Scientists Identify Xylella-Tolerant Olive Varieties and Test Heat Treatments

Summary

The BeXyl project is focused on research­ing resis­tance to Xylella fas­tidiosa and has con­ducted green­house stud­ies to iden­tify resis­tant geno­types in var­i­ous plant species. Researchers have also tested ther­mal treat­ments on dif­fer­ent plant species to com­bat Xylella, with olive prop­a­ga­tion cut­tings show­ing high mor­tal­ity rates but other species respond­ing well to the treat­ment.

Scientists involved in the European Union-backed Beyond Xylella (BeXyl) project are prepar­ing to pub­lish research on the mech­a­nisms behind resis­tance to Xylella fas­tidiosa and to release the first results of a ther­mal treat­ment aimed at com­bat­ing the deadly plant pathogen.

Researchers from across Europe, the United States and Costa Rica have been con­duct­ing live green­house stud­ies to iden­tify resis­tant geno­types. The work is intended to help plant breed­ers repop­u­late Xylella-stricken areas such as south­ern Italy’s Puglia region, sup­ported by mol­e­c­u­lar and phys­i­o­log­i­cal analy­ses.

The resis­tance research exam­ined olives, almonds, cof­fee, per­sim­mon, carob and other plant species known to be sus­cep­ti­ble to Xylella fas­tidiosa. Scientists com­bined phe­no­typic, phys­i­o­log­i­cal, and genetic data to iden­tify genes asso­ci­ated with resis­tance and to guide the breed­ing of more resilient crops.

In olive trees, green­house tri­als showed that the Arbequina and Arbosana vari­eties had high infec­tion rates, par­tic­u­larly when exposed to the pauca strain of the bac­terium.

By con­trast, Leccino, Frantoio, and Gordal exhib­ited lower bac­te­r­ial loads and fewer dis­ease symp­toms, indi­cat­ing greater tol­er­ance.

Since the project began in 2022, researchers have iden­ti­fied more than one mil­lion genetic dif­fer­ences among olive vari­eties, nar­row­ing down genes that may be asso­ci­ated with tol­er­ance to Xylella.

The team has also ana­lyzed the nutri­ent com­po­si­tion of infected plants to under­stand bet­ter how the dis­ease alters olive tree phys­i­ol­ogy and how trees respond to abi­otic stres­sors such as drought.

Xylella fas­tidiosa

Xylella fas­tidiosa is a plant-path­o­genic bac­terium that lives in the xylem (water-con­duct­ing tis­sues) of plants, where it blocks the flow of water and nutri­ents. Spread by sap-feed­ing insects such as spit­tle­bugs and leafhop­pers, it causes seri­ous dis­eases in a wide range of crops and trees — includ­ing olives, grapes, cit­rus and almonds — often lead­ing to leaf scorch, decline and, in severe cases, plant death.

A sci­en­tist involved in the BeXyl project told Olive Oil Times that spe­cific details of indi­vid­ual exper­i­ments could not be dis­closed due to con­fi­den­tial­ity rules gov­ern­ing EU-funded research.

Using insights from the stud­ies, researchers have pro­duced between 300 and 500 new seedlings each year to assess agro­nomic per­for­mance and resis­tance to Xylella.

Among the new crosses already under­go­ing field eval­u­a­tion is a hybrid of Arbosana — widely used in super-high-den­sity groves — and the Xylella-tol­er­ant Leccino vari­ety.

In par­al­lel with green­house tri­als, the project has planted a col­lec­tion of 372 olive trees rep­re­sent­ing 26 cul­ti­vars from 12 coun­tries on the island of Mallorca. The site was cho­sen because all three Xylella fas­tidiosa sub­species known to infect olives — fas­tidiosa, mul­ti­plex and pauca — are nat­u­rally present there.

Researchers expect the col­lec­tion, which will be main­tained beyond the project’s sched­uled con­clu­sion in 2026, to pro­vide valu­able data on cul­ti­var sus­cep­ti­bil­ity to the dif­fer­ent sub­species under nat­ural con­di­tions.

Alongside resis­tance research, another BeXyl team has tested ther­mal treat­ments on sev­eral plant species exposed to Xylella fas­tidiosa, report­ing gen­er­ally pos­i­tive results.

However, the approach proved less effec­tive for olive prop­a­ga­tion cut­tings, which exhib­ited high mor­tal­ity and dif­fi­culty in devel­op­ing roots.

“For olive trees, which are strate­gic in the Mediterranean area, fur­ther test­ing in col­lab­o­ra­tion with com­mer­cial nurs­eries will con­tinue in 2026 to iden­tify pre­cise com­bi­na­tions of tem­per­a­ture, dura­tion and humid­ity that max­imise effec­tive­ness with­out com­pro­mis­ing cut­ting via­bil­ity,” the researchers said.

Beyond olives, mul­berry plants also failed to tol­er­ate the high tem­per­a­tures, but many other orna­men­tal and agri­cul­tural species responded well to ther­mal treat­ment. These included almond and cherry trees, as well as lau­rel, laven­der, ole­an­der and poly­gala.