Q.Why Am I Losing My Tomato Plants To A Horrible Brown Blight?
I try to grow tomatoes each year and the ones I grow in pots are usually healthy and full of fruit but those plants which I put in the ground always suffer with a horrible disease which kills the foliage and turns the tomato’s brown. If I pull all the fruit when I see the first signs of this and try to ripen them on the window sill or the greenhouse they still usually go brown as if whatever has got at them is already sleeping inside the tomato’s just waiting to appear. I’ve photos added below… is it something in my soil which means that I will never be successful in growing them? … or is it an airborne thing, I say this because this year some of my pots have suffered the same fate?

Certified GKH Gardening Expert
This appears to be Late Blight. This is caused by the microbe Phytopthora infestans. Here is an article that will help you to deal with this pathogen:
https://www.gardeningknowhow.com/edible/vegetables/tomato/blight-infected-tomatoes.htm