The why is still very much unknown. The what is painfully clear: Something is rapidly killing apple trees that initially appear to be perfectly healthy. That rapid part is now part of the syndrome's name: rapid apple decline (RAD), with one apple farmer telling the Counter that some of his affected trees went from healthy to dead in two weeks. Modern Farmer reports RAD is a relatively new phenomenon, first detected in Pennsylvania in 2013. Kari Peter, a tree-fruit pathologist at the Penn State Fruit Research and Extension Center, explains the trajectory: "Trees look healthy at the start of the season, look a little off in late July or early August, have a full crop and then boom, they're dead." Peter is working with the Pennsylvania Department of Agriculture to try to get to the root of the problem, and it's possible the roots are involved.
Farmed apple trees start out as a rootstock that is grafted to the scion, a stem or branch clipped from a tree that will becoming the fruit-bearing part. Peter says trees that were grafted on M.9 rootstock—a "popular" choice that has "been around forever"—and in high-density orchards seem to be the most impacted. In those orchards, as many as 2,000 trees can occupy one acre, compared to the 200 to 250 that commercial apple growers used to plant there. It's possible that proximity is leading to shortages in nutrients and water for some trees. As one farmer tells the Counter, in one row "there would be two [dead trees] here, and then five perfectly healthy trees, and then a strip of 18 trees that were all dying or dead." Peter explains in a piece for Penn State the rootstock stays healthy; it's the scion that dies, with necrosis appearing at the graft union and moving up the tree. A three-year federally funded study into RAD is just now getting underway. (Read more apple stories.)