Leaf ionome profile of susceptible and resistant olive cultivars infected by Xylella fastidiosa

Xylella fastidiosa (Xf) is continuing to emerge as a devastating bacterial pathogen for many economically relevant species. Xf subsp. pauca strain De Donno is associated with Olive Quick Decline Syndrome (OQDS), a destructive disease occurring in the southern area of Apulia (Italy). In susceptible olive cultivars, symptoms are characterised by initial leaf scorch and scattered desiccation of small branches that over time worsen and extend to the whole canopy. Greenhouse and field observations revealed that olive trees of the cultivar Leccino show milder symptoms, when compared to those observed in Ogliarola salentina. Ogliarola salentina have progressive, severe scorching and complete dieback. Moreover, the lower bacterial population size in Leccino confirmed the resistance of this cultivar to Xf infection, suggesting that it is able to limit pathogen multiplication.
To understand the role that mineral nutrition may play in host resistance to OQDS, a field survey of the leaf ionome was carried out in trees of two orchards located in the Xf-infected demarcated area, that showed clear differences in response to Xf infection. Infected leaf samples, classified as symptomatic and asymptomatic, were subject to the determination of the ion content by inductively coupled plasma – optical emission spectrometry (ICP-OES). Data were analysed in relation to the different cultivars and the presence or absence of symptoms.
The comparison between symptomatic and asymptomatic samples showed an increase of sodium levels in both cultivars and significantly higher calcium levels in the symptomatic tissues of Leccino, a response that had been found in other Xf-host pathosystems. Otherwise, Leccino trees had a significantly higher content of manganese, in both symptomatic and asymptomatic leaf tissues.
These field observations inform currently ongoing experiments under controlled conditions to investigate the relevance of these mineral ion changes in the development and progression of symptoms, and the potential involvement of manganese in resistance of the Leccino cultivar to Xf infection.