Thank you for your question and for sending pictures of your lilac shrub.

We are seeing damage to shrubs from last November's sudden hard freeze. The pictures do not show the typical signs of any lilac diseases or insect damage. The stunted growth and sparsely growing areas can also be from the frost. Some of the new growth does show flower buds forming. You can prune this area now or wait to see if the flowers continue to bud and bloom, then prune after flowering. I wouldn't give up on the shrub just yet, but try to prune for a more pleasing shape and size. Be sure to give it adequate water this spring and summer. The cooler temperatures and rain we have had this week will help with moisture. Watch the shrub later this spring and summer for any additional problems with insects and/or any signs that the leaves have spots or damage; stressed plants are more susceptible to other problems. If you see something that concerns you please send us additional information and pictures.

Good luck with your lilac.