Pathogen Profile #9

Hello Robigalia Reader and welcome back to another week of Robigalia, delivering the latest in plant pathology directly to your inboxes.

If you’ve noticed the price of olive oil increase dramatically recently, you’re not alone. Global supplies have hit a 10-year low, with recent extreme heat and drought partly to blame. Another culprit is a bacterial pathogen, Xylella fastidiosa.

Xylella fastidiosa is a gram-negative bacterium belonging to the family Xanthomonadaceae. Believed to have originated in the Americas, it was first detected in Europe in 2013 and has since spread to other European countries, including France, Spain, and Portugal. The olive oil industry has been the hardest hit by X. fastidiosa. The introduction into Italy has caused the death of over 20 million olive trees.

The meadow spittlebug (Philaenus spumarius) is a primary vector for X. fastidiosa in Europe. These insects feed on the xylem sap of plants, facilitating the transmission of the bacterium from infected to healthy plants. Once inside the plant, X. fastidiosa colonises the xylem vessels, using enzymes to break through adjacent xylem vessels. Here, it multiplies and forms biofilms, which block the xylem, interfering with the plant's water-conducting system. This blockage is further exacerbated by the plant's defense response, which includes the production of tyloses (balloon-shaped protrusions) in the xylem. These processes block water transport leading to various symptoms, including leaf scorch, wilting, dieback, and plant death.

The impacts of X. fastidiosa are causing pain far beyond the supermarket shelves. A Joint Research Council report from 2019 estimated the spread of X. fastidiosa throughout Europe would cost the EU over €5.5 billion per year due to loss of production. The Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES) estimated that an incursion of X. fastidiosa could cost Australian horticultural industries up to $11.1 billion over 50 years.

Early detection and public reporting are emphasised as critical steps in limiting further spread of X. fastidiosa in Europe and beyond. In Italy, advanced technologies like remote sensing, and mobile apps are being used for surveillance and tree health mapping. The EU conducts surveys in designated quarantine zones around infected areas with efforts are underway to establish high-throughput diagnostic testing capacity. Our furry friends are also being trained to assist with detection. In Xylella-free countries such as Australia, strict biosecurity rules to prevent Xylella from entering the country, and a National Xylella Action Plan has been developed.

If you’re interested in diving into the science about X. fastidiosa, I’ve summarised a few articles below. If you’re a subscriber, scroll down to access my online database which includes full summaries of these, and all other Robigalia featured articles.

Xylella fastidiosa papers

A Colorimetric LAMP Detection of Xylella fastidiosa in Crude Alkaline Sap of Olive Trees in Apulia as a Field-Based Tool for Disease Containment

• Researchers developed a colorimetric Loop-mediated Isothermal Amplification (LAMP) assay for detecting X. fastidiosa in olive trees directly in the field. This method uses crude alkaline sap as a sample and provides results in 40 minutes. It provides a portable, sensitive, and specific diagnostic tool aimed at containing the spread of the disease.

Phenolic Extract from Olive Leaves as a Promising Endotherapeutic Treatment against Xylella fastidiosa in Naturally Infected Olea europaea (var. europaea) Trees

• The study investigates the use of phenolic extracts from olive leaves as a treatment for X. fastidiosa. Results showed that these extracts have bacteriostatic properties and significantly improved vegetative growth in infected trees, suggesting a promising and cost-effective alternative to current treatments.

The impact of xylem geometry on olive cultivar resistance to Xylella fastidiosa: An image-based study

• The study investigates the relationship between xylem vessel geometry and the resistance of different olive cultivars to X. fastidiosa. The study found that cultivars with narrower xylem vessels are more resistant to X. fastidiosa, likely due to reduced pathogen spread and lower embolism risks. This suggests xylem vessel diameter is a key factor in cultivar resistance and can be used for selecting resistant varieties for replanting in infected areas.

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Until next week,

Alyssa