Question

bad leaves   Tomato leaves are discoloring and looking sick and not growing to full size.  They have plenty of water and think I fertilized them well.  Do you have any idea of what could cause this?

Answer
These symptoms are classic for a tomato (or tobacco) virus.  The lesions (yellowing and dying spots) on the leaves suggest a viral infection rather than a fungal infestation.  The two major fungal diseases, fusarium and verticillium, cause major wilting due to decreased flow of water as the fungal infestation grows.  If you see a major wilting (some wilting in the heat of the day is normal), cut through a branch with a sharp knife.  Discoloration will show if infested with fusarium and verticillium.  When purchasing plants, there is a code for VFN, which indicates that these cultivars are resistant to Verticillium, Fusarium, and Nematodes.  

Viruses are much more difficult to identify, as many of the symptoms are the same in the viruses.  Most viruses are transmitted by sucking insect vectors, such as thrips, whiteflies, etc.  The only way that you can identify specifically the viral component would be to send to a lab for an enzyme assay.  That can be very expensive, and if you are not in the industry, probably not worth the effort.  

I am providing you with a link to the Cornell website which may provide you with more information about viral diseases in tomato.  As long as your plants are watered and have sunlight, they can still produce tomatoes, even though infected with a virus.  I would suggest that you NOT plant tomatoes in the same spot in the garden next growing season.  (That minimizes the nematodes and other soil borne diseases.)  

http://vegetablemdonline.ppath.cornell.edu/factsheets/Viruses_Tomato.htm