Switzerland is currently free of Xylella fastidiosa. However, due to the potential damage it could cause to crops, ornamental and forest tree species, it is classed as a priority quarantine organism. To be better prepared for a possible introduction of X. fastidiosa, and to improve its surveillance, woody plant species relevant for Swiss agriculture and forestry were artificially inoculated with different X. fastidiosa subspecies in a biosafety facility.
Seven plant species were selected (Vitis vinifera, Prunus avium, Prunus armeniaca, Olea europaea, Rubus fruticosus, Quercus petraea and Fagus sylvatica), and inoculated on the stem with X. fastidiosa subsp. fastidiosa, subsp. multiplex and subsp. pauca. Leaves were sampled 4 and 11 months after the artificial inoculation and analysed by qPCR. Symptoms were visually assessed at the same timepoints by three collaborators independently.
At the first sampling, X. fastidiosa (all three inoculated subspecies) could be detected in 6-33% of leaves of V. vinifera, O. europaea, R. fruticosus, Q. petraea and F. sylvatica, but not in any sample of the two Prunus species. Surprisingly, subsp. pauca was present in the leaves of 3 out of 5 F. sylvatica seedlings, in the leaf samples of 3 out of 5 Q. petraea and one leaf sample out of 5 R. fruticosus, in addition to all the leaf samples of the O. europaea plants. However, 11 months after the inoculation and after one winter in an unheated quarantine glasshouse, the infection in most of the above-mentioned plants was lost, with only one leaf sample of O. europaea and one leaf sample of Q. petraea still testing positive. Symptoms’ assessment was difficult at the first sampling timepoint, as X. fastidiosa symptoms were very similar to the natural senescence of the plant in autumn. In the few plants that still tested positive for X. fastidiosa 11 months after the inoculation, symptoms were mostly visible and typical for X. fastidiosa. Attempts to re-isolate all the three X. fastidiosa subspecies on semi-selective media PD2 and BCYE were unsuccessful. Our results suggest that the host range of X. fastidiosa subsp. pauca could be broader than previously known and could comprise also deciduous woody species such as F. sylvatica. Moreover, visible symptoms could be an unreliable indicator of an infection, especially for deciduous plants towards the end of the growing season.