Episode 22 – Fighting Xylella, the silent plant killer

Giuseppe (00:00)

It looks like a nuclear war scenario. You go there and you see dead trees, with drying and dying leaves, with yellow leaves, all over the landscape.  

Intro (00:22)

Science on the menu. A podcast by the European Food Safety Authority.  

Ed (00:30)

Hello and welcome to another episode of Science on the menu. 

My name is Ed Bray and I work in the communication’s team at the European Food Safety Authority, EFSA.

Joining me today is Giuseppe Stancanelli. You are Team Leader for the team dealing with the Risk assessment of Plant health at EFSA. Welcome to the podcast, Giuseppe.  

Giuseppe (00:50)

Thank you very much. I'm very happy to be here.  

Ed (00:52)

Today we're going to be talking about a bacteria, a deadly bacteria, one so deadly, in fact, that it has killed more than a third of the olive trees in the southern region of Apulia in Italy.

Giuseppe let's start with Xylella - that's the name of the bacteria - Xylella fastidiosa, I understand is the scientific name. Can you talk about how it affects plants? How does it harm them? 

Giuseppe (01:20)

The name already indicates how these bacteria affect plants. It's called Xylella because it's a bacteria that works all its life in the Xylem vases of the plant, which are the vases that bring water and nutrients from the soil to the green parts of the plants.

How these bacteria affect the plant is by living in these vases, obstructing and closing the communication between the roots and the green part. Therefore, the plant is generally drying or wilting, in some cases like in Apulia the olives at the end are even dying.

Ed (01:58)

Essentially it starves plants of the water they need to survive.

What does it look like? One of these olive trees that has been affected by Xylella, what does it look like?  

Giuseppe (02:13)

For me, it was very striking when, for the first time in Apulia in October 2013, a few weeks after this disease was found, it was communicated, it looked like a nuclear war scenario.

You go there, you see dead trees with dry and dying leaves, with yellow leaves, all over the landscape. From what we discussed also with other scientists, the epidemic in Apulia is the worst epidemic ever reported. Because Xylella has been reported affecting and killing in part also vineyards in California, affecting the almonds in California, in other parts of the world, or the oranges in Brazil, but this is the first time that there was such an evident and striking disease on olives.

Ed (03:15)

Tell us, what did you feel like when you saw that?  

Giuseppe (03:19)

When I first went to Apulia and - you have to think that all my life I worked with plant disease - I felt really like crying for this, for what I was seeing, because I've never seen such a disaster.

Normally when you see a plant epidemic, you see some plants dying here and there, but still within the yield there is normally still a value for the farmer. But when you go to the south of Apulia, you just see a desert of dead trees.

It was really a very striking experience. The good news is that within this desert of dead plants, they found some olive varieties which remained green and can stand the Xylella attack.

These are the variety, for example D’Achino or other variety similar to the Favolosa from the Frantoio - another one that has been identified recently - and these varieties are the hope for the farmers to grow olives in the south of Apulia again.  

Ed (04:23)

The kind of green shoots that give hope for the future.  

Giuseppe (04:26)

Xylella is an American plant pathogen which clearly arrived in Europe with the trade of plants because it moved with the plant as it moves with other plant products and the strain from Apulia is completely identical to a strain from Costa Rica on coffee.

They also reported that this strain is present, for example, in ornamental coffee plants imported to Europe to be used in tropical gardens and zoos. So it’s very likely that's the origin of this pathogen in Europe.  

Ed (05:06)

And there's no cure for the disease that comes, and it arrived then in Europe, and it has devastated this area of Apulia.

Where else has it been found?

Giuseppe (05:16)

It’s also present in the Balearic Islands, on Corsica, in South France, in Alicante province, in Spain, and then also in Portugal, in the Mediterranean. We also have a rather large outbreak in Israel in almonds.  

The most striking epidemic is Apulian olives, but we also have seen almonds, particular almonds which are grown without irrigation, only with rainwater in summer dying in Balearic Islands and in Alicante. In other areas like Corsica, south France, Portugal, you see more this pathogen in the natural vegetation without too much damage.

Ed (06:10)

We said it's a bacteria. How does it spread from plant to plant?

Giuseppe (06:16)

It lives inside the xylematic vases of the plants, but the particular insect inside, which is feeding on this xylematic fluid - like the spittlebug in Europe or the sharpshooter in America - they are capable to suck this xylematic fluid and with that they also suck the bacteria which remain for all the life of the insect in their mouth.

Then by puncturing another plant they transmit the pathogen. This is what happened in Apulia. There was a common insect, the spittlebug, which so far was never reported to cause any damage to the agriculture in Europe. It was very abundant in olives and then it was able to transmit Xylella and, by being so abundant, to transmit the disease in very high proportion to the different fields.  

Ed (07:14)

At a certain moment, a farmer found the symptoms in the olive trees and then what happened? Then researchers came in to have a look? Tell us a bit the story.  

Giuseppe (07:27)

The first symptoms were observed, probably two or three years before the epidemic. There was a very strange disease in olives that was never reported.

Then finally in 2013, the researchers from the National Research County University of Bari were consulted. And by looking at the symptom, they made the hypothesis that it could be Xylella because the symptoms were very similar to what was reported not in olives, but in vineyards and in citruses in other continents. Then they made the test and they found out that unfortunately it was really Xylella.

