[argonaut]

Not in greenhouse yet - but that is where they are intended.

Growing Cucumbers from seeds this year  (normally buy plants)  grew find initially, repotted, again fine, then +2 weeks after poting ... almost overnight the leaves have turned yellow.
Is this expected, is it terminal.
What is the cause ?

They are on the windowsill with about 30 other plants (Chillis & toms) which are doing fine, but not cucumbers.

[MrsPea]

Argonaut they may need a feed of nitrogen or tomato feed, good luck 

[Yorkie]

How wet is the compost?

The edges of the leaves look a little fuzzy - does it look as though there is a mould developing?  Or could you have got water onto the leaves when watering?

Cucumbers are rather fussy!

[jezza]

Hello a good nitrogen feed should sort them out,there could be another problem that we use to have years ago when I worked in a market garden if we  left all the tomatoe and cucumber and pepper plants covered with fleece the tomatoe and cucumbers looked sick the next morning we found out it was a chemical given out by the peppers  jezza

[jaydig3]

It may just be either my eyes or the pics, but is the stem at the surface of the compost looking a little dark, in which case it could be stem rot, something that cucumbers are very prone to?  If this is the case the plant won't survive.
I've had it happen to me on more than one occasion - the cucumbers failing, not that I got stem rot!