Evaluation of vascular occlusions in xylem vessels of olive cultivars infected with Xylella fastidiosa

Understanding the mechanisms of the development of symptoms of Olive Quick Decline Syndrome (OQDS) and the different olive cultivar responses to infections caused by Xylella fastidiosa subsp. pauca, ST53, is fundamental for disease control. The combined action of bacterial aggregates and plant-derived vascular occlusions (tyloses, gums and gels) has been invoked as the cause of the pathogenic alterations occurring in Xylella-infected plants. Conflicting observations were found about the role of vascular occlusion in the disease progression in field-grown olives. In the present work the distribution of vascular occlusions in the secondary xylem vessels of susceptible, Cellina di Nardò, and resistant, Leccino and FS17, cultivars were studied by light microscope observations of toluidine blue stained sections, recovered from stems of greenhouse-grown mock and artificially inoculated olives. One-year-old shoot portions collected from symptomatic or symptomless twigs were first tested by qPCR and used to recover thin sections (0.2 mm thick), with a similar number of vessels inspected for each cultivar. In the sections recovered from the non-infected controls 0.15%, 0.02% and 0.13% of occluded vessels were present, respectively in Cellina, Leccino and FS17. These percentages increased in the infected twigs reaching 9.65%, 6.81% and 1.33% in Cellina, Leccino and FS17, respectively, indicating that Xylella infections (regardless of the cultivar) induced occlusions of the xylem vessels; these were significantly higher in the susceptible cultivar. More specifically, percentages of occluded vessels in the susceptible ranged cultivar from 1% to 34%, while those of the resistant Leccino and FS17 ranged from 0.044% to 14% and 1.09% to 1.53%, respectively. Although a clear-cut difference was observed among the infected cultivars, no significant differences within each cultivar were recorded between symptomatic and non-symptomatic shoots, suggesting that the development of symptoms is a combination of different factors (anatomical, chemical and physical)