Answer: Since they are close together we'll assume the plants were originally of the same type and quality and the growing conditions are similar (same amount of sun, no root competition from nearby trees, no difference in surface drainage, same amount of air circulation, same type of soil) and they have been treated the same way. There is a possibility that the plant is suffering from some type of insect problem such as borer or a disease, possibly a fungal infection or root rot. You mgiht inspect it closely to see if you can find any irregularity. The remaining possibility is that for some reason the roots have failed to take hold and grow through the native soil, thus causing the plant to be stunted. This can be due to encircling roots, poor soil preparation, or in some cases "just happens". To stimulate additional top growth you can cut the plant to the ground now in spring, and see if it regenerates, but if there is an underlying problem it won't help much. You could also dig it up and inspect the roots, possibly untangle them and reprepare the planting area and try replanting it. Finally, you might want to replace it either with a newly purchased plant or with a cutting or sucker taken from the more vigorous plant next to it. In a few years it will catch up to the parent and look about the same. The mildew, while unsightly, is not usually enough to kill a lilac. It can be reduced in some cases by pruning to allow for better air circulation and avoiding wetting the foliage when watering, particularly at night.
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