Soil Nutrient Deficiency (Especially Nitrogen) 

Like many other home-grown plants, cucumber plants require a balanced diet of nutrients, say, nitrogen, potassium, and phosphorus. When encountered with yellowing leaves, you should have a particular emphasis on nitrogen intake.

As one of the three primary macronutrients, Nitrogen takes charge of the synthesis of chlorophyll, the green pigment in plants. Without sufficient Nitrogen, your cucumber leaves are highly likely to turn yellow. Plus, chlorophyll is essential for photosynthesis. 

Later, as cucumbers develop into the flowering stage, you should pay attention to magnesium deficiency in cucumbers. With insufficient magnesium, the leaf veins usually stay green, but the areas between them turn yellow, a condition known as interveinal chlorosis.

How to Solve Nutrient Deficiency

First thing first, ensure your cucumber plants receive a balanced fertilizer containing nitrogen, potassium, and phosphorus. Once you see the signs of yellowing leaves, take action and re-consider the nutrient proportion. For example, you may want to add more nitrogen-driven fertilizer during the vegetative stage and supply enough magnesium during the flowering stage. 

Overwatering 

If the soil nutrient is nothing wrong after checking, how about your watering routine? Did you massively water your cucumber lately? If so, overwatering can cause cucumber leaves to turn yellow. 

When soil is constantly saturated, the roots of the cucumber plant can become deprived of oxygen, leading to root rot. Damaged or rotted roots are less effective at transporting nutrients to the plant, resulting in nutrient deficiencies we mention above. What’s worse, overwatered soil can lead to a leaching effect, where essential nutrients like nitrogen are washed away from the root zone. So basically, it’s still the lack of nutrients that leads to yellowing cucumber leaves. 

How to Avoid Overwatering Cucumber Plants 

1. Before watering your cucumber plants, assess the soil's moisture level by gently inserting your index finger or thumb into the soil, about 1-2 inches deep near the base of the plant. If it feels moist, there’s no rush to water the soil yet. 
2. While watering, do it in the morning, if possible. This allows the foliage to dry out smoothly during the day, reducing the risk of fungal diseases that thrive in moist conditions. Also, water deeply and thoroughly to ensure the water penetrates the soil to reach the root system. 

Pests and Diseases 

One more major cause of cucumber leaves turning yellow is pest infection. When certain pests feed on cucumber leaves, they can disrupt the normal functioning of the plant and lead to a variety of symptoms, including yellowing of the cucumber leaves. Let’s see what are they. 

Aphids

Aphids excrete a sugary substance called honeydew as they feed plant sap. This honeydew can encourage the growth of sooty mold on the leaves, which can block sunlight and inhibit photosynthesis. Reduced photosynthesis then leads to yellowing leaves due to a lack of chlorophyll production.

What’s worse, one mottled discoloration virus named cucumber mosaic virus(CMV) often takes cucumber as the host. Once aphids feed on plants infected with CMV, they ingest the virus particles along with the whole plant sap. CMV can then multiply and replicate within the body of the aphid, circulating and reproducing within the aphid's system.

Spider Mites

Spider mites are tiny arachnids that feed on plant cells by piercing them and sucking out the contents. Their feeding process can damage plant cells and disrupt the flow of nutrients and water within the plant, therefore, leading to cucumber plants turning yellow. 

Additionally, cucumber plants are likely to be attacked by the following pests:

• Whiteflies 
• Leaf Miners
• Cucumber Beetle Larvae
• Cabbage loopers
• Cutworms
• Slugs and snails
• Thrips

How to Recover Yellowing Leaves from Pest Infection 

Unfortunately, cucumber plants are not likely to recover from pest infections. The only remedy you can do is to remove the infected parts as soon as possible before the infection spreads to the whole plant. 

1. First, identify the specific pest responsible for the infestation. If you are not sure about the species, take a picture of the pests and leave a comment below! We’d like to help.  
2. Carefully inspect the plant for yellowing and damaged leaves. Prune and remove these affected leaves from the plant. 

To prevent pest diseases in the future, you can implement appropriate pest control measures to eliminate or manage the infestation, such as:

• Handpicking
• Insecticidal Soap
• Chemical Pesticides

Yellow Spots on Cucumber Leaves, What to Do

Instead of entire leaves turning yellow, sometimes you may notice yellow spots on cucumber leaves. In that case, the cucumbers are possibly experiencing downy mildew, a type of fungal disease. 

Downy mildew often happens in cool, wet, and humid conditions and rapidly spreads across the plant once it takes hold. Leaves affected by downy mildew typically display yellow patches with distinct edges. If you take a closer inspection beneath the leaves, you’ll see a dark purple mildew – this is where the spores form and disperse.

How to Fix Downy Mildew on Cucumber Plants 

Similar to pest infections, there’s no way to fix or cure the fungal-affected cucumber plants. It’s also suggested to get rid of the spotted leaves as soon as possible, just in case of a bigger spread. However, there are indeed some precautions you can take. For example, space plants further apart to enhance air circulation. 

For climbing cucumbers, maintain a distance of 12-18 inches between each plant. For those growing directly on the soil, a spacing of 36 inches or more is preferable, though it's not the most efficient approach.

Should I Cut Off Yellow Cucumber Leaves

It depends on the cause. If your cucumber leaves turn yellow because of nutrient deficiency and overwatering, there’s no need to cut off all yellow leaves. As long as you take remedies, they stand a chance to turn green again. However, if your plant leaves suffer from pest infection or downy mildew, you should cut off yellow cucumber leaves before the virus takes over the whole plant. 

Conclusion

Cucumber leaves turning yellow indicates the plant lacks the green pigment, chlorophyll. Insufficient nutrient supply, overwatering, and pest infection are the three major causes of the absence of chlorophyll in your cucumber plants. Hopefully, by following the solutions offered above, you can fix yellowing leaves on cucumber plants and enjoy a fruitful harvest.