Dying Tomato Plants

I have been growing tomatos for years and they have always been reasonably successful. But this year my tomato plants have been developing grey patches on their stems, the patches grow up the stems and ends up killing the plant.

How do I stop this from happening?

Martyn-if you look through some recent discussions on tomatoes and their problems you may well find the answer-by the sound if it it is fungal disease

But as I say look back over the past couple of days

Martyn, it could be fungal or a wilt. Hard to know without seeing a photo. How does the foliage look? It's rare for disease only to manifest on the stems.

And could you cut off a section of diseased stem, slice it open and have a look inside? Some diseases attack the plant internally and there will be evidence.

Many thanks everyone, sounds and looks like a fungal disease, such a difficult summer, I've not had this problem before, fingers crossed for next year!