I hope this works

Here is a link that might be useful:

Saw the pic. over in the garden clinic forum. Without seeing the foliage up close, it's kinda hard to tell what the problem is. Too much water? Mildew? Try posting a pic. with a better shot of the leaves, if you can.

ummm, is the planting material in the barrel holding too much water? I have found I have to load the bottom of a big barrel with styro or other big material and then plant in just maybe 1/3 of the upper portion. gives the plants room to grow w/o
being in too much moisture. might change your mulch too....that black stuff might have been hiding in some of it waiting to explode. just my '2 cents'.

I also use Neem oil on mine to keep them from getting to fungus-y (I use it on Verbena too.) Plus, you can use it on roses (it's kept my roses blackspot free in Houston!)

Elizabeth who used to post here used to say that Lysol was all you really need for mildew and mold on begonias. I had great success with it. HTH