Preliminary results of the study on spontaneous herbaceous plants as potential hosts of Xilella fastidiosa in the CODIRO outbreak

To investigate the role of herbaceous plants as hosts of Xylella fastidiosa, monthly sampling of the native flora of two heavily infected olive groves and of the side of the adjacent dirt roads was conducted from January 2014 onwards. One of the orchards had not been subjected to weeding, whereas periodic tillage had been carried out in the other. Overall, more than 100 species of 40 monocotyledonous and dicotyledonous families were collected, photographed and identified, their phenological stage was recorded using the Keller-Baggiolini scale, their period of presence and the type of distribution prevailing in the field (e.g. whether the species scattered or concentrated under olive trees or along the edges of the dry stone walls) was assessed, as well as the abundance-dominance indices according to the Braun-Blanquet method. At all sites monitored, Philenus spumarius, the main and most important vector of Xylella fastidiosa so far found in the area of the outbreak, was present with abundant populations on herbaceous species in the spring and on olive trees from the middle of May throughout the summer. All weed samples collected, in pools of 4-5 plants, were tested by DAS-ELISA and the uncertain results were verified by PCR. So far, none of the samples analyzed, in excess of 600, proved to host Xylella fastidiosa, confirming the preliminary observations that, by and large, weeds may not have a major role in the epidemiology of X. fastidiosa in the considered area. However, since sampling of the summer/autumn flora is yet to be done, a better insight into the epidemiological role of these plants will become available in the coming months, in conjunction with the migration of the vectors from the olive trees onto the native flora with the consequent possible inoculation of the bacterium.