SEARCHING FOR OLIVE GENOTYPES RESISTANT TO QUICK DECLINE SYNDROME

The olive quick decline syndrome (OQDS) is a severe vascular disease caused by Xylella fastidiosa subsp. pauca (Xfp), a Gram-negative, quarantine bacterium for the European Union. It first appeared in 2013 in a grove in the Salento peninsula, Apulia region, southeastern Italy, and quickly spread northward. Currently, the pathogen has affected over 40% of the Apulian territory, hosting to more than 21 million olive trees. Utilizing resistant olive varieties is the most effective strategy to mitigate the disease's impact. Extensive surveys were conducted in the infected areas, where OQDS has decimated nearly 90% of olive plants, to identify genotypes displaying resistance. Seventeen olive genotypes, exhibiting minimal or no symptoms of quick decline syndrome, were monitored for Xfp load over the past six years. Samples were collected in April and October and subjected to qPCR detection following accepted protocols. Some genotypes remained uninfected, while others displayed low Xfp loads or declining trends. Ten SSR markers were utilized to genotype a subset of the tested plants, comparing allelic profiles with 244 olive cultivars from the Mediterranean basin, including Putatively Resistant (PR) genotypes, to identify synonyms (LRM analysis) and explore phylogenetic relationships (Unweighted Neighbor-Joining analysis). Additionally, a paternity test was conducted to ascertain the parentage of unknown genotypes. LRM analysis revealed one case of synonymy and high genetic similarity between P4_C and Proto 12 with the resistant cultivar Leccino. Phylogenetic analysis placed the tested samples close to resistant cultivars Leccino and FS17, as well as some PR genotypes, indicating significant genetic affinity. Notably, sample P4_C appeared to derive from self-pollination of the Leccino cultivar. Some samples showed parentage with the Apulian cultivar Rotondella and the Albanian cultivar Kalinjot oval, closely related to the resistant Leccino. Furthermore, certain samples originated from autochthonous cultivars, underscoring the importance of conserving them to identify Xfp-resistant genetic material.