FIELD EVALUATION OF DIFFERENTIAL CULTIVAR RESPONSE TO XYLELLA FASTIDIOSA INFECTION IN OLIVE

While different sources of natural resistance to Xylella fastidiosa (Xf) have been described in grapevines and citrus, lack of consolidated information exists on the wide panel of cultivars characterizing the vast olive germplasm. Preliminary observations and analysis on few cultivars, support the evidence that differential responses to Xf infections exist. To extend these studies to a large panel of cultivars a first experimental plot was realized under natural infection conditions in April 2015, as part of a pilot study commissioned by the European Food Safety Agency (EFSA). This plot, initially including 10 olive cultivars, was further extended, in April 2016, with additional 19 different varieties. The plot is located in the Apulia Region (Italy) in the demarcated infected area, surrounded by Xf-heavily affected olive groves. Twenty-four trees of each of the 29 cultivars were planted in randomized blocks and exposed to Xf infection by the natural vector populations occurring in the area. Indeed, to increase the probability of vector transmission, trees from the first set of 10 cultivars were caged with 15-20 specimens of Philaenus spumarius, collected from the neighboring Xf-infected olive groves. Visual inspections and diagnostic assays are being periodically performed. Laboratory tests confirmed the infectivity of vector populations occurring in the Apulian contaminated area and the Xf susceptibility of the olive cultivars tested. 50% of the trees tested positive to Xf, with an infection incidence ranging from 25% (cv. ‘Leccino’) to 78% (cv. ‘Koroneiki’). Symptoms of shoot dieback started to appear 1-year after planting, limitedly on few replicates of cv. ‘Cellina di Nardò’. In-depth investigations to monitor the bacterial titer and expression of target genes in different infected cultivars, are also underway.