leaf mildew on fothergilla?
Fothergilla are bullet-proof in our experience.
We suspect (since you have shade trees on your lot) that what you are seeing is sooty mold, which is not a disease or insect ON the fothergilla. This is actually a result of being under the shade trees where insects such as aphid or scale are excreting 'honeydew' onto the plants below. Honedew is slightly sweet and clear, so we don't notice it much. Rain washes it off, but if it hangs around for a while, then a fungus starts to grow in it. The fungus is black and known as sooty mold. Sooty mold makes leaves look sort of dirty (sooty!)
Here is more about this common situation (with a photo of sooty mold if you scroll down): https://extension.umd.edu/hgic/topics/soft-scales-trees-and-shrubs
It does not warrant any treatment.
Since you are seeing it only on old leaves, whatever was excreting the honeydew is probably long gone by now.