Characterization of the infection process of forest species by Xylella fastidiosa strains through molecular analysis and physiological stress indicators

Xylella fastidiosa is a quarantine pathogen of increasing concern in Europe due to its rapid spread and severe impact on agricultural and forestry species typical from the Mediterranean. This master thesis aims to characterise the pathogenicity of two European strains of X. fastidiosa subsp. pauca present in Spain; ST53 (CoDiRO) and ST80 (XYL1961/18), in five perennial forest plant species from natural ecosystems in Spain: Arbutus unedo, Juniperus thurifera, Fraxinus angustifolia, Quercus ilex, and Quercus suber. Experimental inoculations of the bacteria were conducted under under controlled and safety conditions, followed by physiological monitoring and molecular detection through quantitative PCR (qPCR). Spectral reflectance data and pigment content were assessed using proximal sensing tools to detect stress indicators associated with pathogen presence. Although symptom development was largely indistinguishable from abiotic stress responses, qPCR analysis confirmed successful colonization by both strains, with ST80 showing significantly lower Cq values, and therefore a higher bacterial content. Additionally, spectral reflectance indexes indicated physiological changes linked to infection, offering potential for non-invasive early detection as well as providing additional information for the involved physiological process in the pathogenicity of the assessed perennial forest plants These findings contribute to the understanding of epidemiology of these X. fastidiosa strains in natural Spanish ecosystems and highlight the risk posed by wild reservoirs plants to susceptible cultivated species, shaping future surveillance and containment strategies.