It sounds like it could be tomato rust, but you didn't get any kind of definitive answer because there's not enough information to do any more than make a guess. Do a search - maybe even go to Google Pics to see if that's it. If not, come back with more info.

You usually find rust if/when the plants are over-watered, foliage is watered, they are in less than full sun, or after periods of cool weather - especially nights.

Al

That sounds like a worm to me. They leave behind black poo.

My guess is fungus. If you don't want to spray with fungicide (I wouldn't, unless I was serving the tomatoes up to people I don't like), try diluted mouthwash (1 part listerine to 2 parts H20) in a spray bottle. Spray all over your 'mater plant, especially under the leaves. Reapply in 4-7 days.

(If you don't have mouthwash, dilluted rubbing alcohol should work.

Look really closely for a huge horn worm that's hiding in the leaves and hard to see and you can use BT to get rid of worms because that one will put you out of business..

Sounds like a worm to me too. They're big, fat and green. You can just pick them off usually. Throw him somewhere the birds can find him!

Worms - my guess too, and they don't have to be obvious. They can be so small that you almost can't see them but in large enough numbers they'll leave lots of "poo". I'd spray with some BT even if you can't see them. If it is worms you'll see them dying the next day, with no deleterious effects to the plant, fruit, or you.