Six years ago, an unpleasant surprise greeted plant pathologist Kari Peter as she inspected a research orchard in Pennsylvania. Young apple trees were dying—and rapidly. At first, she suspected a common pathogen, but chemical treatments didn't help. The next year, she began to hear reports of sudden deaths from across the United States and Canada. In North Carolina, up to 80% of orchards have shown suspicious symptoms. "Rows of trees collapse for what seems like no reason," says Peter, who works at the Pennsylvania State University Fruit Research and Extension Center in Biglerville.
Now, as their trees prepare to blossom, North America's apple producers are bracing for new losses, and scientists are probing possible causes. Apples are one of the continent's most valuable fruit crops, worth some $4 billion last year in the United States alone. Growers are eager to understand whether rapid or sudden apple decline, as it is known, poses a serious new threat to the industry.
Weather-related stress—drought and severe cold—could be an underlying cause, researchers reported this month in PLOS ONE. Early freezes are becoming more common across the eastern United States, for example. But that doesn't appear to be the whole story, and scientists are examining an array of other factors, including pests, pathogens, and the growing use of high-density orchards. "There are a number of things going on that are going to be really difficult to sort out," says David Rosenberger, a retired plant pathologist who worked at Cornell University.
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. The union is vulnerable to late-season freezes because the tissue is the last to go dormant.
A team led by plant pathologist Awais Khan of Cornell found dead tissue just below the graft union in trees from an affected orchard in New York. They suspect the cause was the extremely cold winter of 2014–15, which was followed by a drought. The dying tissue could have weakened the trees, allowing pests or pathogens to invade. But Khan and colleagues could not locate any known culprits in the affected trees or nearby soil, they reported in PLOS ONE.
Observations from other apple-growing regions suggest extreme weather isn't entirely to blame. In Canada, rapid decline "exploded" in British Columbia in the summer of 2018, after a string of unusually mild winters, says Tom Forge, a soil ecologist with Agriculture and Agri-Food Canada in Summerland. These orchards are irrigated, suggesting drought was not a factor.
Some scientists wonder whether certain rootstocks or exposure to herbicides might make trees more susceptible. Decline seems to be more common in trees with a popular rootstock, called M9, which can be slower to go dormant in fall, Peter says. Rosenberger has noticed that decline appears to be more common in orchards with fewer weeds, leading him to suspect herbicides play a role.
Meanwhile, the search for new pathogens is accelerating. Last year, a team that included Peter and plant pathologist Ruhui Li of the U.S. Department of Agriculture in Beltsville, Maryland, reported in Virology Journal that they had found a previously undescribed luteovirus infecting dead trees. Scott Harper, a plant pathologist at Washington State University in Prosser, has also found undescribed viruses in dead trees. Li's group has already infected young trees to see whether its virus is harmful, and Harper is planning similar greenhouse experiments. But getting an answer could take up to 5 years. "In the meantime, people are biting their nails," Rosenberger says.
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, an idea that Sara Villani, a plant pathologist at North Carolina State University in Mills River, and colleagues, will start to test in June. Researchers there will also test way of boosting the trees' immune systems.
Modern apple farming methods could also be a factor. Rapid decline is most common in dense orchards, which are increasingly planted because they are efficient to manage. Instead of about 250 trees per hectare, high-density orchards can have 1200 or more. Tightly packed trees must compete for nutrition and moisture. They also have shallow roots, which make them easier to trellis but more vulnerable to drought. "I'm not criticizing the system," Khan says, "but it's not robust for these kinds of fluctuations."
As studies proceed, researchers remain vigilant. "It wouldn't surprise me," Villani says, "if we get more reports of apple decline."