Can Xylella Be Stopped?

Five years into the Xylella fastidiosa tragedy, scientists fear its continued spread may be inevitable.

Gennaro Santoro (Photos: Cain Burdeau for Olive Oil Times)
May. 4, 2018
By Cain Burdeau
Gennaro Santoro (Photos: Cain Burdeau for Olive Oil Times)

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The set­ting is a mel­low fall day in the vine­yards of the Santoro Tenuta in the Valle d’Itria, an agri­cul­tural won­der­land now the epi­cen­ter of efforts to stop the spread of Xylella fas­tidiosa, a lethal plant bac­terium killing thou­sands of olive trees here in Puglia, the boot-heel of Italy.

This is a stun­ning epi­demic, and it must be dif­fi­cult if not impos­si­ble to over­come the grief over the loss of cul­tural icons that have endured for cen­turies.- Rodrigo Almeida, University of California at Berkeley

Gennaro Santoro, a 75-year-old vint­ner, was clip­ping away at grapes left dan­gling after the har­vest. His vine­yard is sur­rounded by olive orchards and he tends to olives around the tenu­ta’s winery. 

I knew Gennaro from pre­vi­ous vis­its to the Valle d’Itria, and stopped in to hear his thoughts on Xylella, an omi­nous plant dis­ease spread­ing in Europe from the New World that is the sub­ject of an Olive Oil Times series.

Scientists com­pare what is hap­pen­ing today among olive orchards to what hap­pened in the late 1800s in Europe with wine grapes. An aphid-like insect, the grape Phylloxera, was brought to England from the New World and wreaked death in Europe’s vineyards.

Gennaro Santoro’s own vine­yard dates to that period when Phylloxera invaded France and spread to the rest of Europe. For sev­eral decades Puglia was untouched by Phylloxera and became wealthy sup­ply­ing wine-starved Europe.

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Gennaro Santoro (Photos: Cain Burdeau for Olive Oil Times)

Yes, I will talk about that, about Xylella” Gennaro said ami­ably. But let me show you first the autochtho­nous vines we’ve planted!” He’s a learned farmer of Swiss-Italian ori­gins whose fam­ily goes back gen­er­a­tions in this con­trada, an Italian word for a small rural com­mu­nity or village. 

With a back­ground in biol­ogy and some­one steeped in local his­tory, his tour lasted an exhil­a­rat­ing hour. 

He strolled the rows of vines, waved his prun­ing clip­pers in the air, and all the while recounted of feu­dal estates, the masserie founded by ex-Roman sol­diers, and sub­se­quent peas­ant revolts; and how even­tu­ally the brac­cianti, the peas­ants, came into pos­ses­sion of the valley. 

Looking at the green hills run­ning with rock walls, he offered ever more history. 

This was where Byzantine monks once stopped to give water to their herds, dur­ing the tran­shu­mance. You see, a fog­gia is a long ditch they dug to carry rain­wa­ter into the Murgia, where there’s no river, and no wells. There was a fog­gia here, and it was called Sauro.” 

We got back to my vehi­cle in the small park­ing lot behind his fam­i­ly’s win­ery, where a few olive trees grow. The sun was set­ting and din­ner time approached. 

But what about Xylella?” I asked him again. 

He winced. It’s all wrong. You can’t cut down all the olive trees. We must seek to live with the dis­ease, as farm­ers have always done.” 

He was opti­mistic, though, and cer­tainly did­n’t think the olive trees dot­ting his piece of coun­try­side would fall vic­tims to Xylella and die. 

We aren’t wor­ried about our trees dying because we are organic,” he said con­fi­dently. No one uses her­bi­cides around us because it is out­lawed.” Before bid­ding farewell, he added: You can’t com­mand nature. But you can adapt.” 

His sen­ti­ments about Xylella go to the heart of a rag­ing sci­en­tific and agro­nomic debate grip­ping Puglia, Italy’s most pro­duc­tive olive region. 

The last five years have been riven with Xylella head­lines, protests, polit­i­cal intrigue and a razed-earth strat­egy that has seen the clear-cut­ting of thou­sands of olive trees in Salento, a flat olive-rich area of south­ern Puglia. 

