Oh dear! It looks like you might have some sort of blight going on. If so, with the plants so close together, it will spread very easily and quickly.
I've had marginal success stopping bacterial blight with a douse over the whole root system with a dilute hydrogen peroxide solution. Take the stuff you buy at the drug store, and dilute it 32:1 with water. (4oz. per gallon) You'll have to remove that landscape cloth to douse the whole root system of all the plants that are affected. You should use about a gallon of peroxide per plant. It may not help at this point, but it certainly won't hurt.
For future reference, I wouldn't put landscape cloth down under tomato plants except maybe temporarily while they're getting going in the spring time to warm the soil faster. Now, in hot weather it will be making the soil much hotter than it already is, and the roots not able to breathe either. Removing it from the entire bed, and putting down maybe some organic mulch like wood chips might help the whole situation, too.
