Looks like blight. You can remove the bad leaves. I've had that happen with a few leaves and the plant went on to produce a lot of tomatoes. It's normal for lower leaves to turn yellow and fall off as the plant goes into production. The brown spot surrounded by yellow is a sign of the fungus, which the plant can survive if you nip it in the bud. I've found Serenade works well for fungus, or you can try neem.

I am engaged in a research project. Your question caught my eye. My question to you is....have you incorporated your own homemade compost into the soil where this tomato is growing? If so I will be back with more questions.