Here are the common culprits to look at first for the conditions you describe and your location:

Fusarium wilt
Nematodes
Early Blight

bettsyy, the bumps on the stems may be roots trying to form where the stem got wet, tomatoes will try to put out roots along the stem even above ground if the stem is wet enough... do you water the ground below the plant or the actual leaves? Also, if the yellow leaves are relatively spot-free, and they are the lowest leaves, they could just be dying off due to age and lack of sunlight down low on the plant... or they can be yellowing from overwatering. How is the soil you planted in and how have you been watering and have you had much rain?

I try to water at the base of the plant. We have not had any rain for over a month, I am amazed how well everything is growing, I picked 4 zucs today!. I water every other day and water all the tomatoes as evenly as possible. I am debating pulling up these tomatoes and setting out new plants but it is so hot here now, upper 80's, I don't know how well they would do.

A couple of things to consider. Overwatering can give you yellow leaves because it starves the roots of oxygen. I don't know if your plants are in containers or in the ground. If they're in the ground then mix one tablespoon of epsom salt to a gallon of water and apply a half gallon to each plant. If in containers then just give 'em a quart. Epsom salt is a source of magnesium - tomatoes and roses love it. Go easy on fertilizer especially if the tops look good - that tells you they've got enough. Hope this helps you.