What's killing so many young apple trees?

High-density, trellised apple trees
(Photo by Melissa Dobernigg)

Apples are one of the most valuable fruit crops in North America, worth about $4 billion in the U.S. last year alone. That's why scientists and growers are working so hard to figure out why alarming numbers of young apple trees across the U.S. and Canada are rapidly dying off, Erik Stokstad reports for Science.

The phenomenon is widespread -- about 80 percent of North Carolina orchards show suspicious symptoms, for example. Penn State plant pathologist Kari Peter started hearing about it about six years ago, and told Stokstad it happens with startling speed: "Rows of trees collapse for what seems like no reason."

Weather-related stress from drought and/or severe cold could be one cause, especially since the eastern U.S. is seeing more early freezes, but pests, diseases, and the increasing use of high-density, trellised orchards could also be factors, Stokstad reports.

Evidence supporting the severe-cold hypothesis: "One common symptom in trees struck by rapid decline is dead tissue at the graft union, the part of the trunk where the fruit-bearing budwood of an apple variety is joined to hardy rootstock to create new trees," Stokstad reports. "The union is vulnerable to late-season freezes because the tissue is the last to go dormant." However, British Columbia has experienced the problem too, and that region has had several unusually mild winters. And since its orchards are irrigated, drought wasn't likely a factor.

Another theory: certain rootstocks or herbicide exposure might make trees more susceptible. Decline seems more common with trees from a popular rootstock that can be slower to go dormant in the fall. And one scientist noticed that decline seems more common in orchards with fewer weeds, which could point to herbicides, Stokstad reports.

Pests could be a problem too: "In hard-hit North Carolina, researchers have found ambrosia beetles infesting the graft union of dying trees. These stubby insects burrow into weakened trees and cultivate fungus for their larvae to eat. Those fungi or stowaway fungi might harm the trees," Stokstad reports. Researchers will begin testing that theory in June, and will also try to figure out if they can improve trees' immune systems.

Growing apple trees in dense, trellised rows could also be part of the problem. Farmers plant trees like that to get a higher yield per acre, but it results in trees with shallower roots that are more vulnerable to drought, Stokstad reports.