In Majorca, Balearic Islands (Spain), almond trees have experienced severe decline and mortality over the last 15 years. The Almond Leaf Scorch Disease (ALSD) was preliminarily studied from 2008 to 2010 and was associated with a complex of fungal trunk pathogens and their interactions with known disease-predisposing factors such as prolonged drought and tree aging. This early disease diagnosis, however, was recently challenged after the detection of Xylella fastidiosa in Majorca in October 2016.
In the study, XF-Actors researchers show how ALSD was in fact triggered by the introduction of Xylella fastidiosa around 1993 and subsequently spread to grapevines (Pierceʼs disease). So, the pathogen remained undetected for decades in the Mediterranean basin in almond or other hosts, confused for other pathogens or for symptoms of drought stress.
According to the models used in the study the researchers can conclude that both Xylella fastidiosa subspecies found in Majorca, fastidiosa ST1 and multiplex ST81, shared their most recent common ancestors with Californian populations of the bacterium, associated with almonds and grapevines. Consistent with this chronology, Xf-DNA infections were identified in tree rings dating to 1998. These findings uncover a previously unknown scenario in Europe. As far as the EU plant health policy is concerned, the results question the adequacy of eradication and contingency strategies without any prior investigation into the epidemiological history of the outbreaks that are intended to be contained.
Read the full article via Nature: https://doi.org/10.1038/s42003-020-01284-7