I have two rhododendrons that were planted fall 2015. catawbiense grandiflorum. They were doing fine until this summer when branches started dying all the way back to the ground. The plants seem to continue to grow, but then another branch dies. I have been cutting off the dead branches. Should I fertilize them with Holly Tone, or spray them and with what or what else should be done? Also, the plants are in the drip line of the roof of a shed so they get lots of water when it rains. Do I need to put up rain pipes? I have pictures, but I wasn't able to upload them.