Pruning lilacs after they have finished blooming can help them bloom more and resist disease, according to Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle. The time to prune lilacs is after they finish blooming and will result in them becoming tangled, overgrown, and choked with deadwood. To reduce the overall height of a lilac, target the tallest stems for elimination and remove outlying suckers. Plan to do this every year. With the dead wood and oldest stems removed, air will be able to circulate, preventing powdery mildew and other fungal diseases.