Salad cucumbers and the leaves of the cucumber plant can turn yellow for several reasons. To find out the real cause, you have to look at the other symptoms of the plant. 

Too much or too little watering often results in the plant and fruit turning yellow. The leaves first turn yellow, then brown and fall off. If overwatered, the cucumber plant begins to rot at the roots. Too little water saps the plant's strength, so the leaves discolour and fall out. About 3cm above the soil, the cucumber plant should always be moist.

For the plant to thrive and the cucumbers to grow well, cucumber plants need plenty of sun. The plant's location should get at least six hours of sunlight a day. The more shade the plant gets, the more the leaves should be pruned so that the plant can direct its energy to the fruit. 

Nutrient deficiencies of the plant can also cause yellowish discolouration of cucumber and leaves. Nitrogen, magnesium, potassium and iron are particularly important for the plant. The plant should be fertilised regularly with a special fertiliser according to the instructions on the package to keep it healthy.

If the cucumber plant is infested by insects, the leaves can also change colour and make the plant sick, so that the cucumbers cannot ripen properly. The spider mite is a widespread parasite whose infestation can be seen by paper-thin spider webs and orange spots left on the underside of the leaves. There are pesticides against the parasite infestation, but a more environmentally friendly and healthier solution is a lye made from washing-up liquid and water that is sprayed on the cucumber plant.