My four tomato plants are dying off.

One plant is badly affected by grey patches on the stems and the leaves are
shrivelling, turning yellow or just rotting and it appears virtually dead
although the yellow flowers seem unaffected. This is now spreading to the
other three plants. They were all perfectly healthy on weds when I was tying
for support and pinching out side shoots.

Can some one tell me what it may be. Can I save them or do I say goodbye to
tomatoes this year?

Steve