Posted: 13/09/2015 at 11:27
Powdery mildews are host specific - however because it often strikes when plants have been weakened by a cultural problem, it can appear to spread.
Honeysuckle often succumbs to it's own Powdery Mildew when it is suffering from dryness at the roots.
If the amelanchier is also a bit dry at the roots maybe that's why it's own form of mildew has struck? I wouldn't worry about it doing too much harm to the tree at this time of year, the leaves have served their purpose and will soon be falling. I'd gather them up when they fall, and burn them.