Rethinking the Xylella fastidiosa scenario in the Balearic Islands: what epidemiological, phylogenetic and dendrochronological data tell us

Trabajo presentado en la 2nd European conference on Xylella fastidiosa (how research can support solutions), celebrada en Ajaccio el 29 y 30 de octubre de 2019.The emergence of Xylella fastidiosa(Xf) in Europe has been dealt as relatively recent introductions from the American continent. While this seems accurate for highly virulent genotypes such as thestrain currently causing the quick olive decline syndrome in Apulia since ca. 2013, less virulent Xf genotypes might have gone undetected for a long time, being confused with drought or fungal disease symptoms under Mediterranean climatic conditions. Indeed, the current widespread incidence and severity of the Pierce’s disease (PD) and Almond Leaf Scorch Disease (ALSD) in Mallorca Island can only be understood in this context of a 20-year introduction scenario. Our current epidemiological, phylogenetic anddendrochronological data onXf strains belonging to subsp. fastidiosaST1, and Xf subsp. multiplexST81, causing Pierce’s disease (PD) and ALSD, respectively, strongly suggest that they were overlooked for decades. Both subspecies were very likely transported from California to Mallorca with infected almond scions around 1995 and subsequently spread throughout the island by the local vector Philaenus spumarius. Our phylogenetic analysis based on WGS of isolates of both subspecies from Mallorca supports their Californian origin. Congruent with this, Xf DNA was consistently detected in the growing rings of infected almond trees from 2006 to the present and occasionally as far back as 1998. In the main focus in Son Carrió more than 50% of the almond trees diedand 90% of trees showed symptoms compatible with ALSD in 2012, but the aetiology of this problem was attributed to fungal trunk diseases, drought or field abandonment. We hypothesise that during the last 20 years, Xf subsp. multiplexST81 isolates have adapted to wild olive trees, widespread on the island, causing a mild dieback, and later on reaching the island of Menorca very likely on infected P. spumariustransported as a hitchhiker on ships moving between the islands.This research was funded by project E-RTA2017-00004-C06 from AEI-INIA Spain and FEDER and received financial support from the Ministry of Agriculture, Fishery and Food (Dirección General de la Sanidad de la Producción Agrària) of Spain and from ‘Govern de les Illes Balears’