Question
I have two lilac bushes I planted last fall in southern Indiana, and they seemed to be doing very well until late this summer.  The leaves have a moldy, dusty-looking appearance on the tops and some of the limbs are dying off.  It was a fairly mature bush when I planted it - four feet tall.

Answer
Jan,
The dusty-looking stuff is powdery mildew - all lilacs are prone to this, and it seldom harms the plant, but is a cosmetic problem only.  The die-back you're seeing on the tops is probably unrelated.  Did you have a good stretch of HOT weather in Indiana this summer?  If so, my guess is that the lilacs, being recently planted, don't yet have the root system to make it through such a hot spell, especially if they might have dried out at one point or another.

At this point, here is what I'd do if this were my plant: Rake up all fallen leaves in the fall so that fewer mildew spores over-winter in the soil.  If you don't like how the mildewed leaves look, next year as soon as the plant finishes flowering start spraying with a product called "Green Cure"  - if you local garden center doesn't stock it, you can get it online at: http://www.greencure.net/where_to_buy.asp  This product is approved for organic gardening (least-toxic fungicide) and works really well.

As far as the dieback, leave all those branches right now - water the shrub deeply once a week if it doesn't rain at least an inch (water by letting hose trickle at base of the plant for a half hour) and do not fertilize.  (By the way, fertilizer burn causes a plant to die back from the top - if you gave it a bit too much, this might be the cause.)  Next spring you'll be able to see what's living and what's dead once the leaves grow - at that time clip off anything that didn't make it, and I'm sure the plant will re-grow just fine.  It may not bloom as much next year if there is substantial die-back, but it will grow and blooom the following year.

Lilacs don't need a lot of fertilization - an organic product once every two or three years is usually enough.  They like growing in a neutral pH soil, so an annual application of lime or woodashes is good if you have acidic soils.  (Have your soil pH checked at your local cooperative extension.)  

I hope this helps!
C.L.
www.gardenlady.com