Metabarcoding Analysis Reveals Microbial Diversity and Potential Soilborne Pathogens Associated with Almond Dieback and Decline

Almond decline and dieback have become significant challenges in newly established orchards, with symptoms including internal necrosis, canker, and external gummosis. This work aims to explore the potential fungal and bacterial causative agents through metabarcoding and traditional culture plate isolation across six almond cultivars. Our results emphasize the multifactorial nature of almond decline and dieback, with possible co-infections by opportunistic fungi and bacteria playing a central role. Classical isolation identified 47 fungal species or genera, including <i>Diaporthe amygdali</i>, <i>Diplodia corticola</i>, <i>Phytophthora</i> sp., and several <i>Fusarium</i> species. Almond metabarcoding revealed a more diverse microbial community, highlighting the prevalence of soilborne pathogens such as <i>Neocosmospora rubicola</i>, <i>Dactylonectria estremocensis</i>, and <i>Plectosphaerella niemeijerarum</i>. Soil metabarcoding suggested that these pathogens likely originate from nursery substrates or soils shared with other crops, such as olives and vineyards, that serve as a source of inoculum. 'Soleta' generally presented lower richness when compared to the other tested cultivars, suggesting a higher degree of biotic stress and decreased plant resilience. This study highlights the value of integrating NGS approaches to comprehensively study complex diseases and the need for further research on pathogen interactions and cultivar susceptibility for the future development of new sustainable, targeted management strategies in almond orchards.