Ailing Cucumber. Any ideas?

My pet cucumber is sick:( The leaves start to turn yellow at the edges and this spreads to the whole leaf. This leaf yellowing is steadily moving up the plant which is now about 6' tall.
There are no insects present in the greenhouse and I have shading on he roof and sides.
There is another cuc in the greenhouse and it is doing fine as are another six in a polytunnel.
Does anyone have any suggestions as to the cause?
I doubt if there will be a cure as 90% of the plant is effected and it's getting worse by the day, but I'm hoping to salvage one cucumber from it!
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Sorry to see that Dave
I planted four seeds of marketmore and have one left which is now outside and appears to hanging on if a little slow in the growth.
As you know Im no expert but your pictures suggests the leaves have heat stroke.!
Interesting to hear what the experts say
robert

 

What`s it planted in Dave?

 

It's in a pot Dai (pic below taken a month ago when in our porch) and the pot is filled with what is supposed to be good quality grow-bag compost.
I can't see it being a virus as the other cuc in the greenhouse in fine, though smaller and in a slightly more shaded position and my other cucs in a polytunnel all look healthy.
I'm leaning to Robert's suggestion regarding heat stroke as the temps in the GH have been 90F recently and today it was hitting 100F! And that's with door open, floor (concrete) damped and shading on all windows and roof. Polytunnel isn't so hot as I can get a through draught due to doors at each end.

The pot a few weeks ago >

 

Looks to me like classic Red Spider Mite.

But it seems to early in the season.

Only other thought would be a dry atmosphere, the high temps are fine as long as it remainds humid and shady.

 

Don't think it is rsm Pete. I've had it before but later in the season and have dealt with it by introducing predators. I've had a real close look at the leaves and can see no bugs or webs. You may be right about the atmosphere as despite sitting the cuc in a tray which gets filled daily the greenhouse, particularly in the middle where the cuc in question is, tends to get a bit dry. One of the penalties of a concrete floor!

 

I'll go with Robert's Sun-burn too :(

 

Hmmm ... I looked, initially, at the papery patches in the photo, but actually the leaf has gone very yellow between the veins. Might be a virus ... any greenfly about? Best to kill them off as they would be the most likely culprit to spread a virus, I reckon.

 

Ta JWK 7 Kristen.
No bugs at all in the greenhouse at present Kristen and in the garden greenfly has only started to appear on a few roses over the past week.
There's one cucumber almost ready for picking - should be ready in about 5 days, so I'll wait until I've got it picked and if the plant is still getting worse It'll get the chop !

 

Hi Dave,I am no cucumber expert but this disease sprang to mind and have found this on the net.Sorry that there isn't any pictures but there is a good description...
http://www.extento.hawaii.edu/Kbase/Crop/Type/cucvir.htm

 

I don't think it looks like Cucumber Mosaic Virus, however it would be worth Dave sticking "

Cucumber Mosaic Virus" into Google Images and comparing them with his plant