sick Fothergilla 'Gardenii'
My appoximately 4-year old Fothergilla 'Gardenii' has developed brown spots on the leaves. Some leaves have fallen off and some branches have died. I haven't seen any insect pests. The disease is slowly spreading through the entire plant. I have 2 adjacent Fothergilla 'Gardenii' bushes that are fine.
Your fothergilla does look like it is declining. However, the photos do not show a disease or insect problem. It could be a soil problem either too dry or too wet. Many trees and shrubs have suffered from the record-breaking amount of rainfall we had last season. It can take a long while before a shrub shows any negative effects. Things like being planted too deeply and too much mulch can also cause decline. They have a shallow root system so anything that impedes the roots can cause a problem.