Xylella May Not Be Responsible for Olive Tree Devastation in Puglia, Study Finds

New research sug­gests that Xylella fas­tidiosa was respon­si­ble for just a small per­cent­age of the olive trees destroyed by Olive Quick Decline Syndrome (OQDS) in Puglia.

For more than a decade, the pre­vail­ing wis­dom had been that the Xylella fas­tidiosa (Xf) bac­terium infected olive trees across the south­ern Italian region, result­ing in the deadly OQDS.

However, research pub­lished in the Journal of Phytopathology found that slightly less than 23 per­cent of trees killed by OQDS from February 2016 to May 2017 were infected by Xf.

Between May 2021 and February 2022, slightly more than three per­cent of the OQDS-affected olive trees were found to carry Xylella.

If con­firmed – and some sci­en­tists are skep­ti­cal – the find­ings imply that the strate­gies cur­rently employed to con­tain Xf may not effec­tively address the spread of OQDS.

Researchers said the con­clu­sions are based on data gath­ered by regional phy­tosan­i­tary bod­ies and research insti­tu­tions from 2013 to 2023.

The data include the areas mon­i­tored, the num­ber of trees exhibit­ing OQDS symp­toms, the num­ber of plants exam­ined, the num­ber of trees test­ing pos­i­tive for Xylella fas­tidiosa pauca – a strain of the bac­te­ria that infects olive trees – and the num­ber of plants uprooted within the des­ig­nated zones in Puglia.

“Everything we wrote comes from read­ing those num­bers,” said Marco Scortichini, lead olive and fruit crop researcher at the Italian Council for Agricultural Research and Economics (CREA), who co-authored the study.

Current meth­ods for detect­ing Xylella fas­tidiosa have advanced, includ­ing trained dogs and drones.

“These tech­niques have sim­pli­fied the detec­tion of Xylella fas­tidiosa com­pared to ear­lier meth­ods,” Scortichini said. ​“Significant research invest­ment in recent years has led to the devel­op­ment of effec­tive and highly sen­si­tive tools.” 

Local inspec­tors are respon­si­ble for the mon­i­tor­ing tasks, select­ing olive trees for sam­pling by spe­cial­ized labs to detect Xylella fastidiosa’s pres­ence.

“Expectations might sug­gest a high preva­lence of Xylella fas­tidiosa in olive trees from infected zones,” Scortichini said. ​“Yet, we find our­selves at a mere 3.21 per­cent.”

Since its dis­cov­ery in Apulian olive trees in 2013, Xylella fas­tidiosa has been under intense scrutiny by both regional and national author­i­ties.

The bac­terium is clas­si­fied as a List‑A quar­an­tine pathogen, indi­cat­ing it was pre­vi­ously uniden­ti­fied in the region and has caused sig­nif­i­cant dam­age in other areas, includ­ing the Americas.

“Current data val­i­date ini­tial obser­va­tions, indi­cat­ing that the syn­drome and Xylella fas­tidiosa over­lap only in a minor­ity of instances,” said Margherita Ciervo, a study co-author and researcher at the University of Foggia’s Department of Economy, Management and Territory.

“This also sug­gests that Xylella fas­tidiosa is not the pri­mary cause of the trees’ rapid demise,” she added. ​“Xylella fas­tidiosa may exac­er­bate the con­di­tion of trees affected by OQDS but is not the direct cause.” 

Given these find­ings, the researchers advo­cate reassess­ing the mea­sures imple­mented to com­bat Xylella fas­tidiosa.

The exist­ing European Union plant health law man­dates the demar­ca­tion of Xylella-infected zones and the enforce­ment of strict erad­i­ca­tion poli­cies, includ­ing remov­ing an infected olive tree and all oth­ers within a 50-meter radius.

The study’s authors argue that the erad­i­ca­tion mea­sures should be recon­sid­ered in light of their find­ings. ​“Earlier stud­ies have shown that asymp­to­matic olive trees barely con­tribute to the bac­teri­um’s spread,” they wrote. 

In a 2020 study cited by the researchers, the asymp­to­matic stage was found to have low to neg­li­gi­ble infec­tiv­ity. Trees with symp­toms were instead found to be able to spread Xf to an aver­age of 19 other trees per annum.

According to Scortichini and Ciervo, sus­pend­ing the 50-meter radius erad­i­ca­tion rule ​“could pre­serve numer­ous healthy ancient and mon­u­men­tal olive trees and their sig­nif­i­cant con­tri­bu­tion to the land­scape.”

They argue that fur­ther inves­ti­ga­tion should focus on alter­na­tive causes of OQDS. ​“Besides a few stud­ies on Xylella fas­tidiosa and other pathogens, OQDS has received lit­tle atten­tion,” Scortichini said.

He sug­gested that explor­ing OQDS’s devel­op­ment could open new research avenues.

“The past decade has seen a shift from the notion of a sin­gle pathogen caus­ing fatal dis­ease in trees to a more com­plex under­stand­ing,” Scortichini said. ​“We’re grad­u­ally rec­og­niz­ing that var­i­ous microbes, influ­enced by tem­per­a­ture fluc­tu­a­tions and cli­matic dis­rup­tions, can col­lec­tively lead to dis­ease.”

He also high­lighted the poten­tial impact of cli­mate change on soil con­di­tions, mois­ture lev­els and plants’ resilience to drought and heat waves.

“Climate alter­ations might acti­vate dif­fer­ent pathogens that would oth­er­wise be benign or dimin­ish the plants’ capac­ity to resist them,” Scortichini con­cluded.