Leaves turning yellow is a widespread problem with cucumber plants. However, its reason needs to be identified and fixed if you want your plants to produce a good harvest.

First, you have to ask yourself: “What causes my cucumber leaves to turn yellow?”

The main causes of cucumber leaves turning yellow are watering problems, insufficient sunlight exposure, soil deficiencies, and diseases or pests that affect cucumber plants. To fix yellow cucumber leaves, you need to identify the reason and then solve the problem accordingly.

Let’s have a look at each potential cause and how you can fix it.

What causes cucumber leaves to turn yellow	How to fix it
Watering problems	Deepwater your cucumber plants once or twice a week. Water them with about 1 inch (including rainwater) per week. Get a rain gauge to prevent overwatering your vegetables.
Insufficient sunlight	Move your plants to the sunniest spot in your yard if you can. Or remove shade-throwing objects around the plants instead and maximize their exposure to sunlight.
Soil deficiencies	Take a soil test and find out what the deficiencies are. Then supplement the soil accordingly. The most common deficiencies are Nitrogen, Potassium, Magnesium, and Iron.
Diseases	Cucumber mosaic virus can’t be cured. It is best controlled through prevention of the infection and eradication. Remove all weeds and diseased plants from your garden and use clean and sanitized tools. Fusarium wilt can sometimes be treated with fungicide. However, the spread of the fungus can only be controlled by removing all infected plants from the area. The most effective management strategy is to plant resistant varieties, given that the spores can survive in the soil for 5-10 years. Downy mildew -affected cucumber plants must be removed. Apply fungicide to the rest of your plants to prevent any further spreading. The pathogen can only survive on living plant tissue.
Pests	Spider mites, whiteflies, aphids, and potato leafhoppers are easiest to get rid of by treating the cucumber plants with insecticides.

1. Watering problems

Yellow cucumber leaves can be caused by both too much and too little water. 

If your cucumber plant leaves are drying up and turning yellow and brown, your plant might need more water. Cucumber plants grow best when you deep water them about twice a week, letting them receive about 1 inch of water (including rainwater) over that time.

If you overwater cucumber plants, the leaves also start wilting and turn yellow and brown. Other signs of overwatering are stunted slow growth and rotting roots. The plant might even begin to drop old and new leaves simultaneously as a sign of too much water.

How to fix yellow cucumber leaves caused by watering problems

The easiest way to fix yellow leaves caused by too little or too much water is to add water or let the overwatered plant dry accordingly. Then, to prevent further watering problems, regularly check the soil moisture with your fingers. Your plants’ water needs may be very weather dependent.

I recommend getting a rain gauge and putting it next to your vegetables to collect rainwater. The small measuring cup will let you know how much rain your cucumbers have received and how much you still need to water them.

Having good drainage for cucumber plants is also crucial to avoid overwatering. A drip irrigation system can be a great solution too.

Here is a complete guide on how much water your vegetable plants need.

2. Insufficient sunlight

Cucumber plants are fruiting vegetables and therefore require full sun. 8 hours of sunshine per day are ideal for them to grow. Or at least 6 hours, including the stronger afternoon sunlight, are also ok. Anything below that can be a potential cause for yellow leaves on your cucumber plant.

How to fix yellow cucumber leaves caused by insufficient sunlight

If you realize that the yellow leaves are caused by insufficient sunlight for your cucumber plants, you might have a few options to solve the problem. If your cucumbers are growing in containers, you can move them to a spot where they get more sunlight.

If your cucumber plants are growing in your garden soil, obviously, you can’t change their spot easily. However, depending on the situation, you might be able to remove shade-throwing objects from around them to maximize the sunlight exposure hours per day.

Here is a helpful article on how much sunlight your vegetables need.

3. Soil deficiencies

Cucumbers are heavy feeders when it comes to soil nutrients. If the soil lacks certain ones, the cucumber leaves can turn yellow due to the deficiencies. Here are some of the most common ones:

I. Nitrogen deficiency

Nitrogen is essential for plant growth. It supports foliage growth and keeps the leaves green and healthy. If there isn’t enough nitrogen in the soil, the cucumber leaves might start turning yellow.

II. Potassium deficiency

When beginning to flower, cucumbers need an increased amount of potassium. If the cucumber leaves start to turn yellow at the edges and tips, it could signify a potassium deficiency in the soil.

III. Magnesium deficiency

If you find general yellowing of leaves, starting from the bottom of all your cucumber plants, it may be a magnesium deficiency in the soil. The full foliage may look similar, getting yellow in the green throughout the leaf.

IV. Iron deficiency

If the cucumber leaves turn yellow but still have green veins, the plant is likely suffering from an iron deficiency.

