Any ideas?

One of my Japanese maples in training has been suffering recently. It’s a Sango Kaku, famous for their normally pink bark and bright green leaves. Any ideas?

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Discoloration of bark – beginning of dieback?

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Note the withering of leaves and unusual colour of sections of trunk.

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Extreme windburn?

I wonder if it’s a combination of things? It’s been in semi shade and well watered all year but never really took off in spring.  Not much extension growth despite fertiliser being used and in the past couple of weeks the foliage has withered – I also noticed a white discoloration similar to mildew??? I’m familiar with mildew on my oaks and field maples but never my Japanese maples.  I’ve sprayed it anyway and put the tree in the ground to help it recover. Fingers crossed.