`Expert Says Eradication of New Olive Tree Disease in Europe Unlikely - Olive Oil Times
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Expert Says Eradication of New Olive Tree Disease in Europe Unlikely

By Julie Butler
Mar. 29, 2014 09:02 UTC
Donato Boscia

Will Italy be able to erad­i­cate the dis­ease which has already infected 8,000 ha of olive groves in Lecce, on the Salento penin­sula? And what is the risk of the plant pathogen behind it – Xylella fas­tidiosa (Xf), also the cul­prit in Pierce’s dis­ease – spread­ing to olive trees else­where in Italy and wider Europe?
Olive Oil Times put these and other ques­tions to one of the lead­ing experts on the out­break, plant virol­o­gist Dr. Donato Boscia, from the Institute of Plant Virology (IVV) at the National Research Council, in Bari, Italy.

Boscia spoke in the wake of a European Union-wide ban on move­ment of cer­tain plants out of Lecce to help pre­vent spread of the dis­ease. EU mem­ber states must also start annual checks for the pres­ence of Xf.

A major pest in the grapevine and cit­rus indus­tries in trop­i­cal, sub­trop­i­cal and tem­per­ate areas of the Americas, until the Lecce out­break in mid-October there had been no con­firmed reports of Xf in Europe, though there was an uncon­firmed report in vine­yards in Kosovo in the 1990s.

What is the cur­rent extent of the Xf infec­tion in Apulia?

Dr. Donato Boscia: A large regional sur­vey, done by sam­pling and ana­lyz­ing 16,000 plants, has been car­ried out in the Apulia region, in south-east Italy. It was con­firmed that the pathogen is actu­ally con­fined to the province of Lecce, while the remain­ing cen­tral and north­ern part of the region are free.

Xylella fas­tidiosa

Is erad­i­ca­tion pos­si­ble?

I feel that even a severe and ambi­tious pro­gram of erad­i­ca­tion will not have much chance of suc­cess because:

1. The con­t­a­m­i­nated area has a very large exten­sion.
2. The pathogen has sev­eral sus­cep­ti­ble hosts, and surely sev­eral of those are still unknown and may be symp­tom­less.
3. Efficient insect vec­tors (car­ri­ers) are involved.
4. The area is densely pop­u­lated, with an infi­nite num­ber of pri­vate houses with gar­dens, which makes unre­al­is­tic to expect to get all of them sam­pled and even­tu­ally erad­i­cated.

In my per­sonal opin­ion these four ele­ments make it very dif­fi­cult, if not impos­si­ble, for an erad­i­ca­tion pro­gram to suc­ceed.

Is this strain of Xf likely to spread to olive or other trees in other parts of Italy and beyond?

That’s dif­fi­cult to say. Due to the geo­graphic posi­tion of the out­break there is no risk of spread to other parts of Italy or Europe through insect vec­tors (car­ri­ers). However, though move­ment of sus­cep­ti­ble plants is for­bid­den, any ille­gal or uncon­trolled plant move­ment may rep­re­sent a major risk.

What are the symp­toms of Xf?

They con­sist of the with­er­ing and des­ic­ca­tion of ter­mi­nal shoots dis­trib­uted ran­domly but which then expand to the rest of the canopy, thus result­ing in the col­lapse and death of the trees. In the affected groves, the total­ity of the plants are symp­to­matic.

What is cur­rently hap­pen­ing in Apulia in rela­tion to the out­break?

So far there are two main actions being taken by the Regional Plant Protection author­i­ties. First, in the con­t­a­m­i­nated area as well as in the whole province of Lecce, the move­ment of plant mate­ri­als of sus­cep­ti­ble species is pro­hib­ited. Second, a sur­vey of the whole region aimed at iden­ti­fy­ing and delim­it­ing the infested areas, the buffer zones and the secu­rity zones is due to be by April 1.

Until these areas have been delim­ited though a large scale sur­vey, there is no pos­si­bil­ity to start any effec­tive pro­gram of the uproot­ing and removal of infected plants.

It has to be said that the out­break was iden­ti­fied only five months ago, and that the rel­e­vant ter­ri­tory is very large – 8,000 ha of olive orchards dis­trib­uted in a ter­ri­tory at least twice as large. Moreover, other minor spots came out dur­ing the mon­i­tor­ing. This work is almost com­pleted and addi­tional phy­tosan­i­tary mea­sures — such as tree removal — are under dis­cus­sion and nego­ti­a­tion with the European Commission Directorate General for Health and Consumers.

What is the research now focused on?

Epidemiological stud­ies under­way fol­low four dif­fer­ent routes:

1. Identification of nat­ural sources of inocu­lum (nat­ural flora). This is essen­tial to plan fur­ther actions of con­tain­ment, under­stand the real pos­si­bil­i­ties of suc­cess with an erad­i­ca­tion pro­gram, bet­ter under­stand the risk of fur­ther spread, and define the list of species banned from trade.
2. Capture of leafhop­pers thriv­ing on the nat­ural flora, their iden­ti­fi­ca­tion and analy­sis for the pres­ence of Xf
3. Transmission tri­als using Xylella-pos­i­tive insects
4. Placement of bait plants in infected olive groves.

The char­ac­ter­i­za­tion of the geno­type is well advanced, and results will be pub­lished soon.

What strain of Xf is the cause?

DNA analy­sis shows the pop­u­la­tion of X. fas­tidiosa affect­ing olive trees in Italy is an atyp­i­cal vari­ant of the sub­species ​“pauca”, the one known to cause Citrus Variegated Chlorosis in Brazil. Fortunately, our vari­ant is unable to infect cit­rus or grapevine.

When might char­ac­ter­i­sa­tion of the geno­type be com­plete and how might that help?

The sequence of the whole genome is almost com­plete and will prob­a­bly be pub­lished in a cou­ple of months, though a geno­type with the same sequence type has already been iden­ti­fied, giv­ing the pos­si­bil­ity of soon get­ting impor­tant ele­ments to assess the path­way of entrance of the pathogen in Europe and impor­tant infor­ma­tion on its bio­log­i­cal prop­er­ties.

What was the lead-up to the out­break?

A dev­as­tat­ing dis­ease known as ​“quick decline syn­drome of olive” (OQDS) appeared sud­denly a few years ago in olive trees, many of them cen­tury-old trees, in the vicin­ity of Gallipoli, in the Lecce province. By late last year it had exploded in epi­demic form, so as to affect the esti­mated sur­face area of over 8,000 hectares.

Samples from olive trees were sub­jected to mol­e­c­u­lar analy­sis which gave pos­i­tive results for Xf. Almond (Prunus dul­cis) and ole­an­der (Nerium ole­an­der) plants grow­ing near affected olive trees and show­ing symp­toms of leaf scorch also tested pos­i­tive.

In the mid­dle of last October, the Institute of Plant Virology of the National Research Council (CNR) and the University of Bari informed the local phy­tosan­i­tary author­i­ties of the detec­tion of the Xf bac­terium in Lecce and the move­ment of prop­a­ga­tion mate­r­ial of any sus­cep­ti­ble host species from the infected area was promptly pro­hib­ited by the Regional Plant Protection Service.

What is the pur­pose of the com­ing ​“First International Symposium on the European out­break of Xylella fas­tidiosa in olive?”

The sym­po­sium will be held in Gallipoli from October 21 – 22 and be fol­lowed by tech­ni­cal lab­o­ra­tory work­shops from October 23 – 24. It will offer a detailed overview of this emerg­ing threat and pro­vide a great oppor­tu­nity to exchange infor­ma­tion with the main inter­na­tional experts on this topic.

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