[Lardman]

3 or 4 days ago I planted out my cukes in the greenhouse border.

1 in tube of compost
1 in tube of soil
1 in the soil

They had been sown and grown in the greenhouse in 9cm square pots and were nice green health plants.

Now they look like this 

[Trillium]

It could be caused by many simple things like inadequate ventilation, irregular watering, etc. But my  main guess is that its lacking certain elements, specifically potassium and magnesium which yellow older leaves, and the younger ones are showing signs of iron deficiency which will also affect older leaves.

Cure is a dose of basic fertilizer like Gromore, or something like comfrey or nettle tea, even manure tea if you have any. Then regular weekly applications of the latter to keep them in good health.

[Lardman]

I jumped to magnesium deficiency too - but what I don't understand is how fast they're gone like this. I wouldn't have thought they'd have had time to put roots into the new medium.

The soil had a handful of growmore mixed in before I planted and one is in fresh compost, I can't imagine that is lacking already.    They're watered nightly - as they were in their pots and physically they're only about 3 inches lower than they were last week.

Could the roots be cold ? The black pots in the sun must have kept the soil very warm were as the border soil is probably considerably cooler.... transplant shock?

I have plenty of comfrey concentrate to make tea from if you think they warrant it.

[mumofstig]

It may be that the compost around the root ball (the old stuff) has run out of nutrients and the roots haven't had a chance to grow into the new stuff yet. A little feed should be enough to keep them going

Fingers crossed 

[Lardman]

It may be that the compost around the root ball (the old stuff) has run out of nutrients and the roots haven't had a chance to grow into the new stuff yet.

I suppose that could be the case - it does make sense.

You know how frustrated I get when I don't know why its all gone pear shaped.  I've given them (and my slippers  ) a dose of comfrey tea.

[compostqueen]

I think Epsom salts is ok for magnesium deficiency 

[Kristen]

Make sure they have high humidity (mist them regularly; place some shallow containers near their base and keep and inch or two of water in them which can then evaporate).

Check they don't have Red Spider Mite (they are only just visible to the naked eye)