It doesn't look like Powdery Mildew, which can impact apples considerably.
To me, that looks very similar to Leaf Scorch, maybe even from incident light that is too intense. Are these seedlings in a greenhouse, or anywhere that the sun's light could be reflected/refracted into a more concentrated area [right onto those leaves]?
Maybe try turning the plants, or changing their locations if there is a chance that it is the incoming light.
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