Is your soil heavy or light? Too heavy a soil and over watered can cause root rot.

Older leaves, or new growth? Probably dryness, and the tree shedding what it doesn't need. You've had enough rain up there, I believe, but it has been dry, generally this year, and plants have struggled. When you water, is it by the bucketful, which it should be, or a can or two? Always better to water properly (4 gal per sq metre), and leave for a few days or more before repeating, than to water little a day which quickly evaporates.
Phil J