Why Are My Cucumber Leaves Turning Yellow?

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In your vegetable garden, yellow leaves mean something is wrong. In your flower garden, it can mean the same thing, or it could just be the healthy color of the plant. When your cucumber vines or bushes start turning yellow, it’s trying to tell you something isn’t right.

The problem is in trying to figure out what exactly has your cucumbers struggling. It could be a watering issue, too little sunlight, pests, nutrition deficiencies, or diseases.

With all these potential causes, what’s a gardener to do? By the time you figure out what the cause is, the plant could be too far gone to make a comeback. Don’t worry too much, because we are here to help.

Here we’ll go over the possible causes, what the issues can look like, and how to fix the problem. If your cucumber leaves are turning yellow, we’ll help you get them back on track. 

Reasons Cucumber Leaves Are Turning Yellow

One of the biggest causes of yellowing cucumber leaves is a lack of water. These garden veggies are thirsty plants. Cucumbers are made up of around 96% water. That’s even more than watermelon, which comes in at 92%!

It’s pretty difficult to overwater these hefty vines, though it can be done, especially if you have a dense, slow draining soil. But right now we are talking about the lack of sufficient water.

1. Cucumbers Aren’t Getting Enough Water

Depending on the climate, cucumbers tend to need 1 to 2 inches of water per week. If the weather is cooler, it’s humid, or if rain helps, you won’t need to water as much. But cucumbers like hot temperatures, and most areas in summer tend to be dry.

When the summer sun is out, it bakes the ground and evaporates humidity out of the ground. Which means you will have to water your cukes more often. Adding a thick layer of mulch, either straw or wood chips can help reduce this evaporation and reduce your watering chores.

How to Tell if Your Cucumbers Need More Water

Cucumbers can be fickle when it comes to watering. Too little and the leaves wilt, turn yellow, and can fall off. Too much and the leaves can wilt, turn yellow and fall off. Yeah, cucumbers sound a little bit like Goldilocks.

Underwatered cucumber leaves will wilt, turn yellow, and dry up. They’ll usually turn brown before they fall off. The first sign will be wilted leaves through most if not all of the plant.

Check the soil and see if it’s dry. Sometimes the top one half to an inch of the soil can look and feel dry, while it’s still moist underneath, so stick your finger into the dirt. You want to dig down to the first knuckle in your finger. If it’s dry that far down, give your plants a drink.

Consider a Watering Log

Since cucumbers can be fickle when it comes to water, you might want to consider keeping a watering log. Whenever you water your cucumbers, or whenever it rains, be sure to write it down so you can adjust how much water you are giving them.

Be mindful of how and when you’re watering your cukes. You may not want to give them an entire two inches of water in one day, per week, and call it good. They won’t be able to drink up enough to keep them going for a whole week before most of it evaporates off.

You’d be better off watering them twice, or maybe even three times a week. Be sure to take natural rainfall and cloudy days into account as well.

2. The Cucumbers are Getting Too Much Water

Just like insufficient water, getting too much can cause the leaves to turn yellow. Often this will start at the base of the vine and work its way throughout the plant like a slow, creeping disease.

This is caused by root rot, which happens when the plant’s roots are not able to dry out between waterings or get enough oxygen. Depending on the severity, it can be fatal for the plant.

Typically the leaves will turn yellow, start drooping, and the edges can turn brown. The leaves could fall off, or continue to cling on while continuing to turn brown and crispy. If you check the soil and it’s wet, or there’s standing water, let it dry out.

Depending on when you catch it and how damaged the roots are, the plant may come back to life, or it may not. If the plant dies off, check your soil and add some drainage materials like perlite, vermiculite, or some type of clay breaker if your soil is heavy and full of clay. Then try planting them again if you still have time in the growing season.

One of the best ways to water your cucumbers, and nearly all your garden plants is with a soaker hose. Try out this flat soaker hose if you need to get one, LINEX Garden Flat Soaker Hose 1/2″ x 50 ft.

3. Your Cucumbers Plants Aren’t Getting Enough Sunlight

Insufficient sunlight will cause your cucumber’s leaves to turn yellow. Plants produce chlorophyll which gives them the green color, but when they don’t get enough light, the plant doesn’t produce as much of that chemical, and it can start to turn yellow.

This problem is pretty easy to diagnose. The leaves will turn yellow, but won’t typically wilt like they do with watering problems. 

If you suspect a lack of sunlight is the issue, just keep an eye on your cucumber plants during a sunny day. Track how many hours of full sunlight they are getting, and watch for anything that could be shading them. Cucumbers need a minimum of 6 hours, but some do even better with 8 hours of sunlight.

If you’re growing them indoors you’ll need to keep grow lights on for 12 to 16 because they aren’t as strong as natural sunlight.

Cucumbers grown in containers can be moved to a sunnier spot. Vines and bushes grown in the garden on the other hand are pretty much stuck where they are. Cucumbers do not like to have their roots messed with. 

You may be able to position some inexpensive mirrors to reflect more light onto them, just be sure the light isn’t concentrated and burning the plants.

