Why are my Cucumber Leaves Turning Yellow?

Cucumber leaves turn yellow for a variety of reasons from pests to watering troubles. Some of the problems that cause yellow leaves, are preventable, such as over or under-watering your plants, but diseases and pests can often pop up and surprise you by causing your plants to yellow also. 

If you’re trying to figure out why your cucumber plant leaves are turning yellow, here is what you need to know. 

Why Are My Cucumber Leaves Turning Yellow?

One evening, I took a walk through my garden and saw my cucumber plant leaves turning brown and yellow. Something was wrong, but what could be the problem?

After researching and closely inspecting my plants, I found out that my plants had a nitrogen deficiency, an easy problem to fix. Soon, my plants were back on track, and the rest of the leaves stayed their beautiful green color.

Problems with Cucumber Plants

Cucumber plants are prone to a range of problems, and some lead to your cucumber leaves turning yellow. It’s vital to remember that once leaves turn yellow, they won’t turn green again. So, as you work to fix the problem, watch to make sure that no more leaves turn yellow; that indicates you fixed the problem. 

Yellow leaves stay yellow, unfortunately. 

1. Over or Under Watering

The most common reason for cucumber leaves turning yellow is that you are over or under-watering. Everyone knows that failing to give your plants enough water leads to wilting and death, but not everyone realizes providing too much water is just as harmful. 

Cucumber plants need between one and two inches of water per week, including rainfall. Consider keeping a rain gauge in your garden beds to help you measure the water that your plants receive from rain each week. I had a cute rain gauge with a frog on it that catches rain and helps me mindfully track everything. 

Here are some tips to help you water your cucumber plants correctly. 

1. • Stick your finger into the soil and check to see if it's moist two inches down. If it's not, it's time to water your plants again. If it is moist, check again tomorrow. 
   • Soil type is another thing to consider if you have determined that you are watering correctly. Clay soil holds onto moisture too much, while sandy soil allows water to flow through too quickly. 
   • When it's hotter than usual outside, check your soil often. Cucumber plants wilt when exposed to high temperatures without enough water.

2. A Lack of Sunlight

Cucumber plants need to be planted in a location that receives six hours per day. If you accidentally planted them in a spot that doesn’t receive enough sunlight, your plant might end up with yellowing, droopy leaves on your cucumber plant.

Unfortunately, digging up cucumber plants rarely has good results. Fixing this problem is possible in the future, or remove what is casting the shade on your plants. 

3. A Nutrient Deficiency

Cucumbers are heavy feeders, requiring a wide range of nutrients for proper growth. If your plants lack any of the nutrients, it leads to chlorosis, reducing the normal green coloring in leaves. 

The best way to determine if your plant lacks a specific nutrient is to test the soil. You can test your own soil with a simple soil test kit. They cost around $20, but they're comprehensive and provide an in-depth look at your soil quality. 

If you find that your soil is lacking in nutrients, I like this all purpose organic vegetable fertilizer which can help provide proper nutrition to your plants. Adding compost and/or mulch to your garden beds in spring and fall can also help feed the soil.

Here are some of the most common nutrient deficiencies that cucumber plants experience. 

• Nitrogen Deficiency

Having a nitrogen deficiency causes your plants to have yellowing leaves and stunted growth. If the situation is severe, plants die due to a lack of nitrogen. 

One way to tell that your plants have a nitrogen deficiency is to check to see if the older leaves turn yellow along the central veins and at the tips. At the same time, the newer leaves look green. This is a classic sign of a nitrogen deficiency. 

• Iron Deficiency 

When cucumber plants have an iron deficiency, the yellowing leaves have green veins, but only on the new leaves. The older leaves stay green.

This is a classic sign of iron problems, which are easily treated once identified. 

• Potassium Deficiency

Cucumber plants are different than others because they need a lot of potassium to help the fruiting process. They're one of the only plants that need more potassium than nitrogen. Since many fertilizers cater to plants that require more nitrogen, it's easy for a potassium deficiency to occur. 

Plants exhibit potassium deficiencies by causing the leaves to turn yellow at the tips and edges. The young leaves will be small with a dull look with puckered edges. 

• Zinc Deficiency 

A zinc deficiency isn’t as common as a potassium one. You will know that your cucumbers lack zinc when the older leaves start to yellow between the veins. A zinc deficiency causes smaller leaves and restricted growth.  

4. Cucumber Diseases That Cause Yellow Plant Leaves

As we work our way down the list of reasons for cucumber leaves turning yellow, cucumber diseases are another problem that gardeners need to understand. A few diseases cause streaks or yellow spots on cucumber leaves. 

Check your plants to see if they have any signs that might indicate a disease is the cause of your yellow cucumber plants. 

• Cucumber Mosaic Virus

This disease causes the leaves to wrinkle and curve downward with yellow spots. 

