powdered sulfur works magic, I use it indoors as well.
it kills russet mites, spider mites, and all fungus growing on the leaf.
and its only 12 bucks for a couple pounds at english gardens
trick is to use 2-3 tablespoons per gl of warm water with some coco wet.
also once you hit them a couple times with the sulfur, its best to use some beneficials to boost the life back. the sulfur will kill damn near everything.
The sulphur and copper should keep it from spreading, that’s about all you can do once it’s there.
There are a few bacillus strains of bacteria that have been effective on fungus and mildew problems, Cease is one brand name that has it, but there are several.
It could also be bacterial leaf spot which is pretty common, need an antibiotic for that. Streptomycin sulfate is available, people use it on roses and fruit trees primarily for fire blight.
thanks guys