Overwatering your lilac leads to waterlogged soil and suffocation of the roots. This causes yellowing and wilting of leaves due to root rot. It also causes blackening of roots, stunted growth as the plant struggles to absorb nutrients and foul odor emanating from the soil due to anaerobic conditions.

If the lilac’s roots are in waterlogged soil, this promotes the conditions for fungal disease pathogens, which prevent the lilacs' roots from taking up moisture and nutrients, causing the leaves to turn yellow or brown with a dying appearance.

The reason for your lilac dying is usually because the soil drains too slowly for the lilac to tolerate. Lilacs require well-draining soil, and if the soil is waterlogged around the roots, this promotes the conditions for root rot and Verticillium wilt, which causes the leaves to turn brown and drop off with a dying appearance.

Lilacs thrive in well-draining, moist soil but will show signs of wilting if the soil becomes waterlogged and soggy for long periods of time. Many times homeowners over water the area where lilacs grow, believing more water will increase growth and health.

If your lilac bush is shedding brown leaves too soon, don't despair. This could be a sign of lilac blight, a common bacterial disease that spreads through wind and rain. Watch for buds may blacken and brown spots on the leaves that can later grow into sizable blight areas that extend to the stems and produce canker sores.