

What makes it so difficult to fight?

There’s currently no product that can reach the xylem and kill the bacteria. This means we can’t eradicate it. The key is to keep plantations healthy by irrigating, fertilising and pruning them. In well-managed crops, the disease progresses much more slowly.

And those were some of the main findings of the project?

Yes, although the project lasted three years, which is a short time for a bacterium like this. We study seed banks with many varieties of almond trees, vines and olive trees. In almonds, for example, the variety 'Vairo' showed low affectation even after several years. But we also saw that external factors, such as climate, management, etc., have a strong influence.

Could climate change be increasing the spread of Xylella?

Yes. Intense cold can kill the bacteria, but with the mild winters we’re having, we aren’t reaching the temperatures needed to stop it.

You mentioned irrigation and fertilisation earlier, how exactly do they play a role?

A lot. Plantations suffer less when they’re properly watered and fertilised. All have the potential to become infected, but in a well-tended plot, disease progression is greatly slowed down.

So, should we assume that we must live with Xylella?

Exactly. In the Balearic Islands we’re now in a phase of containment, not eradication. In America, they’ve been living with this bacterium for many years. We have to change our mindset: plantations will no longer last 40 or 50 years. Trees will need to be replaced as they die.

Are these studies of any practical use for farmers?

Yes, a great deal. They help in deciding which varieties to plant and how to manage them. Moreover, the main carrier of the disease, the insect Philaenus spumarius, can be targeted with specific treatments.

What was it like to collaborate with other scientific institutions on this project?

It was hugely enriching. I only did a fraction of the work. Institutions such as the Cordoba branch of the Spanish National Research Council (CSIC) were involved, each with a different approach. Collaboration was key to tackling such a complex problem from different fields.

What lines of research should be developed now?

A very useful one would be to analyse the genetics of less vulnerable varieties. If we identify key genes for resistance, we could work on breeding programmes to make crops more resilient to Xylella.