Further acquisition on the response of a large number of olive cultivars to infections caused by Xylella fastidiosa subsp. pauca, ST53

Deployment of resistant varieties is a key strategy for mitigating economic losses due to Xylella fastidiosa infections in crops, particularly in those areas where the bacterium finds favourable conditions for its persistence and infections are associated with severe diseases. As such, genetic traits that confer resistance/tolerance have been identified in the past in grapes, citrus and, more recently, in olives. Although understanding the molecular pathways involved in the resistance is a primary target of many studies, nowadays supported by the powerful resolution of the high-throughput sequencing technologies, biological confirmation remains a paramount tool to ultimately define the response of a given plant genotype. In this context, the emergence of X. fastidiosa in olives in southern Italy, prompted for large surveys and biological screening tests of olive germplasm in open fields and in greenhouses. Test plants are monitored for symptoms, incidence of bacterial infections, estimation of the bacterial population size and, more recently, analysed for the expression of LRR-RLKs- and ABA-related genes. The results so far collected highlighted the following aspects: (i) under controlled conditions the latent period of the infections in olives is as short as one year in the most susceptible cultivars, whereas under field conditions a first reliable estimation of the symptoms associated with the infections could be retrieved upon at least three years of exposure to the natural pressure of inoculum; (ii) cultivars under testing could be firstly distinguished in highly susceptible’ those that developed symptoms in the shortest time and those ‘potentially tolerant/resistant’ deserving further investigation; (iii) upon needle-inoculations or field exposure, cultivars could be differentiated based on the rate of systemic infections, as highly infected vs low-infected. Interestingly, several cultivars either in greenhouses or in field testing produced results similar to those recorded for the cultivar Leccino, used as resistant control; indeed cultivars harbouring high bacterial titers but not showing symptoms (i.e. with traits of tolerance) have also been identified. Although preliminary, these studies allowed to gather information on the spectrum of tolerance/resistance of additional olive cultivars