Post additional information: what is your zone and where are you located; when did the problem start; have you amended the soil recently with acidifying compounds (too much can burn the roots and can impact the leaves); have you recently added fertilizers and how much (this causes fertilizer burn on the leaves); have you sprayed the plant or nearby ones during the last 2 weeks (fungicides and insecticides can cause all kinds of leaf problems); what is the soil pH where you live (just check to make sure it is not too high or too low for safety's sake); is the soil currently dry or wet? is it usually wet?; how has the weather been during the last 2 weeks; etc.

Leaf disease info:

http://www.aces.edu/pubs/docs/A/ANR-1212/ANR-1212.pdf

Thanks for the feedback, really appreciate the help. We live in southeastern OH, zone 6a. This is a newly planted limelight tree that we purchased from a local nursery going out of business. We noticed it was very dry when we purchased it so we watered the soil thoroughly when we planted. Actually, at first it occurred to me that the leaf problem might be from lack of watering, but have discarded that theory. It has rained hard every day since we planted it. It has been in the ground one week. We have around 15 - 20 hydrangeas, and I've never seen this issue before with any of them. Yesterday, we decided it was a fungal issue, caught a break in the weather, & sprayed with Ortho Systemic Rose & Flower Insect & Disease Control. If it is a fungal problem, do you think that will help? By the way, it's not bottom leaves, just scattered leaves throughout the tree. No soil amendments or fertilizer have been used.

Some nursery stock has fungal issues. Indeed all you might be able to do is fight it, as in removing the damage and treating the healthy parts. Next year should be a "hard reset" after winter. Either way, you'll probably still get nice blooms this year, minus some leaves.

Thanks, Springwood. I have been so exited about that tree. The leaf issue took the wind out of my sails a little.

Stick your finger well into the root ball to be sure it feels moist. Even with a lot of rain, if the medium was peat-based and thoroughly dry, it might still be dry since dry peat doesn't rewet readily, and there may not be good water movement between OH soil and the potting soil due to textural differences. Since the roots are still in the original media if it is dry, the roots are dry. If that is the case, I would dig it up since it is newly planted and plunge it into a bucket until bubbles stop coming out of the root ball and then replant. Mulch well (but not right against the stem) so it doesn't dry out again and keep close attention to moisture levels in the soil.

If when you stick your finger into the root ball it is moist, you can ignore all this. It might just be damage from the original dryness showing now, depending on how long it's been since you planted it.

Great idea, thanks! Checked this morning, root ball is moist. We have been doing some supplemental watering. It really does make sense that we could be dealing with damage from the original dryness. Thanks for the advice! Will continue to monitor closely. It's in ICU. :)

I'd say its in the progressive ICU and not in the AICU or SICU :)
I just wanted to mention that the transplanting process could have added some stressed on top of the dryness you mentioned, which may have caused the LL to brown or drop stressed leaves. I agree w/ SPringwood that next year it should be established and ready to grow strong.

Thanks for the encouragement! Hoping for the best!