Acer - Pink Wilson - Leaf Drop/Mildew

I planted back in May into the ground near another acer that was in the ground.

I gave it some john innes no3 and used organic manure mulch to top dress.

The area is fairly well sheltered from high winds and is in partial sun (about half the day)

Last month i noticed there was a slight hint of mildew on the leaves. This seemed to fade afer a while, but now the leaves are all dropping off. The other acer which is possibly 1 meter away (and has been there for a considerable time) is doing really well.

Any ideas what might have gone wrong?