I am thinking that they could be some sort of fungal growth. They seemed almost powdery when I first inspected them. I live in a very cool and humid environment that molds and fungus really love (northern coastal California), but these spots only just started appearing a week or two ago. Any and all help is appreciated, especially for my silver spot begonia (pictures 3 and 4), she has such massive beautiful leaves that I would love to save!
Just to add, I did peruse the ABS website and couldn't find anything definitive which is why I'm posting here. Apologies if this same question is asked frequently!
by Pleopeltis_