Dusty and spotted crabapple tree leaves
It sounds as if the tree may have Botrysphaeria, a fungus which produces the kind of dusty appearance and spots you describe. You say the tree is small, so applying a fungicide such as captan or thiophanate-methyl, which are effective if applied early and at regular intervals throughout the season, might be the way to treat this. The other recommended treatment is to prune the infected branches and twigs and completely remove all of the debris, but you do not seem to indicate that you see any signs of infestation on the anything but the leaves. I would check withthe Delaware County Cooperative Extension Service, 20 Papermill Rd., Springfield, 610-690-2655. DelawareExt@psu.edu to confirm that this is what is infecting the tree.