The most common causes of yellowing leaves on ornamental trees and shrubs such as lilac are overwatering or poorly draining soils and an iron deficiency called iron chlorosis. Overwatering results in yellowing, then browning leaves and leaf drop. Chlorosis, caused by a lack of iron in the soil or alkaline soil that blocks the uptake of the iron in the soil, results in yellow leaves with faintly green veins and then completely yellow or whitish leaves. Each can be tested for and readily remedied.
Dig down 12 inches in the soil 2 feet out from the trunk of your lilac. Feel the soil for wetness that is beyond moist and to see if there is pooling of water in your hole. If the soil is very wet and you have not watered and it has not rained in the last few days, the tree is likely overwatered. When the roots are forced to sit in pooled water, they suffocate.
Allow the soil to dry out a bit at least 8 to 10 inches down before watering deeply and refrain from frequent shallow watering sessions. Allow the surface soil to dry out again before watering and do not water for several days to a week after a substantial rain.
Topdress the soil around your lilac with iron sulfate when the soil is alkaline and has a pH of more than 7.0 to treat chlorosis. Cast the amendment over the soil in the dose recommended on the package label for your size of lilac and planting area and water in well. Repeat as recommended and needed, which may be as frequently as every one to three years.