leaves turned yellow, then looked burned

[emmaloufudge]

I have a Sophie's Choice I planted in a barrel on May 20. It started out looking very healthy. All of a sudden the leaves, beginning at the bottom and moving upward through the center of plant, started turning yellow. This was a bright yellow about the same color as the little smiley faces we can insert in the messages. After a week or so, the leaves started disappearing, it was like the heat was just causing them to go away. I have one very large tomato on the plant.
there is a Carmello and a Martino's Roma on either side.
I have attached a photo that was taken on June 20 when the plant looked better than it does now.
Does anyone know what this problem is?
Marion