Authorities in Puglia Confirm Culprit in Olive Tree Devastation

Authorities in the south­ern Italian region of Puglia have pub­lished the entire dataset used to mon­i­tor and con­trol the spread of the dis­ease caused by Xylella fas­tidiosa.

The regional admin­is­tra­tion said the data clearly show that the pauca sub­species of Xylella fas­tidiosa is the lead­ing cause of Olive Quick Decline Syndrome (OQDS), which has killed tens of mil­lions of olive trees in the region.

Before the arrival of Xylella fas­tidiosa, Italy rou­tinely pro­duced more than 500,000 tons of olive oil annu­ally. Now, a har­vest beyond 300,000 tons is con­sid­ered a boun­ti­ful crop.

The data span the first mon­i­tor­ing oper­a­tions in 2013 through 2022, and their pub­li­ca­tion comes shortly after a study pub­lished in the Journal of Phytopathology indi­cat­ing that Xylella fas­tidiosa was respon­si­ble for only a minor­ity of OQDS cases.

Over the past decade, Xylella fas­tidiosa pauca has spread across south­ern Puglia due to vec­tor insects, such as the spit­tle­bug, which carry the bac­te­ria from one olive tree to the next.

The epi­demic prompted Italian and European author­i­ties to con­tin­u­ously define and update the bound­aries of infected and neigh­bor­ing areas, enact­ing numer­ous con­tain­ment mea­sures to pre­vent the spread of the bac­te­ria.

As a result, offi­cials said the most highly mon­i­tored parts of Puglia were along the bor­ders of these con­tain­ment zones, where the per­cent­age of Xylella fas­tidiosa-infected olive trees and other plants was expected to be min­i­mal.

As Xylella fas­tidiosa pauca spread north from Puglia’s south­ern­most penin­sula, so did the bor­ders of the infected and buffer areas.

“The goal of the mon­i­tor­ing oper­a­tions… is not to inven­tory the infected plants but to antic­i­pate the bac­terium and iden­tify and con­tain or elim­i­nate new out­breaks at the out­set to reduce the inoc­u­la­tion pres­sure on the dis­ease-free zone and slow down the expan­sion of the infected area as much as pos­si­ble,” author­i­ties wrote in a note pub­lished on a ded­i­cated web­site to mon­i­tor­ing the spread of Xylella fas­tidiosa.

“Therefore, mon­i­tor­ing data can­not pro­vide indi­ca­tions on the fre­quency of the bac­terium in the infected zone, as most of its sur­face is no longer sub­ject to sur­veil­lance by the phy­tosan­i­tary obser­va­tory,” they added.

The state­ment explic­itly chal­lenges the con­clu­sions of the Journal of Phytopathology study, which found that only a frac­tion of the olive trees affected by OQDS were infected by Xylella fas­tidiosa pauca.

By assum­ing that OQDS killed mil­lions of olive trees that were not infected with Xylella fas­tidiosa pauca, the research authors implied that the two were largely unre­lated.

According to the regional author­i­ties and lead­ing experts in the field, this con­clu­sion is incor­rect.

“The read­ing of the data by the authors of that research is not reli­able,” Donato Boscia, direc­tor of the National Research Council’s (CNR) Institute for the Sustainable Protection of Plants, told Olive Oil Times.

“Should we go and test olive trees in the infected area, we would find Xylella fas­tidiosa pauca in 90 to 100 per­cent of the cases,” he added.

Boscia, whose work made it pos­si­ble to iden­tify the pres­ence of Xylella fas­tidiosa pauca in 2013, said the data ana­lyzed in the study to con­firm that Xylella fas­tidiosa is rarely present in OQDS-hit olive trees just can­not be used to reach such a con­clu­sion.

“It is true that over ten years, about a mil­lion plants were ana­lyzed,” Boscia said. ​“But almost all of them were located where the bac­te­ria was least expected, the so-called buffer zone and the upper end of the infected area. It is nor­mal to find very low per­cent­ages of Xylella fas­tidiosa pauca-infected plants there.”

According to Boscia, the authors of that study also mis­tak­enly inter­preted symp­toms on mon­i­tored olive trees as proof of the pres­ence of OQDS, which their paper com­pared to the pres­ence of Xylella fas­tidiosa.

“If you look at the data­base, at some point, you will see that for each mon­i­tored plant, there is a def­i­n­i­tion of ​‘symp­toms present’ or ​‘symp­toms absent,’” he said. ​“That is just an indi­ca­tion for researchers. Symptoms present is not syn­ony­mous with Olive Quick Decline Syndrome, as my col­leagues wrote in their study.”

Boscia added that the con­tro­ver­sial study did not con­sider the dif­fer­ent meth­ods applied dur­ing mon­i­tor­ing oper­a­tions over the years.

“If we con­sider data from 2015, we are talk­ing of a period when analy­ses were also car­ried out in the areas where the des­ic­ca­tion of the olive trees was already clearly vis­i­ble,” he said. ​“That means that in that year, the per­cent­age of plants with ​‘symp­toms present’ could reach 70 per­cent.”

The data may have been skewed fur­ther by the rapid spread of the dis­ease in the early years, lead­ing author­i­ties to move the buffer and con­tain­ment zones far­ther north from areas densely pop­u­lated with olive trees to those less so.

As a result, many olive trees in this area that died from OQDS were never tested for Xylella fas­tidiosa. ​“Those per­cent­ages drop to about 20 per­cent,” Boscia said.

“If one looks at the 2021 data, you can see that per­cent­age drop­ping fur­ther. The rea­son is that in 2020, E.U. reg­u­la­tions halved the con­tain­ment area to be mon­i­tored. Instead of mon­i­tor­ing the last ten kilo­me­ters of the red zone, it was reduced to kilo­me­ters,” Boscia added.

“If you look at the whole pic­ture, the con­clu­sions go in the oppo­site direc­tion of what the study’s authors wrote,” he con­cluded. ​“If today, in the areas bor­der­ing the infected zone, we have a very low inci­dence of Xylella fas­tidiosa, that means that the con­tain­ment strat­egy adopted by the European Union is work­ing.”