This is unfortunately common as lilacs mature. The soil around the roots becomes compacted making dry summer conditions even worse and the branch structure becomes congested, holding in humidity that promotes fungal disease. Luckily, it is usually possible to remedy both of these situations.

Normal annual pruning of a lilac involves cutting out some of the older stems from the base of the plant every year immediately after blooming. That keeps the plant vigorous and uncongested. It is too late to do that in the fall without losing flowers and risking loss of tender new growth. You can, however, work on improving the soil now and then make a more drastic, rejuvenation pruning in the late winter/ early spring. 

To rejuvenate an older, overgrown lilac, you have two options. The drastic measure is to cut the entire shrub back to eight inches from the ground in late winter (February/March). The lilac will resprout in the first year. Thin out sprouts to form a good structure.  It will take the lilac two to three years to regain some of its former glory and to flower. The more cautious approach is to cut out ⅓ of the old wood of the plant every year for three years. Thin out older wood to create good circulation and an open structure. Shorten any large branches that are drooping or top-heavy by cutting back to a strong side-shoot.  This gradual renovation pruning can take place either in late winter or immediately after flowering.

To improve soil conditions now, gently break up the surface soil around the shrub going out to beyond its full reach by about a foot. Remove any grass or other plants that grow in that zone. Then add some good quality organic compost 2 inches deep around the base of the plant. In the spring, feed the plant once with a balanced organic fertilizer. Take care to keep any lawn fertilizer from getting on the roots of this plant - it can keep the lilac from flowering. If there are other plants that have grown larger next to or above the lilac and now crowd the plant, prune them back too so that there is full sun and excellent air circulation around your lilac to reduce the likelihood of fungal disease. 

Next season, you can add more organic compost to the base of the plant in the spring and then give it supplemental water any week that there is less than an inch of rain. Lilacs are not especially affected by drought but when it gets really dry, we need to step in and help.

Courtesy of NYBG Plant Information Service