Can't directly answer your question, om. But one thing I thought of while reading is to ask you, what is the pH of your water (If you happen to know)? In some cases, plants like river birch, which have a decided preference for acidic soil conditions, can be harmed by high-pH water. Just a thought, and not really pointed at those dead leaves. +oM

Have you checked that branch where the dead starts? We do have what they call a twig girder. They will make a cut around a branch and lay eggs on the branch. If it is you will see the bark removed in a ring where the branch starts to die. I do not know if they are in your area but here in Texas they are well known to many.

That's a tough one to address without a picture. My first inclination is to say, plant it, WATER it and keep watering it because I think the garden specialist might be right. Sounds like neglect...too much sun without enough water. Since Endless Summer blooms on new wood, it should rebound fairly quickly. Remove and dispose of the unsightly leaves and cut off any dead stems down to (just above)a healthy looking set of leaf buds, but first make sure they are actually dead by scraping the 'bark' with your fingernail. If there is green underneath, leave the stem alone as it is alive and may yet produce some buds. If it is dead all the way down, then cut the whole stem right down to the crown. Many hydrangeas have brownish/reddish speckles on the stems as a natural characteristic so I wouldn't worry about that. For $2.00, it's worth a try as a new one can cost up to $30.00 or so. I think it might pleasantly surprise you, but if you could post a picture, you would be able to get a much better analysis.....yg

Lynn, I've seen pictures on the web of sunburned leaves...sounds similar to what you have. And given the temperatures we've had lately, and the fact that the problem came up at the same time, this is what I'd suspect. I know you're internet search-weary by now, but try googling "sunburned leaves" and see what you find. Good luck! Lynne