A condition known as rapid apple decline has afflicted orchards in several regions of the U.S. in recent years. A new study by researchers in North Carolina finds fungus-growing ambrosia beetles are often associated with rapid apple decline, but they do not appear to be a primary culprit driving the condition, and a variety of other factors are likely at play. Here, an orchard in Henderson County, North Carolina, exhibits rapid decline in the health of apple trees. (Photo courtesy of Sara Villani, Ph.D.)

By John P. Roche, Ph.D.

Several regions of the U.S. have observed rapid declines in the health of cultivated apple trees, a phenomenon known as rapid apple decline. Multiple factors may contribute to rapid apple decline in apple orchards, including infestations with insect pests. One pest of apple trees is ambrosia beetles, which dig gallery chambers in tree tissue, introduce symbiotic filamentous fungi, and introduce offspring that consume the fungi as food.

To examine the degree to which declining apple trees in North Carolina are infested with ambrosia beetles and associated filamentous fungi, a research team surveyed apple orchards in western North Carolina from 2017 to 2019. In an article published in September in the Journal of Economic Entomology, they conclude that, while ambrosia beetles might contribute to rapid apple decline, they are not the sole culprit, and onset of the condition is driven by a combination of biotic and abiotic factors.

Ambrosia beetles are a type of bark beetle in the subfamily Scolytinae. Two species of ambrosia beetle, the granulate ambrosia beetle (Xylosandrus crassiusculus) and Xylosandrus germanus, sometimes known as the black stem borer, have been pests of ornamental trees in North America for a long time. Recently, these species have also been discovered infesting fruit trees. Xylosandrus crassiusculus was identified infesting peach trees in South Carolina in 1985, and Xylosandrus germanus was found infesting apple trees in New York in 2015 and 2017 and in Michigan in 2016.

Xylosandrus germanus, sometimes known as the black stem borer was a dominant species of ambrosia beetle observed in the study. (Photo by Rachel Osborn, Michigan State University, Bugwood.org)

The current study was conducted by Sean Gresham, Ph.D., James Walgenbach, Ph.D., and Sara Villani, Ph.D., of North Carolina State University, along with Christopher Ranger, Ph.D., at the U.S. Department of Agriculture’s Agricultural Research Service. They evaluated 169 apple trees with rapid apple decline symptoms in four commercial orchards and one research orchard in western North Carolina in 2017. They extracted a total of 313 ambrosia beetles from infected trees, 282 of which they identified by species. Twenty-six percent were Xylosandrus crassiusculus, 34% were Xylosandrus germanus, and 31% were Xyleborinus saxesenii, sometimes known as the fruit-tree pin hole borer. The investigators also collected ambrosia beetles by placing traps baited with ethanol (which attracts ambrosia beetles) in commercial orchards from 2017 to 2019. They found that captures were primarily the same beetles that were found in the examination of infested apple trees: Xylosandrus crassiusculus, Xylosandrus germanus, and Xyleborinus saxesenii. These three species made up 99% of trapped ambrosia beetles in 2017, 84% of trapped beetles in 2018, and 93% of trapped beetles in 2019.

In North Carolina, Xylosandrus crassiusculus, Xylosandrus germanus, and Xyleborinus saxesenii were the predominant species attacking apple trees. “This is in contrast to similar surveys in New York and New England,” Villani says, “where Xylosandrus germanus (the black stem borer) has been observed to be the primary species recovered from apple orchards and in Georgia where Xylosandrus crassiusculus has been identified as the main species in fruit and nut orchards.”

Sara Villani, Ph.D. (left), and Sean Gresham, Ph.D. (right), examine an apple tree in an orchard in North Caroline, during he course of a study examing the role of ambrosia beetles in rapid apple decline. (Photo courtesy of Sara Villani, Ph.D.)

In sampling fungal species, Gresham and colleagues found 62 species from 31 different genera. Fungi from two genera were only found on gallery tissue: one genus was Ambrosiella, which is symbiotic with Xylosandrus germanus and Xylosandrus crassiusculus, and the second was Chaetomium. Species of Fusarium, Botryosphaeria, and Diaporthe fungi were often discovered in galleries; 67% of fungi collected were in these genera. “It’s important to note that there was no phytopathogen that was consistently recovered from trees symptomatic of rapid apple decline,” Villani says.

Ambrosia beetles are often associated with declining apple trees, but determining whether the beetles contribute to the decline is difficult, and the degree to which the ambrosia beetles Xylosandrus crassiusculus and Xylosandrus germanus might affect declining apple trees is not yet known. Additional work is needed to explore the effects of ambrosia beetles on apple trees.

Abiotic factors, including stress events such as cold, drought, and flooding, could contribute to rapid apple decline. In their study, Gresham and colleagues observed apple tree declines following stress from drought and rapid changes in temperature. To make it even more complex, abiotic stressors don’t only directly affect the health of trees—they also increase infestation by pests. Stressed trees release ethanol, which attracts attack by ambrosia beetles. Abiotic environmental factors were also found to facilitate infection by pathogenic fungi, such as Fusarium, Diaporthe, and Botryosphaeria.

Sentinel trees planted along the margin of an apple orchard are used to detect arrival of insect pests. (Photo courtesy of Sara Villani, Ph.D.)

“The causes of rapid (or sudden) apple decline are complicated and appear to be related to a combination of biotic and abiotic factors,” Villani says. “In western North Carolina, a combination of rapid temperature swings as trees were approaching dormancy, as well as when trees were coming out of dormancy in the early spring, likely increased tree vulnerability to opportunistic fungal phytopathogen colonization and symptom expression, as well as ambrosia beetle attack.”

“More broadly,” Villani says, “in regard to rapid apple decline, the challenge will be separating the variables of ambrosia beetles, associated phytopathogens, and stress events.”

Efforts to determine the effects of ambrosia beetles on declining apple trees face challenges. “In large scale field studies,” Villani says, “reproducing the exact type or degree of a stress event makes it difficult to study the impact of ambrosia beetles year after year.” But the current study provides an important step in determining the dominant species of ambrosia beetles present in four declining orchards in North Carolina and determining the most prevalent species of filamentous fungi. The researchers’ conclusion that in North Carolina, rapid apple decline is probably due to an interacting network of both biotic and abiotic stressors provides a useful road map to help inform future studies.

John P. Roche, Ph.D., is an author, biologist, and science writer with a Ph.D. and postdoctoral fellowship in the biological sciences and a dedication to creating compelling narratives for readers. He authors books and writes materials for universities, scientific societies, and publishers. Professional experience includes serving as a scientist and scientific writer at Indiana University, Boston College, and the UMass Chan Medical School; as a visiting professor at four tier-one schools; and as editor of periodicals at Indiana University and Boston College.