indoor plant disease
Thank you for your question. First, we believe your beautiful plant to be a Clivia (https://www.chicagobotanic.org/plantinfo/clivia), also called a bush lily (although they are related to amaryllis). The spot(s) you see may be of fungal or bacterial origin, but its shape militates toward a fungal infection. I suggest you remove the leaf, and take it to your county Extension office (https://extension.oregonstate.edu/lincoln) where staff can examine it under a microscope. Good luck!