How to fix yellow cucumber leaves caused by soil deficiencies

First, you will need to take a soil test and determine what is lacking and what deficiencies your plants are suffering from. Then, you can amend your soil with fertilizer according to specific needs.

If your soil lacks nitrogen, you could also fix it by adding compost or coffee grounds. Those are both nitrogen-rich substances.

A well-balanced fertilizer best solves potassium deficiency.

Yellow cucumber leaves caused by magnesium deficiency can be treated with Epsom Salt. Put about 2 tablespoons of Epsom Salt per gallon of water in a sprayer and apply directly to the leaves. The magnesium will be absorbed quickly, and the leaves will turn green again.

Lacking iron in the soil can be remedied by either spraying the foliage on the cucumber plant with a liquid iron or by sprinkling granular iron around the plant’s roots.

4. Diseases causing cucumber leaves to turn yellow

There are quite some diseases that can infect cucumber plants and, as a result, will let their leaves turn yellow. Cucumber mosaic virus, fusarium wilt, and downy mildew are the most common ones. Let’s have a closer look:

I. Cucumber Mosaic Virus (CMV)

The cucumber mosaic virus is a small RNA virus capable of infecting cucumbers and a wide variety of plant species. Typical symptoms of this virus include yellowing of leaves and turning them mosaic, appearing ring spots, growth stunting, and flower and fruit distortion.

So besides yellowing the cucumber leaves and turning them mosaic, CMV can cause the cucumbers themselves to turn pale and bumpy. Usually, the overall growth of affected plants is stunted, and fewer flowers are produced.

Aphids spread the virus. Within seconds a cucumber plant can be infected, and the disease can be distributed around a garden within a few hours.

II. Fusarium wilt

Fusarium wilt is caused by a fungus. It affects the vascular system of the cucumber plant. That causes yellowing and wilting in the leaves of the plant. Usually, the whole plant growth is stunted as well. An affected cucumber plant usually dies within 3-5 days after the first symptoms.

The fungus can persist for long periods in the soil as spores and is, therefore, a potential reoccurring problem in a garden.

III. Downy mildew

Downy mildew is caused by a fungus-like pathogen. It thrives in humid and wet weather conditions.

Symptoms of downy mildew on cucumber plants are yellow spots appearing on the surface of the leaves. They increase in size over time and eventually turn the entire leaves brown. Usually, older leaves are attacked first.

How to fix yellow cucumber leaves caused by diseases

Once you have found the disease causing your cucumber leaves to turn yellow, you might have a few options:

Unfortunately, the cucumber mosaic virus can’t be cured. It is best controlled through prevention of the infection and eradication. Remove all weeds and diseased plants from your garden and use clean and sanitized tools to start over. If you have the option, it is best to use resistant varieties, or you could look into planting so-called “trap crops” alongside your cucumbers.

Fusarium wilt can sometimes be treated with fungicide. However, the spread of the fungus can only be controlled by removing all infected plants from the area. The most effective management strategy here is also to plant resistant varieties, given that the spores can survive in the soil for 5-10 years.

Downy mildew affected cucumber plants must be removed as soon as you discover them. Try to treat the rest of your plants with a fungicide to help prevent further spread. Thankfully, the pathogen can only survive on living plant tissue. However, it is also best to plant varieties with a high resistance to prevent the disease.

5. Pests causing cucumber leaves to turn yellow

Spider mites, whiteflies, and aphids are three types of insects that feed on the sap in cucumber leaves. That can cause them to turn yellow and might eventually kill the whole plant.

Another common pest on cucumber plants is the potato leafhopper. It also feeds on the sap in the leaves. However, it also injects watery saliva and, when doing so, damages the leaf. The leaf slowly starts turning yellow, dies, and falls off the plant.

Besides yellowing, another sign of pests attacking your cucumber plants is holes in the leaves. Check out the most common causes of holes in cucumber leaves here.

How to fix yellow cucumber leaves caused by pests

Unfortunately, pests’ damage done to your cucumber plants can’t be reversed. However, your best way forward is to get rid of the pests as fast as possible.

To get rid of spider mites, whiteflies, and aphids on cucumber plants, you can use an insecticidal soap that you spray on the leaves.

Insecticides also work to get rid of potato leafhoppers. Or you could use row covers to protect the leaves from the dangerous attacks of the little bugs.

So, no matter what has caused your cucumber leaves to turn yellow, once you have identified the reason behind it, it is crucial to take action and fix the problem. Healthy plants make for a healthy garden and a great harvest. The faster you identify and eliminate problems, the better your cucumbers will grow.

Related question:

Should you cut yellow leaves off cucumber plants?

In most cases, cutting off yellow leaves on cucumber plants won’t solve the problem. First, you need to identify the cause of the yellow leaves and then treat the plant accordingly. In some cases, that could mean that you have to remove the entire plant from your garden.