4. Check Your Cucumbers for Pests

There are several tiny pests that will suck the moisture out of your cucumber (and other garden vegetables) plants. This will cause the leaves to turn yellow, wilt, turn brown, and if the infestation is severe enough, ruin the entire plant.

The major culprits that could cause cucumber leaves to turn yellow are aphids, spider mites, whiteflies, potato leafhoppers, the cucumber beetle, and the southern corn rootworm.

Aphids on Your Cucumbers

When you plant anything in the ground aside from grass, you’ll probably be visited by these tiny, sap sucking insects. They’re everywhere! Fortunately, they aren’t very difficult to deal with.

Aphids will hide in the crevices and the undersides of the leaves. They are tiny, soft bodied insects that suck the juices from plants. This causes the leaves to turn yellow, wilt, and eventually curl up and turn crispy brown as they’re drained.

Aphids can be white, green, red, brown, or other colors, but they can be seen by the hundreds and are very slow moving bugs. Ants also often accompany aphids and transfer them from plant to plant. The ants will also violently defend the aphids like they are a herd of livestock.

The aphids excrete a sticky substance called honeydew, which feeds the ants. That’s why they will defend their tiny friends.

You can treat aphids with diatomaceous earth such as HARRIS Diatomaceous Earth Food Grade, 2lb, insecticidal soap, or spray them off with a jet from the water hose. If they are only on a few leaves, you can cut the leaves off, seal them in a plastic bag and throw them into the garbage.

Once the aphids are taken care of, the plants should go back to normal. Keep an eye on your cucumber plants for a reinfestation or the occurrence of black sooty mold which will grow on the honeydew excretions.

Whiteflies Are Similar to Aphids

Another type of sap sucking insects, similar in habits and size to aphids, are whiteflies. These tiny insects are about the same size, except they are white and triangular shaped. As their name suggests, they can fly too.

You’ll often see tiny white insects flying around the leaves during the day, especially if the plant is disturbed in any way. Whiteflies hang out on the undersides of the leaves and suck the juices from the plant too. These also secrete honeydew.

The symptoms and treatment for whiteflies are basically the same as treating aphids. Only you want to hit them in the early morning so they don’t fly away. Much like me before coffee, whiteflies are sluggish and won’t fly in the early morning hours.

Spider Mites on Your Cucumber Plants

You may not be able to see these miniscule bugs on your plants. You will see yellow, unhealthy looking leaves though. But the most telling sign you have spider mites are the super fine webs they leave on the plants.

Spider mites are most active during June and August when the weather is hot and dry. They look like tiny specks of dust and you may need a magnifying glass to see them.

Ladybugs, green lacewings, and predatory wasps will help take care of most of these insects. If you don’t want to wait around for nature to take its course, Neem oil such as Neem Bliss – Pure Neem Oil for Plants, is a great treatment for spider mites.

When using insecticidal soaps, or sprays such as neem oil, be sure to spray the entire plant, paying particular attention to the undersides of the leaves, and the tiny crevices of new growth.

Potato Leafhoppers Will Attack Cucumbers

While these little buggers will feed on potato plants, they don’t discriminate when it comes to cucumber plants. These insects also feed on the juicy sap of the plant, but while feeding, they have a bad habit of backwashing.

The saliva that gets injected back into the cucumber plant causes the leaves to fade to yellow, and will eventually drop to the ground.

Potato leafhoppers won’t usually appear in large numbers like aphids, whiteflies, or spider mites, but if enough show up, they can severely damage your crops. The best way to treat them is with prevention.

They won’t attack plants that are well spaced apart and have ample airflow. Weeding around your cucumber plants will help keep these bugs away too. But if you do have a large number of leafhoppers on your cucumbers, try sprinkling DE (diatomaceous earth) on them. Encouraging beneficial insects such as ladybugs, and minute pirate bugs will help as they eat leafhoppers and their offspring.

Watch Out for Cucumber Beetles

Cucumber beetles are a big threat to cucumber plants. These insects not only turn the leaves into skeletons, but they carry a host of cucumber diseases. While these pests don’t really turn cucumber leaves yellow, the diseases they spread can.

These insects can be a big problem so that’s why I’ve added them. Cucumber beetles can look similar to ladybugs, but they are more elongated and lighter in color. They can also have black spots (like ladybugs) or stripes.

Telltale signs of these beetles are the bugs themselves, or a multitude of holes between the veins of the leaves. To treat them, you’ll have to pluck them off by hand and drop them into a container of soapy water, DE, encourage braconid wasps, or resort to pesticides.

Cucumber beetles can be difficult to treat, so be diligent and keep checking your plants.

Southern Corn Rootworm

Technically this worm is a grub and the juvenile form of the cucumber beetle. The reason they get their own section is because of their habits, and how you go about treating them.

These little wigglers hatch in the ground, but when they emerge, they will burrow into cucumber plants (and corn) at the ground level and cause trouble with your plants from the inside.

Inside the cucumber vine, these bugs eat the soft tissue and interrupt water and nutrients from being transported to the rest of the plant. This will cause yellowing of the leaves, wilting, and eventually the death of the plant.

