Xylella fastidiosa in the Balearic Islands: a genetic diversity hotspot in Europe

The Balearic Islands hold one of the major genetic diversity of the pathogen Xylella fastidiosa in Europe. To date four genetic lineages, ST1, ST7, ST 80 and ST81, belonging to the three formally known subspecies, fastidiosa, multiplex and pauca have been detected in the islands. Recently, we have unravelled the origin of two of these introductions in the island of Mallorca, the ST1 of subsp. fastidiosa and the ST81 of subsp. multiplex. They were associated with a long overlooked outbreak of almond leaf scorch disease traced back to 1993 (Moralejo et al. 2020). Less is known about the origin of the ST80 (subsp. pauca) in Ibiza, although there is circumstantial evidence that would indicate it was introduced through the ornamental plant trade at least one decade ago. The ST80 produces a lethal dieback of olive trees; however, they seem less virulent towards olive trees than its counterpart ST53 in Apulia, Italy. Nonetheless, it poses a considerable threat to mainland olive oil production. The other main threat is the ST1 responsible of Pierce’s disease, which is still confined in the island of Mallorca (Moralejo et al. 2019). We used a large database of more than 13,000 samples analysed since 2016 to highlight general aspects of Xylella’s biology, including population genetics, epidemiology, landscape impact, dispersal, phylogenetic signal in host range and disease control. Most of the characteristics related to Pierce’s disease and almond leaf scorch disease repeat those general patterns observed in California, but others seem to be specific to the Balearic Islands.