Hi there, I live in San Francisco and planted a fuschia from South America. It is not doing well and I noticed some black spot under the leaves which eventually turned silver then die. 3/4 of the leaves are now gone. I could not see the rust color that your site is referencing to about Fuschia rust and I could not see the symptoms of Botrytis that you mentioned as well in the list of diseases. Here are some photos and maybe you could help me diagnose the problem and suggest a remediation plan. Thank you.
That appears to be spider mite damage, which is common with non-native plants. There are several things you can do to stop this including: Neem oil, garlic oil, lavender oil, rosemary, and many others. This link will help you: https://www.gardeningknowhow.com/plant-problems/pests/insects/two-spotted-spider-mite-control.htm