Hello lee,
I have the same problem with my tuberous begonias, and I am not sure of the cause. Parts of the leaves turn yellow, then spotted with brown, and then fall off. I believe the cause to be too much water, so I let the soil dry, and will water them with a water/fertilizer mix. I will let you know if they do well. I have heard that by slitting the veins of a begonia leaf, and placing a rock or other weighted object on it partially, that it will root, but I have yet to try this. I hope this helps, and I will post later!