Ed (08:07)

At what point did you come into the story Giuseppe?  

Giuseppe (08:11)

We came at the beginning, within the first few weeks from the announcement that it was Xylella that led to an important quarantine pathogen for Europe. We went to Apulia, we observed the situation in the field, and we drew up a fast report on what can be done to stop the spread of this pathogen in Europe. Then we continued working since 2013, more than ten years that we work on Xylella and we did the risk assessment for Europe.  

We now are in the progress of updating it with new information.

Ed (08:48)

When we talk about the control measures that can be taken, essentially, we're talking about drastic measures, digging up the trees. Is that right?

Giuseppe (08:56)

Unfortunately, there is no product at the moment which is available for field application to be used to eliminate Xylella from inside the plant. What we need to do is to control the spread of this pathogen to avoid it to go to new plantations, to new areas. 

Something you can do is work on the plant production to ensure that the plants producing the nurturing are healthy. This you can do by testing them, by producing them in greenhouses with nets that avoid this insect, the spittlebug can’t come in and transmit the disease.  

But you can also treat the plant, for example, the grape, the vineyard plants, during the period of dormancy, when they don't have leaves, they can be soaked in hot water for a certain period. This doesn't kill the plants if you do it right but kills the Xylella inside. This system called thermal therapy can be used for woody plants.  

Ed (09:59)

I was reading the economic damage of Xylella in Apulia region is something in the region of one and a half billion euros and I guess if Xylella goes to another economic crop, let's say grapes, the wine sector, the economic damage would be huge.  

Giuseppe (10:18)

We do not expect that the grape epidemic in Europe would cause so much damage. I think the olives epidemic has really been the perfect storm, because it found very abundant factors, very susceptible olive varieties, very good climatic condition, all in the same area. The estimation of potential damage of Xylella on vineyards in Europe is around 3% of the total production. Which is a lot.  

I think the impact of Xylella is not only the yield loss, but also the impact on cultural heritage, on the tradition. Because the moment you have an attack on a vineyard, you look for new varieties and all our wine production is based on the local variety. That means the impact is not only on plants, but on all the all the sectors.

Ed (11:28)

We've talked a lot there about the case of Xylella, the story, etc., and the damage that it's done.  

Can we draw some wider lessons about plant health from this?  

Giuseppe (11:41)

Trading plants it's a very risky activity because a plant is a living organism, it doesn't come alone it can bring a community of other living organisms.  

The trade of plants is regulated, after Xylella in particular in Europe, we have a new plant health law where a lot of effort has been put by Member States, by the European Commission, by EFSA, in reducing the risk for entering new pests with plants.

Somehow, we've learned a lesson. You cannot bring the risk to zero. To bring the risk to zero, you should stop any traveling, any trade, which we cannot do. We have other advantages from the free trade, from the free travel, but we have reduced the risk and this could be done by the effort of the Member States with the European Commission by more surveillance, more control there at the borders, also by the behaviours of people. I think is very important to know that when you travel, you shouldn’t bring plants with you.

We had one case of Xylella, there was one greenhouse in Germany, an area which is normally not very suitable for Xylella, but it had an outbreak in Saxony because one tourist brought back one plant for his collection from the Southern area, we don't know from where.

This is something that can happen so we should do our part by not doing it.

Ed (13:19)

EFSA has a campaign, the Plant Health for Life campaign.

We have a representative here - Pesty, he’s hiding back here - who is a reminder that actually if you bring plants, you risk bringing pests like Pesty traveling and then maybe bringing something. There's something that can be done - communicating to people.  

One final area of discussion. You talked at the beginning about green shoots of hope for the Apulia region. Can you talk more about that and what's being done in the area of research to find hope in the case of Xylella that we could see olive trees again in some areas of Apulia?  

Giuseppe (14:00)

We have been fighting with research Xylella in Europe since 2013. There have been several European projects funded by the European Commission. EFSA has also funded some research activities to solve particular issues.  

So now we have hope because we have already identified four varieties of olives which are resistant or tolerant to Xylella. Four is not many because the population of varieties of olives in the Mediterranean area is more than 1000, but from the screening done so far, all we have identified is four and more are the studies so there is even more hope for this. 

The same happened for almonds. Almonds have big problems because of Xylella in Spain, but also in Israel in the other areas of the Mediterranean and already we have one variety of almond identified as resistant.  

For vineyards in California, a few years ago it has been registered the first two commercial wine varieties resistant to Xylella which were simply produced by introducing, with traditional breeding, the resistant gene from the American grapes.

American grapes have been living with Xylella for a thousand years and they are resistant to this pathogen.  

Ed (15:26)

The hope is that you could then plant these resistant varieties and essentially, even if Xylella is present in the environment, the trees would live, they'll still be able to survive.

Giuseppe (15:41)

Yes. Several hundred thousand resistant plants of olives have been planted in Apulia.  

Ed (15:49)

The landscapes that you described, this kind of nuclear wasteland landscape will change.

Giuseppe (15:55)

Somehow the situation in south of Apulia is better now compared to the last ten years.  

Ed (16:08)

I'd just like to thank you very much, Giuseppe, for joining us and for your expertise.

And to all our listeners, thank you very much for listening.  

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Thank you very much from me and also thank you from Giuseppe.  

Giuseppe (16:45)

Thank you!

Ed (16:47)

See you again soon on the next episode of Science on the menu.