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.

Thanks