Start by giving the plant a shower, washing both the top and bottom of the leaves. I would take a sample of the infected leaf to a garden center that sells begonias. If it is mildew, the spots would have a white powder like substance on them, you will need to spray them with a fungicide labeled for use on house plants. Best And Happy Yardening, Nancy.
Begonia Leaves Have Spots That Look Like Mold Or Mildew
Question From: M. Anderson - NEW YORK
Q: Help! My begonias which I brought in last month have been looking great but then today they have what appear to be spots on the leaves that look like mildew or spots of mold that are from dime size to half dollar size. What can I do?