When this tiny grub gets into the plant, the only thing you can do is remove the plant and dispose of it. Turning the soil consistently and exposing the eggs, larvae, and beetles to sunlight or plunking them into soapy water is the way to get rid of them.

When you see the cucumber beetle, if you don’t get rid of them early, they can lay eggs in the soil. The adults will overwinter in the soil, then lay eggs in the spring. A few weeks later the eggs hatch, and the grubs will search for a juicy plant to burrow into.

5. Nutrient Deficiencies Can Turn Cucumber Leaves Yellow

Cucumbers are not only heavy drinkers, but they are also heavy feeders. They will leach the soil of potassium, nitrogen, and other nutrients as they grow to lengths up to 8 feet long and spread wide. But it can be difficult to determine exactly what nutrients your cukes are needing without a soil test.

A nitrogen deficiency is the typical cause of yellowing leaves if sunlight and water are not an issue. But if you add too much nitrogen, your plants could grow large and healthy, but they won’t produce.

After a soil test is conducted and you determine that you need more nitrogen in the soil, just find a fertilizer that has the nutrients you need and add it to the soil.

Of course, you could find out that your cucumbers are lacking potassium. These plants start utilizing a lot of potassium when they start to flower. A deficiency in this mineral could manifest in yellow highlighted edges of the leaves.

To supply more potassium, add a layer of compost to the soil before you water, or find an organic, all purpose fertilizer. Look for a 4-4-4 or similar. If you have plenty of nitrogen but are lacking other nutrients, you’ll need something with a lower first number.

When looking at fertilizers, the three numbers represent NPK, or (N)itrogen, (P)hosphorus, or (K)potassium. Down to Earth Organic Vegetable Garden Fertilizer is a great, organic, all purpose fertilizer.

The final nutrient deficiency that could cause yellowing cucumber leaves is iron. Again, a soil test will be needed to see if this is the case. But if you know a lack of iron is the cause, you can supplement it with a liquid or granular product.

Iron deficiency could cause the leaves to turn yellow, but have green veins.

6. Cucumber Diseases That Cause Yellow Leaves

When you can’t find anything else wrong with your cucumbers, but the leaves are still discolored, then the last thing to consider is a plant infection. There are several viruses, fungal, and bacterial infections that can affect your cucumber plants. These include but are not limited to cucumber mosaic virus, downy mildew, and fusarium wilt.

Cucumber Mosaic Virus On Your Plants

As the name suggests, if you see yellowish mottling or a mosaic looking pattern on your cucumber leaves, it’s probably because of this virus. The cucumber mosaic virus is spread by aphids, and the only way to treat it is to remove the plant to keep it from spreading.

You’ll have to remove any plants or weeds that have this virus and you shouldn’t plant anything else that is susceptible to it in that space because it lives in the ground. These plants include tomatoes, lettuce, legumes, spinach, celery, and several flowering plants.

So far, there isn’t a way to treat this virus if it gets into the ground.

Downy Mildew On Cucumber Plants

Not to be confused with powdery mildew, which causes dusty looking, white, fuzzy patches on cucumbers and squash plants. Downy mildew will cause your cucumber leaves to have yellow patches attached to the veins, and unlike powdery mildew, the downy version is fatal to plants if it’s not treated. 

You can treat downy mildew with fungicides if it’s caught early. For plants in the advanced stages, you’ll have to remove them to prevent the spread.

To help prevent downy mildew in the first place, make sure your plants are spaced far enough apart for good airflow, or purchase mildew resistant varieties, such as Eureka, Spacemaster, and the funny looking cucamelon.

Finding Fusarium Wilt on Your Cucumber Plants

This is a fungus that gets into the plant’s vascular system. It can occur in seeds, seedlings, or established plants. While this can cause the leaves to turn yellow, the biggest indication is a normally healthy plant that suddenly becomes wilted and weak for seemingly no reason.

If you have several cucumber plants in a row, and they are all doing fine except for one that was healthy just a little while ago, you probably have a case of fusarium wilt. Another sign of this fungal infection is damping off. The otherwise healthy plant will wilt, could turn yellow, and die within 3 to 5 days.

There isn’t any treatment for fusarium wilt. You should remove any affected plants as soon as you suspect fusarium wilt. You might try to replace the soil too because the spores can linger in the ground for years.

FAQs

Don’t Let Yellow Cucumber Leaves Turn You Blue

When your cucumbers start turning yellow, they are trying to tell you something is off. Unfortunately, they can’t spell it out exactly for you, and you’ll have to do some sleuthing to find out what exactly is the problem.

First, make sure they are getting the proper amount of water and sunlight. If these are in the right proportion, check for bugs that might be bugging your plants. Next, get a soil test done to see if the cucumbers are needing the proper nutrients.

Finally, if everything else looks good, you could be dealing with a plant illness. Some can be treated with fungicides if caught early, while others will require the disposal of the plant, and possibly soil replacement.

With a little know how and some tenacity, you can make sure your cucumbers don’t turn yellow.

More cucumber guides

• What is causing holes in my cucumber leaves?
• Types of cucumber varieties
• Best companion plants for cucumbers