This is a stun­ning epi­demic, and it must be dif­fi­cult if not impos­si­ble to over­come the grief over the loss of cul­tural icons that have endured for cen­turies,” said Rodrigo Almeida, a Xylella expert at the University of California at Berkeley. 

Now, five years into the tragedy, it’s becom­ing ever more omi­nous that the bat­tle to erad­i­cate Xylella may have been lost and sci­en­tists fear its spread may now be unstop­pable and poten­tially even spread faster. 

Scientists analyze olive samples for Xylella fastidiosa

Alexander Purcell, another Xylella expert at the UC-Berkeley, said that the dis­ease is so preva­lent in most of Salento that erad­i­ca­tion of Xylella is no longer con­sid­ered possible.” 

In December, Vytenis Andriukaitis, the European Commissioner-des­ig­nate respon­si­ble for Health and Food Safety, warned that Xylella had become the biggest phy­tosan­i­tary cri­sis con­fronting the EU for many years.” He made his com­ments in Paris. 

What to do then? Cut or not cut sick olive trees and their neigh­bors? It’s here that sci­en­tists are divided. 

For now, the strat­egy remains to cut down and erad­i­cate sick trees in areas where author­i­ties are seek­ing to stem the dis­ease’s spread, an area where Gennaro Santoro’s vine­yard and olive trees are. 

The dis­ease appeared in the region about five years ago when farm­ers and sci­en­tists started inves­ti­gat­ing the sud­den brown­ing of olive leaves on trees near the port of Gallipoli in Salento. 

The iconic olive tree is an ever­green – so any brown­ing is cause for alarm. 

Since Xylella fas­tidiosa was declared the cul­prit in 2013, the EU has demanded Italy under­take a rad­i­cal erad­i­ca­tion pro­gram to stop the spread.

Whether it’s lack of action by Italy or the sheer nature of the bac­terium, erad­i­ca­tion efforts have failed. And Xylella is on the march. 

So far this year, hun­dreds of new trees are being cut down in areas not far from the Santoro vineyards. 

The num­ber of infected trees has quadru­pled in one year in the crit­i­cal con­tain­ment zone, accord­ing to ANSA, the Italian news agency. 

Another omi­nous devel­op­ment is tak­ing place across the Mediterranean: Olive trees in main­land Spain are begin­ning to die due to Xylella, accord­ing to Spanish author­i­ties and news reports. Spain is the world’s largest olive oil producer. 

So far, though, Puglia is where death has been fiercest. 

Tens of thou­sands of trees have been infected and thou­sands either cut down to pre­vent the spread of the dis­ease or are dying due to infestation. 

The only way to fight it is the com­plete erad­i­ca­tion of infected trees and their sur­round­ings as so far no treat­ment exists for the bac­te­ria,” Andriukaitis, the EU Commissioner, said in an email to Olive Oil Times. 

He added that the dis­ease could worsen due to cli­mate change and glob­al­iza­tion of trade. 

Since pests respect no bor­ders every­one must play their part to keep plants healthy across the EU and to avoid seri­ous con­se­quences for our agri­cul­ture, our econ­omy and our local com­mu­ni­ties,” he said. 

In Puglia’s areas where the bac­terium has already wreaked havoc and where sci­en­tists say it can no longer be erad­i­cated, farm­ers are left hav­ing to cope with the bacterium. 

Co-exis­tence with this sick­ness is a goal the whole sec­tor wants to get to,” said Marco Scortichini, a plant bac­te­ri­ol­o­gist at the Council for Agricultural Research and the Analysis of Agricultural Economics, an Italian research arm that spe­cial­izes in olive cultivation. 

Reduction of bac­terium inci­dence will not be impos­si­ble,” he said in a tele­phone inter­view. Co-exis­tence is some­thing that can be achieved.” 

I don’t doubt it’s Xylella (killing trees),” said Christos Xiloyannis, a plant phys­i­ol­o­gist at the University of Basilicata, in a tele­phone inter­view. It’s a ques­tion about how to intervene.” 

