Lower half of plant turning brown...Oklahoma

by Chris
(Oklahoma City)

I have 3 tomato plants that are about up to my belly button. They have not yet produced any tomoto's but have flowers on them. A couple weeks ago they started turned brown on the bottom and the leaves are wilting. I have been watering them about every other day since it has been in the 100's almost every day. Any idea why they are turning brown on the bottom? See pictures.