I have an angel winged begonia that was originally from a cutting. I have had it for approximately 5 years. I have never pruned it, so it is one big cane! It is getting quite scraggly now, but I have never had any problems with it. It has been blooming for about two years now during summer and always blooms close to the tip. This last bloom was absolutely spectacular! but now... the leaves near the bottom of the plant are yellowing. They are not drying out though, just yellowing as if they wear changing colors for fall. Also, the point where the next leaf would have opened has now browned. What is happening, and how can I save the plant? I do not know how to prune it, or even if that is the right thing to do.

thanks