Take a close look at your plant; does this description fit what you notice on your plant? 

Unfortunately, the cucumber mosaic virus doesn’t have a cure or a way to stop the disease from progressing. The recommended course of action is to remove the infected plants to prevent them from spreading throughout your other plants.

Aphids and leafhoppers spread this disease, so check your garden for those pests and work towards removing them. 

• Fusarium Wilt 

Gardeners dread fusarium wilt. It starts with the bottom leaves turning yellow first and gradually progresses up the plant. Fusarium wilt causes the leaves to turn yellow, starting the edges and spreading inward. 

Take a close look at your plants; fusarium wilt is easy to identify once you know the characteristics. 

Cucumber beetle larvae spread this disease as they eat the roots. Unfortunately, fusarium wilt has no treatment plan. Most gardeners find that they need to get rid of their plants to prevent this disease from killing the other plants in their garden. 

• Downy Mildew

Check your plants and look for yellow spots along the upper surface of the leaves. Over time, those spots turn brown. The veins bound the spots, so it has a strange, patchwork look.

If that sounds like your cucumber plants, chances are you have downy mildew. If your area has high humidity problems, you might also find a grey fuzz on the leaves' underside, which is how this disease received its name. 

Downy mildew is manageable. Here are a few tips.

• Space your plants better. You might have to remove a few plants to increase the air circulation between the leaves. 
• Water the base of the plant rather than getting water on the leaves.
• If you see a plant with downy mildew, remove it from your garden. It will spread quickly, but gardeners manage it once they know it’s present. 

If you determine that you do in fact have a plant disease turning your cucumber leaves yellow, a copper fungicide is a great safe solution for combatting the problem. I love this small hand held garden sprayer if you need one similar to the one in the photo above.

5. Cucumber Pests That Cause Yellowing Plant Leaves 

Cucumber plants are prone to a variety of pests. Last year, I thought I did everything right, and my plants ended up covered in cucumber beetles. Pests love cucumber plants. 

Some pests suck the sap out of the leaves, causing leaf discoloration and yellowing. If you notice that your cucumber leaves turn yellow, closely inspect your plants. 

Many pests are tiny and like to hang out on the leaves' underside, so flip them over and look to see if you find clusters of insects or groups of eggs. Eggs also indicate a pest problem. 

Here are a few of the most common cucumber pests that cause yellowing leaves. 

• Striped Cucumber Beetles

Adult cucumber beetles feed on the leaves, causing holes, wilting and yellowing of the leaves. Larvae of the cucumber beetles feed on the roots of the plants. Even worse, these critters can carry the bacteria of bacterium wilt, which is even more dangerous than the pest itself. 

• Aphids

A small aphid infestation rarely causes problems, but as the infestation grows, the issues begin. Aphids are small, oval-shaped insects that stick to the underside of the leaves. They suck out the sap from the leaves, leaving behind a sticky substance called honeydew. 

Aphids can be light green to black in color. The best way to solve for them, is to just spray them off the plant with water, or squish them. Other options include introducing lady bugs to your garden, as they feed on aphids.

I also like to plant a lot of nasturtiums around as a sacrificial plant, as the aphids seem to prefer them above all else. I also love how easily the "nasties" as I call them, reseed themselves for years to come, and both the flowers and leaves are edible. So plant them heavy one year, and they will pop up all over in years to come.

• Spider Mites

Another pest that likes to suck out the sap from leaves is spider mites. They cause yellow stippling on the leaves. Identifying spider mites is tricky because they’re so small, so look for a silvery, fine webbing on the underside of the leaves. 

• Whiteflies

Give your plant a good shake, and watch to see if small, white-winged insects fly out. If they do, then you have a whitefly problem that needs to be addressed. Like aphids, whiteflies suck out the sap from the leaves and leave a trail of honeydew behind. 

• Potato Leafhoppers

Potato leafhoppers are larger insects, so they’re easier to identify. Keep an eye out for these pests because they also suck out the sap from your plants.

The biggest problem is potato leafhoppers is that they inject toxins into your plant as they feed, which causes the yellowing. Over time, the damage gets worse, and the leaves fall off of the plant. 

If you determine that pests are the problem with your cucumber plants, neem oil is a great safe solution to use on them. Just remember to do a couple of repeat applications for the next few weeks, so that any eggs that were laid, are handled when they hatch or you will just have another infestation.

Final Thoughts on Cucumbers Leaves Turning Yellow

Seeing your cucumber leaves turning yellow is no fun, especially after all of the hard work that you put into your garden. I hope this article has helped you to solve any cucumber plant problems that you may have.

Take a look at your plants regularly and inspect them for any signs of pests, diseases, or nutrient deficiencies. Make sure the plants get enough sunlight and avoid over and under watering them.

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