Funds have poured into under­stand­ing and com­bat­ing the dis­ease and sci­en­tific stud­ies have been pub­lished look­ing at Xylella from nearly every angle: How does it spread? How does it stran­gle plant tis­sues? What vari­eties are resis­tant? How best to spray against the bac­terium? How to graft infected trees? How to mon­i­tor the infestation? 

Science has answers to a lot of these press­ing ques­tions and impor­tant break­throughs have been made. Perhaps the most impor­tant dis­cov­ery has been that some vari­eties of olives are nat­u­rally resis­tant to Xylella — a fact giv­ing hope to many farm­ers who are now replant­ing with a resis­tant Leccino variety. 

But there are other projects under­way too. For exam­ple, some farm­ers are seek­ing to replen­ish their orchards by graft­ing Xylella-resis­tant stocks onto old trunks. 

Measures to com­bat the vec­tor, a com­mon spit­tle­bug, have been imple­mented too. Landowners now face hefty fines for not till­ing or cut­ting the grass in their fields to kill the spit­tle­bugs that grow in weeds. 

An experiment to artificially infect a tree with Xf, part of efforts to find resistant olive varieties

Among the promis­ing field­work is one Scortichini is work­ing on in Salento. His lat­est study, which appeared in April in the sci­ence jour­nal Phytopathologia Mediterranea, showed that a cop­per-based spray was show­ing pos­i­tive results in com­bat­ing the bac­terium. Other sci­en­tists have ques­tioned the study’s find­ings as far from conclusive. 

When it comes to trees, forests, you can’t cut them all down,” Scortichini said. 

This is a com­mon state­ment among agron­o­mists, sci­en­tists and farm­ers. They say olive trees are unlike other infected crops – whether ani­mal or plant.

Why? Because they are unique ever­green fruit trees that can sur­vive cen­turies. In this sense, this olive sick­ness is at a dif­fer­ent scale than Mad Cow Disease in Great Britain and even the Phylloxera out­break. Under this view, cut­ting down trees is not the solu­tion and it’s not viable. 

Our advice is to return to the coun­try­side and spend time cul­ti­vat­ing,” Xiloyannis said. We’ve never been able to erad­i­cate dis­eases that have appeared in the last 30 – 40 years.” 

He said it was very likely that Xylella broke out in Salento because olive orchards had become aban­doned and poorly man­aged, mak­ing them sus­cep­ti­ble to pathogens. 

In part, the European Union is to blame for this, he said, because it was encour­ag­ing farm­ers to cul­ti­vate in this manner. 

Xiloyannis said he is work­ing with farm­ers to improve their land prac­tices to fend off the dis­ease – an echo of what Gennaro Santoro, the vint­ner, said about his con­fi­dence in his trees. As of April, the trees on the Santoro vine­yard had not been infected, his son, Marco Emilio Santoro, told Olive Oil Times. 

Xiloyannis said cut­ting and replant­ing new olives is sim­ply not fea­si­ble in many areas of Puglia and there is no guar­an­tee that cut­ting old trees and replac­ing them with a resis­tant-vari­ety will work in tough terrains. 

Still, plant­ing the Leccino and pos­si­bly other bac­terium-resis­tant vari­eties, for now, appears to be the only way of com­bat­ing Xylella, some sci­en­tists say. 

At present, the only pos­si­ble thing which seems to work, albeit data are still pre­lim­i­nary, seems to be plant­ing resis­tant olive vari­eties,” said Enrico Bucci, a sys­tems biol­ogy researcher at Temple University, in an email. 

This is the focus of Donato Boscia, a lead­ing researcher into the Xylella out­break at the Institute for Sustainable Plant Protection in Bari. 

There is no cure for Xylella at this point,” he said in a tele­phone interview. 

For now, he is work­ing in Salento to find olive cul­ti­vars resis­tant to the bac­terium. He said research has shown the Leccino and Favolosa vari­eties are resis­tant and he is opti­mistic many more may be too. 

The prospect of this rem­edy eerily hear­kens back to Phylloxera. 

Both pests are native to the Americas. Both cause leaves to brown and die, stran­gling and killing its host plant, and both move on to find the next victim.

In the end, Europe learned to live with Phylloxera, but only after nearly every vine­yard was replanted with wild American root­stocks that are resis­tant to the root-destroy­ing bugs.

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