The biggest problem with rexes is mildew. Even if you keep the leaves "dry" does not mean that mildew spores are not on the leaf surfaces. Don't you still get wet leaves from the dew?

Anyway, water is not the issue, it is mildew and other fungal organisms. Water may help them germinate but in my experience it is the cool temps that must be part of the trigger as well - I've never seen mildew in the summer, only fall. It seems like spraying the leaves off with a blast of water would help dislodge any dirt and spores. Perhaps a soapy mixture to begin with and then a rinse. I have been spraying all my begonias with Orchard Spray this year but not as religiously as I guess I should have. I have some ugly spots appearing on some rexes (mostly red leafed varieties) in recent weeks and that is due to the very cool temps starting in September.

As an aside, most of these leaves today will be long gone before next spring when the cycle begins anew.

Here is a pot of 3 red rexes (or degrees of red) the end of August.

And the same pot 4 days ago. It got its last spray the week before and the pot is indoors since yesterday. If I have room, I may try to propagate from a lot of those old leaves in the near future. The problem is what to do with all the excess I start and grow.

Another pot of Bashful Bandit and 3 rexes that I started from wedge cuttings back in late Feb.

July 24 (I moved them into this square pot from 3 inch pots 9 days earlier.

September 16

4 days ago.

This was 10/9 showing the other side of the pot.

You can see some spots on the Super Curl leaf half hidden by the most forward leaf of Bashful Bandit. I could cut that leaf off but for now I am going to let it there as I may use it for propagation.
PS I put all my plants out in the open air since I don't have enough space to keep them away from rain and usually when I water I water pots if they are wilted and usually give them a good overhead gentle spray as well.

All these photos below are from four days ago (the last time I took pictures)

Escargot showing some of the better leaves. Escargot is infamous for mildew magnet. I hope this year it will make it through winter.

Beautiful!

Have you ever tried neem for mildew, on rexes?

Yes, I've used Neem oil - it's good but smelly. I've used Milstop as well. I hate spraying indoors but it is that or nothing. Neem just lingers for days after spraying it. Spraying under the leaves and getting most of the stems is a big problem as I am not going to move any plants for photos or spraying - just too many to do that.