By Erik Stokstad
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.