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Defining factors associated with rapid apple decline in the Southeastern United States

Sara Villani: North Carolina State University

<div>Rapid Apple Decline (RAD) is an emerging and poorly understood phenomenon that results in the sudden death of apparently healthy, young dwarfing apple trees in high density (HD) planting systems. Erratic environmental conditions, including water stress, are believed to be largely responsible for RAD in Western North Carolina. In 2017 a survey was initiated to evaluate the extent of RAD in NC. Nearly 80% of young, HD orchards in Western NC had trees that were declining or had recently died from RAD. Within several of these orchards, tree loss exceeded 30%. In addition, biotic agents associated with RAD in NC were investigated. Ambrosia beetles were identified in 100% of declining trees with <em>Xylosandrus crassiusculus, X. germanus,</em> and<em> Xyleborinus saxeseni </em>accounting for the majority of species observed. The association between fungi and RAD was also determined. From 28 orchard locations, 582 fungal isolates were recovered from the tissue of 3 regions of the tree: graft union, beetle galleries, and scion. <em>Botryosphaeria dothidea, Phomopsis </em>spp., and <em>Fusarium </em>spp., were most frequently recovered, with the majority of <em>B. dothidea </em>and <em>Phomopsis </em>spp. isolated from the graft union. Ambrosia beetle fungal symbionts, <em>Ambrosiella grosmanniae </em>and <em>F. solani</em> represented 13.4% and 16.9% of fungal isolates recovered from beetle galleries, respectively. Further elucidation of the association of recovered fungal isolates under varying degrees of water stress is ongoing.</div>