This fall the leaves on both of my rhododendrons are turning from yellow to brown and black with holes in centers. The edges also have gray powder looking stuff on leaves. These plants are about 10 years old and were transplanted a year ago in the fall.
Certified GKH Gardening Expert
It sounds like a sooty mold problem, brought about by insects. Neem oil is an effective treatment. Here is more information: https://www.gardeningknowhow.com/ornamental/shrubs/rhododendron/rhododendron-sooty-mold.htm