I'm having a similar problem. I'm growing tomatoes in a pot on my patio and they've been showing strange symptoms this past month. I've done some searches but can't pinpoint a single reason or how to make it better.
Here are my symptoms:
-leaves are turning yellow and drying up...most from bottom up but also a few branches in between
-tomatoes have dark spots on the bottom...almost as if they're rotting
-leaves aren't as lively as they used to be...they're wilting
-the drained water in the tray is a light brown...all my other plants have clear water.
I have no idea if this is a disease or calcium deficiency or insects.