Mildew. Spray with Copper sulphtae. This is allowed (or was) under organic growing rules. In any case it does not harm any beneficial insects. Or you could try 1 part full cream milk to 9 parts water as a spray. Never used that one myself but it has been recommended by others. Mildew on apple trees is a serious illness. Sadly more often than not the cure takes a long while.

If copper sulfate is no longer allowed, look for copper soap. At least, that's the only copper product still allowed in the USA. Another possibility to look for is potassium bicarbonate, especially since copper can damage some breeds of apple.

Funny enough, it is the sulphur which is the most active fungicide in the mix. I do know they still sell Bordeaux mixture here which is basically the same thing.
Again some apple trees are sulphur shy.
Trouble is the only way to get rid of apple mildew is to know exactly how and when to prune out the affected ashoots before thespores on the mildew are ripe enough to spread to the rest of the tree and it is very hard to describe in words what the shoots look like at that stage and how to prune them.

Huh! I didn't think of the sulphur being the main actor! I do know that copper soap works fine on rose mildew--not many apple trees here, and they never get mildew here (touch wood) anyway. Bordeaux mix is, alas, also illegal here.

thanks to users who replied... another mystery solved!!