You might have some mildew but hard to tell with the photos you posted. I see a lot of mechanical damage (broken leaves, scratches, etc) that are invitations for germs to invade open wounds.

I noticed on GW that pictures shot in portrait mode do not enlarge when I click on the photo, but usually are a smaller version than the image embedded. If the picture is in landscape mode though, the enlargement is bigger. Weird, huh?

I would remove leaves with a lot of damage. The stems are what you should be more concerned with.

Here is a plant I grew from maculata seeds many years ago.

Here it is 3 days ago in the backyard with filtered sunlight. Portrait mode to get the full height of the plant.

Landscape mode with a bigger begonia next to it and bracken ferns that are popping up all over the area.

Here is the same plant in December last year. What I don't have is photos of all the leaves I removed due to heavy mildew infestation. I sprayed it twice with Orchard spray to save the stems. I had to cut off two blackened stems due to mildew.

Hey hc mcdole, thank you so much! Your begonias look amazing!

I’ve been planning to cut the damaged leaves but I’m not sure if the plant will continue growing with just the small, new leaves? Maybe 3/4 new leaves, the older ones were pretty badly damaged during transport. :(

Also, do you do any proactive treatment for powdery mildew? I cleaned off the leaves a while ago and didn’t feel the... powdery stuff lol the stains came off easily (dried water stains)

I sometimes trim all the leaves off (many) different plants if they are bad looking. The plant has to put out new growth if it wants to live. The reason most plants are easy (semi-easy) to propagate is their ability to put out new growth if they get broken, eaten, mowed, or dumped on.

Indoors for winter, I really cut back on watering which seems to help a lot on mildew outbreaks. Being in a basement is conducive for mildew (damp, cool) but most come through winter okay. More light would help but my spaces are crowded so most of the canes go on the floor near a stand with the lights. Spraying with a fungicide will help tremendously but I really hate to spray if I don't have to. The mildew got so bad on my maculata (hybrids) that I had to do something as a lot of leaves had already fallen and I picked the rest of them off if they even had a smidgen of mildew on them.

PS: I find the maculata hybrids to be more susceptible to mildew than other types of canes.

This parviflora had so many damaged leaves that I cut/pinched most off May 23rd after putting it in a larger pot (I want to get a big plant again)

This is what it looked like July 18th with a few pinholes either from insects or falling debris/maybe hard rain.

This is what it looked like when I had it in a huge pot with other canes back in October, 2015.