I have a fairly young 'Annabelle' hydrangea in my front yard. Today, I noticed with much astonishment that practically all of the leaves on the plant had disappeared. The very few that remained were browning at the edges and slightly wilted. The reason why I hadn't noticed this earlier was because the plant still has very large, healthy looking blooms and from far away the leaves (or lack thereof) are hard to notice up against the foliage of other plants. It's essentially long, bare stems with flowers at the end. I have also noticed that the stems have become significantly more "woodier" with the outer layer of bark rather brittle. About two meters away is another hydrangea plant that is doing wonderfully which leads me to suspect some kind of disease or pest. Also - we have had a recent spell of extreme heat (the other hydrangea was unaffected). What could be the issue?