Search for crop species immune to Xylella fastidiosa subsp. pauca, ST53

Up to date 35 different host species have been reported susceptible to Xylella fastidiosa, subsp. pauca, ST53 (Xfp), in the Apulia region (southern Italy). Xfp is genetically related to coffee strains of the subsp. pauca, particularly aggressive on olives, the predominant crop species in the area, and on other species (i.e. oleander, Acacia spp, Polygala myrtifolia). On the other hand, our previous investigations demonstrated that this genotype was not able to infect Vitis spp., several species of Citrus and the Prunus hybrid GF677. In the last 3 years, we extended the investigations by performing a series of artificial inoculations on different species, with the aim of identifying additional immune species. These included Pyrus communis, Juglans regia, Ceratonia siliqua, Mespilus germanica, Diospyros kaki, Citrus limon, Punica granatum, Persea americana. At least 10 plants for each species were needle-inoculated in 2017, along with susceptible controls (periwinkle and olive). Vector-mediated transmission was also used for P. americana, by caging the plants with Xfp-infected specimen of Philaenus spumarius. Inoculated plants were periodically inspected and sampled to assess bacterial multiplication and host colonization. Overall, the diagnostic tests performed during the 3 years post-inoculation did not reveal the presence of the bacterium in the tissues above (>10cm) the inoculation points of any of the inoculated plants. Detection occurred only when leaf petioles from the inoculation points were tested. Similarly, no positive detections occurred in the plants of P. americana exposed to infected insects. In addition, for C. limon, P. granatum, P. communis and C. siliqua surveys conducted in these last 3 years in the heavily infected area did not reveal the occurrence of natural infected plants. Overall, the data suggest that these species do not support the multiplication and the spread of the bacterium. However, even if for P. Americana needle-inoculations were complemented with vector transmissions, for the remaining species mechanical inoculations should be integrated by vector-mediated transmission tests for conclusive assessment of their immunity.