Have you got other tomato plants which are clear so far? If so I'd put a binbag over the affected plant, pull it up, seal it in the bag and get rid of it somewhere, to try not to affect the others. If the stem is already affected I wouldn't hold out much hope for it.
I've got 6 Marmandes and 7 Red Alerts in pots outside - the Marmandes have got a few well developed fruit and a lot of flower, and for the last week I've noticed a few small signs of blight on some leaves. I cut them off immediately, seal them in a plastic bag and they go in the bin. I've moved the plants so that there's more distance between them and check them 3 times a day removing any affected leaves and keep all my fingers crossed. So far so good.
It's so annoying 'cos in goodness knows how many years of gardening, these are the best looking tomato plants I've ever grown - so far, and in such a rotten year too!
The Red Alerts, which are next to the Marmandes, are showing no signs of anything - so far!