[Carollan]

I`ve tried to attach the photo of the dying  soon to be binned  tomato plant.I`m hoping it doesn`t have anything too dreadful,it started to look ill a couple of weeks ago,so I moved it well away from the others.It had the same treatment of compost/ indoors/outdoors/no feed

Anyone like to try a diagnosis ?

[arugula]

Magnesium deficiency?

[DD.]

It looks like it, but why just the one?

[JayG]

Bit strange if it has been treated exactly the same as the others; could a cat or dog have decided it made a good spraying/peeing target?

[Aunt Sally]

I'd say magnesium deficiency too. 

Are you keeping it too wet though.  You should allow them to dry out a little bit between watering to stop it from waterlogging.

[Bing]

Michael Jackson syndrome. 

[mobilekat]

before binning I would repot and give it a meal!

if it then continues to die say good bye, but looks in need of TLC first!

(I never give up until a plant is TOTALLY dead!!)

[8doubles]

Looks badly sun-scorched to me. Mag deficiency normally affects the older lower leaves worst.
Was the plant in a position where it was getting extra reflected sunlight?

[Nige2Plots]

before binning I would repot and give it a meal!

if it then continues to die say good bye, but looks in need of TLC first!

(I never give up until a plant is TOTALLY dead!!)

My sentiment exactly!

[mumofstig]

A couple of mine went like that, I'm ashamed to say 
They were badly potbound 
I potted them on into much deeper pots so that I could bury some of the stem. They've had a weak feed as well and now a week later they look completely different with baby toms on

[Carollan]

Thankyou for your input everyone,there  is the possibility that you are all right!! Yes,I have a male dog ,that could be strong urination problem on the plant if I wasn`t watching...yes to the too wet,I have a tendancy to overwater I admit,yes to the magnesium deficiency,I`ll try feeding it before I bin it...sunscorched,very likely after all this hot weather we`ve been having and potbound,yes,the others have been potted on and are thriving,this poor plant has been,well,neglected I guess in that department.

I shall report back and let you know whether the patient survives,with food prices escalating the way they are,every plant we have is precious!!