I purchased two Southern Living Dear Delores repeat bloomer hydrangeas several months ago. They have bloomed beautifully over the past month. I came back from vacation and noticed a thick brown and white powdery substance below the blooms on the stems. It was only on one of the hydrangeas. Underweath the white powdery substance is a brown mud-like gunk. I am completely clueless as to what it is. Any ideas?