Improving Xylella sampling in Mallorca

Abstract

Xylella fastidiosawas detected in Majorca in late 2016 (Olmo et al. 2017). Since then, accomplishing EU regulatory, a huge number of samples have been analysed. Poster sessions71| P a g eParticularly, the Mallorca outbreak is different to others in Europe, because of the coexistence of subspecies multiplex(ST81, ST7) in almond trees, olive trees and other species, and subspecies fastidiosa(ST1) in almond trees and grapevine plants among others.As alreadyknown, detection tests of X. fastidiosaare conditioned by the sampling date. However, it is not well known if the optimal dates for sampling are the same for all host species, if it is viable to pool several samples, or if the bacterium can be detected in old wood samples.In this study, we have focused the X. fastidiosamonitoring on almond, grapevines and olive trees. Monthly percentages of positives and average Ct value of more than 2,000 analyses of each of these crops were compared. Overall, we observed that for almond trees the best results were obtained from samples collected from June to August, whereas in grapevine the optimum period was from August to October. In olive trees early spring resulted in the most favourable time for detection, with an increase in Ct and decrease of positive cases in summer.In the assays conducted with pooled almond leaf samples, mixing one infected plant extract with the same volume of up to four negative extracts, showed an average Ct increase of 2.7 cycles, whereas for olive and grapevine samples the Ct increase was > 4 cycles.By default, X. fastidiosais normally analysed in leaf midribs and petioles. We also tested the potential use of wood samples from trunks,sampling different tree rings. Although this sampling procedure is not recommended for routine surveys, it allowed us to establish a potential infection chronology of the number of years since the first infection might have occuredin the Balearic Islands.