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

Rapid apple decline is a syndrome primarily affecting young (<6 yr old) apple trees planted in high-density orchards throughout the eastern United States and Canada. Despite the sudden death of thousands of apple trees in North America, no definitive biotic or abiotic cause of rapid apple decline has been identified. Surveys of apple orchards in North Carolina experiencing rapid apple decline symptoms in 2017 found that ambrosia beetle infestations were associated with dead 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 United States and are of increasing concern in other regions throughout the world. Trapping surveys across western North Carolina apple orchards over 3 yr revealed that X. crassiusculus, X. germanus, and Xyleborinus saxesenii (Ratzeburg) were the dominant species captured, with annual variation in species composition. The same 3 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. The establishment of several genera of opportunistic fungal pathogens and ambrosia beetles in rapidly declining apple trees suggests that rapid apple decline in North Carolina has no single cause and is likely due to a combination of abiotic and biotic stress events.