Olive Trees on Corfu Face Threat from Wood-Eating Insects, Experts Rule Out Xylella

On Corfu, the north­ern­most Greek island in the Ionian Sea, olive trees are affected by twig dieback, which leaves parts of the trees com­pletely des­ic­cated.

The phe­nom­e­non has so far been observed in small pock­ets in the island’s cen­tral and north­ern pro­duc­ing regions.

The local depart­ment of agri­cul­ture sent spec­i­mens of the infected olive tree branches to a spe­cial­ized lab­o­ra­tory in Athens, which ruled out the pos­si­bil­ity that the dieback was caused by a strain of the Xylella fas­tidiosa bac­terium.

In two con­sec­u­tive announce­ments, the local depart­ment said the lab analy­sis found no pres­ence of the Xylella fas­tidiosa bac­terium and attrib­uted the twig dieback to wood-eat­ing insects.

“No infec­tion by the Xylella fas­tidiosa bac­terium has been observed in the olive groves of Corfu,” the depart­ment said. ​“According to the macro­scopic assess­ment of [our] agron­o­mists, the dry­ing of the tree branches is due to the action of wood-eat­ing insects, espe­cially the polyphagous moth Zeuzera pyrina.”

“Nevertheless, the risk of the Xylella fas­tidiosa pathogen enter­ing our coun­try remains, and there­fore, any impor­ta­tion of uncer­ti­fied plant mate­r­ial should be avoided,” they added.

Corfiot olive oil pro­duc­ers con­firmed that the symp­toms indi­cate a dif­fer­ent infec­tion than that caused by the Xylella bac­terium.

“The phe­nom­e­non started about two years ago,” said local pro­ducer Dimitris Andriotis. ​“I have seen what Xylella does to olive groves in Italy, and I can say that what we are expe­ri­enc­ing here is not Xylella.”

The Xylella fas­tidiosa bac­terium, which has emerged as a global threat to olive pro­duc­tion, is asso­ci­ated with Olive Quick Decline Syndrome (OQDS), a dis­ease of olive trees that causes dieback of leaves, twigs and branches, ren­der­ing the trees unpro­duc­tive and even­tu­ally dead.

While Xylella fas­tidiosa has caused exten­sive dam­age to olive groves in south­ern Italy and other olive oil-pro­duc­ing regions, espe­cially in the Mediterranean, Greek olive groves have remained unaf­fected.

Zeuzera pyrina, also known as the leop­ard moth, is a white insect spot­ted with black rings that can grow up to five cen­time­ters long. The moth’s lar­vae bur­row into the branches of olive and other fruit trees and ulti­mately kill the trees if left untreated.

Emmanouil Roditakis, a pro­fes­sor of agri­cul­tural ento­mol­ogy at the University of Crete, argued that a dif­fer­ent pest is most likely caus­ing the dieback of tree branches in Corfu’s olive groves.

“My first impres­sion is that the cul­prit is the olive bark midge and not the leop­ard moth,” Roditakis told Olive Oil Times after exam­in­ing pho­tos of the affected olive trees. ​“If con­firmed, the infected twigs and branches should be removed to be burned or crushed next spring.”

The lar­vae of the olive bark midge (Resseliella oleisuga) develop under the olive tree bark and feed on the plant tis­sue. The infested twigs and branches may wither and die if not treated prop­erly.

Roditakis added that some field research is required in Corfu’s olive groves to pin­point the exact cause of the branch dieback.

Scientists have iden­ti­fied more than 100 olive tree pathogens. However, accord­ing to rel­e­vant research, only a few can cause sig­nif­i­cant eco­nomic losses to olive farm­ing.

Olive tree infes­ta­tions in the Mediterranean basin by the olive bark midge have been par­tic­u­larly preva­lent since 2010, and some sci­en­tists have linked the prob­lem to a warm­ing cli­mate.

On Corfu, Andriotis said the dis­ease is becom­ing more wide­spread, puz­zling the local olive oil sec­tor.

“Some of my olive trees are already affected,” he said. ​“The same has hap­pened to other pro­duc­ers in the area. The trees are not dying, but some branches and twigs are com­pletely with­ered.”

“It is a con­cern­ing event that needs to be dealt with,” he added. ​“None of the island’s older olive grow­ers remem­ber any­thing sim­i­lar to what hap­pened in the past.”

• Agrotypos
• EIP-AGRI