Scientists Defend Study Finding Xylella Not Responsible for Most OQDS

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.

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.