Pathogenicity of Xylella fastidiosa subsp. multiplex isolates from Alicante outbreak (mainland Spain) on different hosts

In the outbreak of Xylella fastidiosa in the province of Alicante (mainland Spain), all isolates characterized so far belong to subsp. multiplex and ST6, and the main host is Prunus dulcis. Two different strains (IVIA 5901 and ESVL), isolated from almond trees in two municipalities of the Demarcated Area, were used to carry out pathogenicity tests on different plant species. The Italian strain De Donno (subsp. pauca, ST53) and the Spanish strain IVIA 5770 (subsp. fastidiosa, ST1) were also included. Five cultivars of Prunus dulcis, eight of Olea europaea, four of Vitis vinifera, Citrus limon, Citrus sinensis, Citrus reticulata, Diospyros kaki, Eriobotrya japonica, and the forest species Quercus suber and Q. ilex were challenged. A total of 35 plants for each species or cultivar were inoculated. At 1, 3, 6, 12 and 18 months post-inoculation (mpi) plants were sampled and analyzed by real-time PCR, to assess bacterial movement over time. The results show that, in general, no clear pattern of movement was found in the different plant species. Interestingly, in olive and grape, regardless the cultivar, the subsp. multiplex ST6 Alicante isolates were hardly detected from 12 mpi onwards, i.e. the systemic movement was very slow. In fact, they were only detected in less than 4% of the infected plants and with very late Cq values. In citrus plants, detection of these isolates failed since 3 mpi, as occurred in kaki and loquat trees. As for Quercus species, the detection rate was higher in Q. suber than in Q. ilex. Only in almond and cork oak trees, leaf scorch symptoms could be correlated with the presence of X. fastidiosa subsp. multiplex. Moreover, in these cases the pathogen was re-isolated. The timing and the level of detection for all the strains throughout the trial obtained so far are presented.