Azalea stress

Hi all,

There are some deciduous azaleas in a client's garden that started showing discolouration in June. Now they have severe powdery mildew. They're in varying degrees of part shade in the garden, and are not over or under watered as far as I know. The garden is below a giant spruce tree. Vaccinium ovatum is also in the garden and took awhile to get established. Could it be soil pH?
She may also have fertilized, I'm just checking.
Any ideas?

Thanks!

The garden was planted in spring of 2018.