Scientists Defend Study Finding Xylella Not Responsible for Most OQDS

Margherita Ciervo and Marco Scortichini defend their research that found Xylella fastidiosa was not responsible for most Apulian olive tree deaths over the past decade.
Puglia, Italy
By Margherita Ciervo and Marco Scortichini
Mar. 25, 2024 19:19 UTC

The rebut­tal to our arti­cle from Donato Boscia, the unit man­ager of the Institute for Sustainable Plant Protection in Bari of the National Research Council, requires some clar­i­fi­ca­tion

It should be stressed that the ini­tial stud­ies con­cern­ing the olive decline in Salento attrib­uted the phe­nom­e­non to a num­ber of pathogens: the com­plex” dis­ease, includ­ing also some fungi. But, upon the iden­ti­fi­ca­tion of Xylella fas­tidiosa subsp. pauca, the bac­terium has been retained as the sole causal agent of Olive Quick Decline Syndrome (OQDS).

Consequently, from 2015 onward, every decline symp­tom iden­ti­fied on olive trees, such as leaf, twig and branch dieback, was attrib­uted to the bac­terium, and this assump­tion was largely con­veyed to farm­ers, agron­o­mists, jour­nal­ists and politi­cians.

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

During the mon­i­tor­ing sur­veys, the regional inspec­tors have, con­se­quently, col­lected sam­ples from olive trees that vis­i­bly show some of such symp­toms just because they are sus­pected to be caused by Xylella fas­tidiosa.

By check­ing the data col­lected by the regional phy­tosan­i­tary inspec­tors of Puglia, one ques­tion arises. If, from 2016 until 2022, the per­cent­age of olive trees that vis­i­bly show symp­toms of decline tested pos­i­tive for the occur­rence of Xylella fas­tidiosa is in a range of 22.5 per­cent to 3.21 per­cent, which other pathogens caused the symp­toms in the remain­ing 78 to 97 per­cent of olive trees?

Within this sce­nario, it should be out­lined that, in the path­o­genic­ity tests, Xylella fas­tidiosa subsp. pauca causes some leaf wilt­ing on the inoc­u­lated plants more than one year after the inoc­u­la­tion, whereas fungi belong­ing to Neofusicoccum spp., iso­lated in the same areas of Salento affected by the olive dis­ease, are capa­ble of killing the whole plant in two to three weeks (Scortchini et al., 2023).

The wide occur­rence of fungi and their aggres­sive­ness could explain the vast major­ity of symp­to­matic olive trees sam­pled and resulted in neg­a­tive for the bac­terium.

It is pos­si­ble that in the infected areas of Salento, Xylella fas­tidiosa occurs to a great extent but what is observed is that phy­topath­o­genic fungi are also con­tem­po­ra­ne­ously present in the same tree that hosts Xylella fas­tidiosa.

A more com­pre­hen­sive approach to the study con­cern­ing the olive decline should be to also con­sider other phy­topathogens involved in the com­plex dis­ease cur­rently affect­ing olive trees in Puglia.

It should be added, that, nowa­days, many patho­log­i­cal emer­gen­cies that affect woody species are caused by a num­ber of phy­topathogens that can act between them and in com­bi­na­tion also with abi­otic pre­dis­pos­ing fac­tors.

In any case, in Salento, nei­ther Xylella fas­tidiosa nor OQDS could have killed tens of mil­lions” of olive trees, con­sid­er­ing that in the province of Lecce, the olive trees are esti­mated to be only” 11 mil­lion, and many of these are still vis­i­bly healthy and pro­duc­tive. Therefore, this nar­ra­tive is absolutely unfounded.

A sec­ond aspect of the rebut­tal con­cerns the low inci­dence of Xylella fas­tidiosa in the demar­cated areas. We do not deny that the aim of the mon­i­tor­ing sur­veys is to find infected trees and that a low occur­rence of the bac­terium has to be expected within the buffer” areas. We point out the unnec­es­sary sac­ri­fice of asymp­to­matic cen­ten­nial and mil­len­nial olive trees that sur­round the infected one within a radius of 50 meters.

According to epi­demi­o­log­i­cal mod­els that revealed the neg­li­gi­ble role of the asymp­to­matic trees” in fur­ther spread­ing the dis­ease, the addi­tional uproot­ing would not seem use­ful.

It should also be added that the blind” tree uproot­ing, irre­spec­tive of the real occur­rence of the bac­terium within the olive crown, appears quite obso­lete, espe­cially by tak­ing into con­sid­er­a­tion the great num­ber of very sen­si­tive and reli­able detec­tion tech­niques recently devel­oped for Xylella fas­tidiosa.

Moreover, many autochtho­nous olive trees in Salento, which have been infected since 2015, are per­fectly healthy and pro­duc­tive today.

Some years ago, it was pre­dicted that in the whole of Salento will remain just 50 olive trees, a sort of museum of the past”.


Nowadays, such a state­ment appears quite far from the real­ity since, thanks to field man­age­ment strate­gies that allow the olive groves to veg­e­tate and yield, many farm­ers are con­tin­u­ing to take care of their olive groves planted with Ogliarola salentina and Cellina di Nardò.

In addi­tion, a vast phe­nom­e­non of resilience is cur­rently observed in Salento, also includ­ing the areas where the ini­tial out­break of the dis­ease was reported.

Margherita Ciervo is a researcher and pro­fes­sor at the University of Foggia’s Department of Economy, Management and Territory.

Marco Scortichini is a researcher at the Council for Agricultural Research and Economics’ (CREA) research cen­ter for olive, fruit and cit­rus crops in Rome.


Related Articles