Thought to have arrived in Italy on a plant imported from Costa Rica in 2008, the plant pathogen Xylella fastidiosa was first detected there in 2013. Its subsequent unchecked spread resulted in the loss of millions of olive trees across Southern Apulia, a region in Italy responsible for the production of roughly 12% of the world’s olive oil (5). The pathogen moved swiftly and, to date, a total of 20 million olive trees have been infected across Europe.
The economic implications of the spread are dire: in Italy, X. fastidiosa infection in the olive orchards of Apulia is projected to cost the country €5.2 billion over the next 50 years if the trees can’t be replaced (1). This would in turn likely impact the price and availability of olive oil for consumers. Depending on the rate of disease spread, the estimated decline in welfare for European consumers over the next 50 years ranges from €4.1 to €10.3 billion (7).
X. fastidiosa isn’t exactly picky—it’s also responsible for significant yield loss in other plant species, including grapevines, almond and citrus trees. The pathogen is common throughout the Americas, where it originated, as well as in the Mediterranean. So, what exactly is X. fastidiosa, and how does it spread?
Benign Commensal, or Ruthless Parasite
X. fastidiosa is a gram-negative bacterium that feeds on the xylem sap of its hosts. The xylem is the vascular tissue of a plant which transports a sap comprising some nutrients but mostly water from the plant’s roots into its branches and leaves. This network is a series of vessels connected by pit membranes, which are composed primarily of nonliving plant cell wall and allow for the passage of xylem sap but preclude the transit of pathogens. X. fastidiosa erodes these pit membranes through enzymes that target cell wall tissues, diminishing the plant’s ability to block pathogen migration and allowing the bacterium to move freely throughout the plant. In its spread, X. fastidiosa produces a biofilm that obstructs the vessels and occludes the pathways that provide the upper reaches of the plant with water, inevitably killing its host (1, 3, 5).
X. fastidiosa infection can lead to a variety of plant diseases in numerous different species, including Pierce’s disease in grapevines, phony peach disease, plum leaf scald, citrus variegated chlorosis disease, olive quick decline syndrome and leaf scorch (3). A particularly insidious feature of this pathogen’s spread is its broad host range. While capable of infecting over 600 different plant species, only some of these species will develop disease as a result of X. fastidiosa infection. In other species, the bacterium exists as a harmless commensal—feeding on, but neither hurting nor helping, its host. This makes it particularly tricky to contain as many infected plants will not show symptoms, allowing X. fastidiosa to proliferate undetected.
The Vector
X. fastidiosa is transmitted from one plant to another via sap-feeding insects such as leafhoppers and spittlebugs (4). After biting the leaf of an infected plant, the insect will carry X. fastidiosa in its foregut and, when chewing on the next plant, will deposit the bacterium into the healthy plant’s xylem tissue. With no incubation period required in the vector, X. fastidiosa can be transmitted instantaneously. Once infected, an adult insect can continue to transmit the bacterium throughout its lifetime (2).
These insects serve as vectors for X. fastidiosa across the Americas and Europe. Specifically, the spittlebug P. spumarius has been identified as a critical vector in the Italian outbreak and is notable for being present and active as a vector in both regions.
Detection and Containment
Containing the spread of X. fastidiosa presents a host of challenges due to the wide variety of impacted plant species, as well as issues with vector control, climactic factors, and difficulties in enforcing quarantines and movement restrictions. Getting buy-in from affected stakeholders is another obstacle. While prevention is key, plants that have already been infected are marked for destruction, which can be hard for those who grow and rely on these plants to accept. Particularly in places like Italy, where olive trees are not only vital to the economy but also possess deep cultural and societal value, it can be challenging to effectively connect with farmers over the social and political noise that surrounds the issue (4, 5). Some of these trees are several hundreds or even more than a thousand years old, making them particularly difficult for communities to part with.
Given all of these factors, controlling X. fastidiosa comes down to rigorous surveillance programs, rapid diagnostic measures, effective strategies for vector control and campaigns to combat misinformation and raise public awareness. Diagnostics and detection can be performed through serological tests including ELISAs, or molecular tests like Loop-mediated Isothermal Amplification (LAMP) and real-time polymerase chain reaction (qPCR). Due to its high sensitivity compared to other molecular tests, qPCR is preferred where X. fastidiosa has either not been detected or in plants that may be asymptomatic (2).
Since X. fastidiosa spreads rapidly and infected plants do not always exhibit visible symptoms, automated nucleic acid extraction can help labs perform qPCR and detect X. fastidiosa quickly. An interlaboratory comparison of molecular methods identified the Promega Maxwell automated extraction method as a highly sensitive, accurate and reproducible way to extract and purify X. fastidiosa DNA from infected plant tissue and vectors (6). The primary tools employed in this method include the Maxwell® RSC Instruments with the Maxwell® RSC PureFood GMO and Authentication Kit, or the Maxwell® HT Environmental TNA high-throughput chemistries. The Promega Maxwell method is recognized as a validated DNA extraction method according to guidelines for X. fastidiosa detection put forth by the European and Mediterranean Plant Protection Organization (EPP0) (2) and France’s national food, environmental and occupational health and safety organization, ANSES (8).
Moving Forward
Despite efforts to control X. fastidiosa, this disease continues to spread, leaving devastation in its wake. This pathogen requires a multifaceted approach for effective management. Advanced diagnostic tools coupled with proactive surveillance, stringent quarantine measures, the swift elimination of infected plants and improved public awareness can help combat X. fastidiosa. Hope is certainly not lost—with action and support at the government level as well as from scientists and communities, we can work toward building out and implementing strategies that may eventually lead to the salvation of pivotal plant species like the widely revered olive tree.
Works Cited:
Additional Sources:
How Xylella came to Apulia (nature.com)
Xylella fastidiosa (arcgis.com)
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