Factors appear to be drought and early freezes, and may involve new viruses and pathogens. The practice of high-density planting is also blamed. Trees on M9 have been the worst hit. Reports of sudden deaths have come from across the United States and Canada. Ours is not a high density orchard, and we have very little on M9, but it is worrying news.
Thanks for the link to the article in Science. Lots of research and discussion about the Rapid decline of dwarf apple trees in commercial orchards, sometime called RAD or SAD. The research done in my state (NC) points to a relationship between RAD and Ambrosia beetles but fails to conclude that Ambrosia beetles are the cause for the disease. From what I understand, apple trees under stress produce ethanol which attracts the beetles. Many potential causes for the stress on the trees and the researches continue to investigate the problem.
According to this interview it looks like they are not convinced that any particular virus is responsible. They gloss over the subject by saying that several viruses have been detected but none are proven to be the cause. I’m just glad to not be affected so far.
It’s true that there are 3 deleterious viruses found in apple orchards east of the Rockies and in Europe. It’s also true that beetles may be a vector in the easternmost U.S. states.
Of the 3 viruses, one is (now) known to cause loss of branch strength and the second to cause leaf dysfunction. In plant pathology publications this year, the third (ALV-1) has been identified as causing loss of metabolism in Malus domestica followed by rapid decline.