Dear Sarah, 

Thank you for contacting Ask Extension about your lilac. I'm going to assume that the entire tree exhibits the wilting shown in your photos. Please let me know if that is incorrect.

The wilting of the entire tree could be caused by environmental factors like drought, however, that is not likely in this case. The other possibility is chemical damage. Do you recall using any sort of chemicals in the area of the lilac? Airborne herbicides may have drifted from a neighbor, or a chemical used on the lawn or surrounding plants may migrate through the soil to injure a tree. This can happen with dusts, mists, liquids, and granular applications.

The ability of a tree to recover from chemical injury depends on the chemical involved, the manner of uptake, growth rate of the tree, temperature, and humidity. Ironically, flushing the soil with water can actually worsen the damage caused by some chemicals. The only thing I can suggest is that you add a layer of mature compost and an organic mulch, then wait to see if the tree recovers. Sometimes a plant will lose all of it's leaves but is still able to regrow during the season. 

If you are able to identify the source and type of possible contamination, please respond to this email and we may be able to advise you further. 

Dear Sarah,

I will admit I am quite at a loss. There are no pests or diseases that would cause that kind of wilting, which leaves us with something in the environment... weather, chemicals, or perhaps mechanical damage to the roots. Without a lab analysis, it is not possible to confirm the chemical theory. 

So, I would suggest compost and mulch. Do not water more than once per week to avoid root rot and shallow roots. I wish we could provide more information. Good luck with your shrub.