Why do the tomato plant leaves go all brown sometimes?

Last year, I lost all my tomato plants to this as the brown dryness spread from the leaves to the stem until the entire plant was covered.

Why do the tomato plant leaves go all brown sometimes?
Its a ground borne fungus that spreads to the leaves when heavy rains splash as they hit the ground. First it hits the lowest leaves and spreads up. There is not much you can do to decontaminate the soil. What you can do is mulch with about 4 inches of straw so the soil won't splash onto the leaves. If the lower leaves get infected, pick them off and don't put them in the compost pile. Dispose of them far away from your garden. When picking tomatoes ar doing garden chores work on the affected plants last so you don't transfer it to other plants. If possible, don't plant tomatoes in the same spot for at least three years.