My serviceberry has developed a yellowish powder on the undersides of the leaves, and those leaves yellow and drop off. What is this and what can I do about it?
Serviceberries are subject to mildew and to cedar rust, both fungal diseases. Although normally downy mildew is more white than yellow, in this case, I think it is more likely the culprit in your case, since the cedar rust starts as a very orange powder then produces quite ugly orange / brown protruding growths (spore cases), primarily on the berries. In both cases, fungicide treatments can be effective if the disease is caught early.
For more information please see this past question: