Verticillium Wilt?

[Labradors2]

I think this is Verticillium Wilt. It is affecting two tomato plants of the same variety that are not growing side-by-side. I suspect that it is a problem in the soil as I had a slightly different problem last year in about the same two areas. Last year, there was some rot on the stems around the soil line. I pulled the plants, examined the roots (which looked fine) and chopped the stems to reveal dark lines in the stems. Although I never noticed the leaves turning yellow last year, the plants drooped badly during the day and revived overnight. This year I've noticed a bit of drooping in the affected plants, but nothing drastic.

1. Should I save seeds from these plants?

2. I was thinking of excavating the soil in the affected areas, and replacing it with aged cow manure. Is this a feasible plan? If so, how deep would I need to go?

3. I was actually thinking of removing 6" of soil in the whole tomato/pepper/eggplant-growing area, since we have a tractor with a bucket, and access to two trailer loads of free aged cow manure. Would 6" be deep enough? It is clay underneath.

Linda