I'm thinking too that now would be a good time to transplant it into a larger container.
In my experience, cukes don't do very well when they get root bound, and it looks like that is a definite possibility for you.
gardening.stackexchange.com/users/1447/michelle
What you have is classic Chlorosis.
Leaves dark near the veins, yellowing in the areas between.
Basically, it's the equivalent of anemia.
The chlorophyll in plants is related to hemoglobin in blood in that it requires iron.
This plant needs to be in a much larger pot with some actual soil in it.
Potting soil while fluffy and great for getting plants started, is often nutrient poor when the plant is past the stage where it's expended its seed nutrients.
Plants displaying Chlorosis in new growth leaves can be recovered with the use of iron sulphate solution which gives off enough free iron to give the plant what it needs.
In the case of cucumbers, not a whole lot as having too much iron can bind other nutrients.
Chlorosis can, in plants like blueberries be an indicator of both magnesium and iron lack, the acid soil they love harbors bacteria that release these elements from the soil particles.
In older leaves, chlorosis is an indication that the leaf is spent and occurs as sugars and starches build up that the aging leaf cannot move to the plant.
These being older leaves, probably are a good indicator that a transplant into a pot with healthy soil is more likely to be the ultimate cure.
gardening.stackexchange.com/users/2197/Fiasco Labs
Check if the back of the leaves hides little parasites.
Check if it could had had too cold or changes of temperature.
Give manure and sequestrene (iron), maybe it has grown too quickly and has used all the nutrients.
They need to be watered 2 times a day, only on the ground and not on the leaves.
If everything you checked is good, try some anti fungal, it is very difficult to make a quick diagnosis only from a photo.
Your plant seems to be into a room.
It needs more light, sun, heat.
It needs to be transplanted, since the pot is small.
Your clima is a bit to cold now, so wait for better weather.
gardening.stackexchange.com/users/2052/violadaprile
OK it's food, green leaves at the top, leaves yellowing at the bottom and working its way up, until the leaf goes brown, drops off or you cut it off because it looks ugly and makes the plant look sick.
THE CURE Magnesium sulphate 100% in Epsom salts from your chemists, 2 tablespoons full in 1 gallon of water, soak the leaves in it, not using a watering can, but not when the sun is on the leaves, because cucumber plants like heat but not direct sunlight, which is why they are usually grown in greenhouses, in their own container, then pour some onto the soil and the plant will store it for later.
gardening.stackexchange.com/users/2478/hudson.hawk
doesn't look like nutrients...
usually the old leaves would still be green and the new ones would be chlorotic...
Looks honestly like heat stress, was it in a hot window?
gardening.stackexchange.com/users/1158/Grady Player
Calcium deficiency.
In addition to the answer about chlorosis being like anemia, that's really not as far off as it may sound since anemia is in essence a shortage of Iron.
An iron deficiency can be the root cause of multiple nutrient deficiencies, even when those nutrients are in the soil and available to the plants.
Iron is what allows a plant to absorb many of those other nutrients.
If there is not enough iron in your soil, an addition of chilated iron to the soil can help the plants uptake the other nutrients the plant may be deficient in.
This isn't usually the case for plants growing directly in the ground, as there is usually plenty of free and available iron in top soil so long as the pH isn't too high.
Since this is not a potted plant it is perhaps not worth getting the soil analysed, but if you do, then don't ignore values for Iron or Calcium.
If you need more iron, this is introduced in the form of chilated iron.
Tossing iron shavings into the soil won't do diddly squat in the short-term.
I like to say that Calcium is perhaps one of the most valuable yet overlooked nutrient, and I have to say that Iron is too, however in most circumstances you don't have to worry about iron deficiencies unless you are doing hydroponics or constantly reusing potting soil without refreshing the organic nutritive content from time to time.
You can do that best by tossing your potting soil in the compost heap.
Calcium on the otherhand is consumed by the plant and then reaped when you pick your fruit and veg.
A lot of calcium goes into those fruits and veg you eat.
gardening.stackexchange.com/users/10805/Escoce
My cucumber plant's leaves are turning yellow.  Could it need nutrients already? It only germinated 24 days ago, but is my biggest plant already.
Why are my cucumber leaves turning yellow?
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<diseases><cucumbers>
gardening.stackexchange.com/questions/7217
gardening.stackexchange.com/users/2064/skerit
This is called Chlorosis, which is a calcium deficiency. It is probably a good time to transplant into a bigger container and give it more heat, sun and light. This is classic Chlorosis. This plant needs to be in a larger pot with more soil in it and it needs more light, sun, heat.
This plant needs to be in a larger pot with more soil in it. This is classic Chlorosis. It needs more light, sun, heat.