A New Study Indicates Presence Of Natural Traits Of Resistance To Xylella Fastidiosa In Leccino Offspring
The recent publication ‘A survey in natural olive resources exposed to high inoculum pressure indicates the presence of traits of resistance to Xylella fastidiosa in Leccino offspring’ highlights the progress made in fighting Xylella fastidiosa. BIOVEXO partner Mr. Pasquale Saldarelli from CNR, co-author of this publication, contributed with his research significantly to this publication – this is a must read if you are concerned about the spread of Xylella and the health of olive trees.
Background to modern farming practices
The cultivated olive (Olea europaea, subsp. europaea, var. europaea) is a vital crop, especially in the Mediterranean Basin, which accounts for 95% of global olive oil production. The olive tree represents a rich genetic diversity, with over 1,200 recognized cultivars and thousands of minor ones. However, modern, intensive farming practices are narrowing this biodiversity, with fewer traditional varieties used in new cultivation methods. These traditional varieties, which have been selected and tested over centuries by local farmers for their resilience to climate change and lower resource requirements, are crucial for sustainable olive farming. Unfortunately, this biodiversity is often underexplored, as seen in regions like Apulia, where local varieties have been grafted with more commercially viable cultivars, reducing genetic diversity.
The greatest threat to olives: Xylella
One of the most significant threats to olive trees today is the Xylella fastidiosa bacterium, which causes the olive quick decline syndrome (OQDS). This disease was first discovered in southern Italy in 2013 and has since caused severe epidemics, devastating millions of olive trees, including centuries-old specimens. The bacterium, spread by the spittlebug (Philaenus spumarius), has found ideal conditions in the Mediterranean climate, leading to large-scale outbreaks in the Apulia region, where susceptible cultivars like Cellina di Nardò and Ogliarola salentina are predominant. However, isolates of Xylella fastidiosa subspecies pauca have also been detected in South America, specifically in Brazil and Argentina, associated to similar symptoms in olives. In contrast, California has reported only mild symptoms in olive trees, due to the presence of a different Xylella subspecies.
At present, no curative treatments exist for Xylella-infected plants. Current control measures focus on reducing insect vector populations and removing infected trees, though these strategies are only effective if implemented promptly. A promising long-term approach is the use of genetic resistance, as seen in other Xylella-susceptible crops like grapes and citrus. In these crops, certain genotypes demonstrate natural resistance to the bacterium, offering a path forward for breeding resistant varieties. For example, some wild relatives of the domesticated grapevine (Vitis vinifera) and specific citrus varieties exhibit strong resistance to Xylella infection, and this resistance has been linked to specific genetic loci, such as the PdR1 locus in grapevines.
Signs for optimism
In olives, two cultivars, Leccino and FS-17, have demonstrated a degree of resistance to Xylella. Trees of these varieties show lower bacterial loads and fewer symptoms compared to other cultivars. Research suggests that these resistant cultivars are able to control the replication of the bacterium in xylem vessels and manage drought stress more effectively. However, despite the rich genetic diversity within the olive germplasm, large-scale screenings for resistance have not yet identified any cultivars with higher resistance than Leccino and FS-17. This highlights the difficulties in conducting olive breeding programs, which face challenges due to the long time required for the selection of resistant genotypes.
Recognizing the urgency of the situation, researchers have conducted extensive surveys in the Apulia region, which has been under high Xylella pressure for years. These surveys, conducted over six years, identified 171 spontaneous olive genotypes that showed no visible symptoms of Xylella infection. Over time, some genotypes did develop symptoms and were excluded from further research, but many remained asymptomatic or exhibited only mild symptoms. The parentage of these resistant genotypes was analysed to determine which cultivars contributed to their resilience. A few promising genotypes underwent more in-depth studies, including transcriptomic profiling and controlled artificial bacterial inoculations to better understand their resistance mechanisms.
This work is part of broader research Italian (REACH-XY, RESIXO) and European (BeXyl) efforts, such as the BIOVEXO project, which has contributed significantly to understanding and combating Xylella infections in olives and other crops. BIOVEXO focuses on developing sustainable, bio-based pesticides to address the Xylella problem. As research continues, the identification of resistant genotypes and the development of effective biopesticides will be key to managing Xylella outbreaks and ensuring the long-term sustainability of olive cultivation.
To access the article, follow this LINK.
