Are my cucumbers ok?

This is my first attempt at cucumber. I am growing them in the greenhouse and have already had my first small one. Delicious it was too. I am a bit concerned with my leaves. Are they ok?

That looks like magnesium defficiency. Any foliar feed with magnesium in it, read the label, or epsom salts.

Surprised its just the one leaf, Id have thought Mg problems would have sown across the entire plant ?

I  now have my ES. How much do I put in a gallon watering can?

No I got it from the pharmacy dept in the supermarket. Hope its the same as the Westland ES.