Answer: Stress - either environmental or cultural - can cause the leaf problems. Weather, watering practices, fertilizing and even failure of the roots to establish can all contribute to the symptoms you describe. I'd give them another year to become established. Don't prune either until after they have finished flowering next year. If they don't flower, it would be safe to prune them both in early June. Pruning will encourage healthy new growth which should bloom the following spring.
Water stress should be avoided this year. To make sure your crabapple and lilac get enough, but not too much water, make a watering well or water basin beneath each (mound up 2-3 inches of soil, in a circle, about 12" out from the main trunk or stem). Fill this basin with water, allow to drain, then fill a second time. Do this once each week. Watering in this way concentrates the moisture directly over the root mass and allows it to trickle down and wet the entire root system.
Best wishes with your landscape!
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