Exotic ambrosia beetles and phytopathogenic fungi associated with rapid apple decline in North Carolina

Authors: GRESHAM, SEAN - North Carolina State University; Ranger, Christopher; WALGENBACH, JAMES - North Carolina State University; VILLANI, SARA - North Carolina State University

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2025
Publication Date: 10/27/2025
Citation: Gresham, S., Ranger, C.M., Walgenbach, J., Villani, S. 2025. Exotic ambrosia beetles and phytopathogenic fungi associated with rapid apple decline in North Carolina. Journal of Economic Entomology. 118(6):3029-3041. https://doi.org/10.1093/jee/toaf191.
DOI: https://doi.org/10.1093/jee/toaf191

Interpretive Summary: Rapid apple decline (RAD) is a syndrome primarily affecting young (<6 years old) apple trees planted in high-density orchards throughout the eastern USA and Canada. Despite the sudden death of thousands of apple trees in North America, no definitive biotic or abiotic cause of RAD has been identified. Surveys of apple orchards in North Carolina experiencing RAD symptoms in 2017 found that ambrosia beetle infestations were associated with dead and or declining trees. Exotic ambrosia beetles have been implicated in the damage and decline of ornamental, fruit, and nut trees throughout the USA and are of increasing concern in other regions throughout the world. Trapping surveys across western North Carolina apple orchards over three years revealed that three exotic species of ambrosia beetles were the dominant species captured. The same three species of beetles were found within declining apple trees collected from commercial orchards. A large diversity of fungi was associated with the beetle galleries and non-gallery tissue within declining trees, including opportunistic phytopathogens. However, no phytopathogenic fungi were exclusively associated with the gallery tissue. These results indicate that RAD is a complex issue.

Technical Abstract: Rapid apple decline (RAD) is a syndrome primarily affecting young (<6 years old) apple trees planted in high-density orchards throughout the eastern USA and Canada. Despite the sudden death of thousands of apple trees in North America, no definitive biotic or abiotic cause of RAD has been identified. Surveys of apple orchards in North Carolina experiencing RAD symptoms in 2017 found that ambrosia beetle attacks infestations were associated with dead and or declining trees. The exotic ambrosia beetles (Coleoptera: Scolytinae), Xylosandrus crassiusculus (Motschulsky) and X. germanus (Blandford),, have been implicated in damage and decline of ornamental, fruit, and nut trees throughout the USA and are of increasing concern in other regions throughout the world. Trapping surveys across western North Carolina apple orchards over three years revealed that X. crassiusculus, X. germanus, and Xyleborinus saxesenii (Ratzeburg) were the dominant species captured, with annual variation in species composition. The same three species of beetles were found inside the scion, rootstock, and graft union of declining apple trees collected from commercial orchards. A large diversity of fungi was associated with beetle gallery and non-gallery tissue on declining trees, including opportunistic phytopathogenic genera such as Botryosphaeria, Fusarium, and Diaporthe. No phytopathogenic fungi appeared to be exclusively associated with gallery tissue.