Question

Lilac bushes   We are in northeast Ohio and have a 5 y/o lilac that has an abundance of flowers, but very sparse leaves.  It is shown in a picture next to two other trees that look normal.  The tree itself looks healthy, including the flowers and what few leaves there are.  I have only been able to find info on bushes with leaves and no flowers.  Mine appears just the opposite.  We do not want to lose this tree.  Could you provide us with any information and direction?  -thanks

Answer
Ok, lets begin with a few questions.  

Some I think are obvious answers, but difficult to tell for sure from the photo.

Keep in mind that lilac flowers actually bud the year before, so those buds on both plants would have begun last year.

It is quite likely there are no insect issues, as it is impacting only one tree.  Neither appear to have any suckers growing, which is also good.

Do the leaves have any visible blotches or spotting on them at all, or are they all normal looking healthy looking leaves.  From your statement, it sounds like the leaves it has are healthy looking.

If you take your finger and lightly scratch the bark on the trunk, is it green and healthy looking beneath the surface?

Is there any damage to the trunk anywhere, bark torn off, anything at all.  Look right down to the ground area.

It sounds like a root related issue, but at this point trying to find additional information.

There has been no digging or any work around the root area at all?

When was the last time these were pruned?  (note any pruning should be done immediately after flowers drop off but before new buds set)

Have you had any other trees, fruit trees or any others which have become ill or died in the past recent years?  Especially ones fairly close to this one.

Is there anything you can think of that is different with this tree than the other one, fertilizer, insecticide treatment, pesticides?  Anything at all, no matter how unrelated it may seem.  Something is different, and the photo is not giving all we need.

Do you remember any mushrooms growing around the bottom of the tree last fall?  (late summer to fall timeframe - golden honey colored)

Is there any signs of branches dying.  Do you see any liquid that is coming from bark near the lower part of trunk.

I am not certain at this point, but it is possible that we have Armillaria mellea also known as Armillaria root rot or Honey Fungus.

You will see listed here.  3rd one down.  This shows as new in Ohio, but was 15 years ago.  Lilac is very susceptible to this fungus.

http://ohioline.osu.edu/sc165/sc165_9.html

Here is some additional information on Armillaria mellea.

http://ipm.illinois.edu/landturf/diseases/armillaria/