Why Are My Cucumber Plants Turning Yellow? 7 Causes (and Solutions!)

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Yellow Cucumber Plants

“Why are my cucumber plants turning yellow?” This is a common question among gardeners, so don’t worry- you’re in good company in wondering about this!

Your yellowing problems could be due to several reasons:

  • Lack of sunlight
  • Harmful insects
  • Insufficient nutrition
  • Too much water or not enough
  • Plant diseases

In this article, we’ll cover each one individually, along with the appropriate course of action to get back the green!

Ready? Let’s get started!

RELATED: Cucumbers aren’t the only plants that can take on a yellowish hue. Visit our post on Mint Leaves Turning Yellow to find out more!

1. Not Enough Sunlight

A healthy plant’s vibrant green color is due to chlorophyll. This substance absorbs sunlight and converts it into plant-friendly energy. 

If your plants aren’t getting enough sunlight, chlorophyll can’t do its job. As a result, you’ll see yellow leaves, stunted growth and minimal to no blossoms or fruit. 

Solution: Provide sufficient sunlight. Cucumbers are a sun-loving crop, and most varieties like to have at least 6 hours of direct sunlight daily. 

If you’re growing your cucumber plants in containers, try moving them to a sunnier spot. For in-ground gardens near a shade tree or dappled sunlight, trim as many overgrown branches as you can. 

If you can’t remedy the light situation, you may have to accept that you’ll have a small harvest this season.

But there’s always next year! If you have it available, choose a sunnier location for next summer’s crop of cucumbers.

2. Aphids, Spider Mites and Whiteflies

These little pests have ravenous appetites for your plants’ nutritional sap. Due to their small size, these insects can easily go undetected until you have a serious problem.

Aphids are oval-shaped insects that leave a trail of yucky black waste in their wake. 

Thanks to their green coloring and tendency to live on the underside of leaves, they can be hard to spot. 

Spider mites are reddish insects that often show up in large numbers.

They get their name from the delicate, thready nests they weave on the underside of plant leaves.  

Whiteflies are another type of pest that can severely damage your plants when they form a large community.

They are winged insects that have a white body.  

Solution: Apply insecticidal soap. Whether your problem is due to aphids, spider mites or whiteflies, you can usually correct the situation if you catch it early. These insects won’t go away on their own, and an infested plant will eventually die. 

Spraying the affected plants with insecticidal soap is the most effective treatment. 

Commercial formulas are readily available online or at big box stores. This one from Safer Brand is a good option. 

Make sure to follow the package directions for how much and how often to apply the product. 

You can also make DIY insecticidal soap. The process is easy, inexpensive and gives you more control over the substances you put on your cucumbers.

During application, be careful to spray both the top and the underside of the affected leaves. Especially if you have a heavy infestation, you may need to apply insecticidal soap weekly or bi-weekly. 

Depending on the severity of the infestation, some leaves may be damaged beyond repair. Cut these leaves off and discard them. 

3. Southern Corn Rootworm

These little pests are spotted cucumber beetles in larva form, and they make their home in your garden soil. 

Southern corn rootworms eat through plant stems at ground level, thus slowing growth and turning foliage yellow. 

Solution: Pull affected plants and turn the soil regularly. Unfortunately, there isn’t much you can do about the plant damage once it has occurred. Remove affected stems and leaves and discard them. 

Since southern corn rootworms live in the ground, you can try to loosen the soil around your cucumber plants. However, it’s unlikely that you’ll see much improvement in your current crop. 

The best way to deal with southern corn rootworms is to interrupt their life cycle early in the spring.

Make it a point to thoroughly cultivate your garden before you sow seeds. This is especially critical if you’ve had past problems with spotted cucumber beetles in the past year or two. 

4. Potato Leafhoppers

As the name suggests, these insects usually attack potato plants. But they also won’t pass up the chance to suck the sap from cucumber plants as well. 

A green potato leafhopper sits on the edge of a plant leaf.
Potato leafhopper

To make matters worse, potato leafhoppers leave behind a toxic substance that leads to yellowing and leaf drop. 

Solution: Don’t overcrowd your garden and use insecticides carefully. Crowded vegetable patches can make an inviting home for potato leafhoppers. So remove as many weeds as possible from your cucumber plants. 

Commercial insecticides can also rid your cucumbers of potato leafhoppers. But these chemicals are often harsh and can contaminate your food, so you may want to proceed with caution.

If you’d prefer a more natural insecticide, this formula could be a good choice. But you’ll still need to follow the package directions carefully. 

5. Nutrient Deficiencies

If your cucumber is low on essential nutrients, it won’t be able to produce vibrant green foliage (or a good harvest.)

Potassium, phosphorus and nitrogen are the three main nutrients most plants need to thrive, and cucumbers are no exception. But cucumbers prefer a nutrient balance that’s a bit different than many other vegetables.

Relative to other plants, cucumbers require high amounts of both phosphorus and potassium to produce blossoms that result in fruit. And while they definitely need some nitrogen, cukes don’t require as much as many other plants.

In fact, using too much nitrogen has a couple of potential downsides:

  • Your cucumber may go wild in producing excessive foliage but no fruit. 
  • All that extra nitrogen can make its way into the groundwater and become a contaminant. 

Solution: Fertilize carefully. Ideally, you should have your soil tested before applying any additional fertilizers. But if you can’t, these guidelines are a good general rule of thumb.

Many pre-made fertilizers are rich in nitrogen. Look for a formula with higher levels of potassium and phosphorous than nitrogen.

Most commercial formulas list their ingredients in this order: Nitrogen, phosphorous, potassium. A fertilizer with a 5-10-10 label should be about right.

If you prefer an alternative to commercial fertilizer, compost is perfect. Naturally low in nitrogen, compost feeds your cukes with the nutrients they need. 

Bonus: Compost can also act as a light mulch. 

RELATED: We go into a lot more detail in our post about using compost as mulch. Stop by to learn more!

6. Improper Soil Moisture

Here’s the thing with this issue: Yellow leaves could be due to either overwatering or under-watering. 

If you let your cukes get too dry, they will respond with wilted, droopy stems and leaf discoloration. 

Perhaps you got a little overzealous with their watering. If so, yellow leaves can be the result of the water-logged soil blocking roots from absorbing necessary nutrients. 

Solution: Adjust your watering routine. So how much moisture is just right?

Typically, cucumbers grow best with about 1 to 2 inches of water every week. Remember to account for rainwater in your total moisture tally!

Before watering, check to see how moist the soil feels. Stick your finger into the soil, and if you don’t feel any moisture about 1 inch deep, give your cucumbers a drink. 

If your cucumbers are too wet, water them less frequently or use smaller amounts.

Are they too dry? Water more often or give a larger drink at each session.

7. Viral, Bacterial or Fungal Diseases

Unfortunately, plant diseases happen to the best of us. And if you don’t act quickly to address the problem, you risk losing your entire harvest. 

Here’s a closer look at the diseases that might be responsible for your cucumber’s unhealthy-looking foliage. 

Mosaic Virus

This viral disease leaves your cukes with yellow spots in a random, mosaic-like pattern.

A leaf with yellow mosaic virus on it

Aphids and certain weeds are usually to blame for spreading the virus.

Depending on when your plant gets infected with mosaic virus, it may produce no fruit at all or very little. Any cucumbers you do manage to grow may be small, misshapen or have yellow spots of their own.

Solution: Pull affected plants. There are no chemical treatments available for mosaic virus. Pull and destroy any infected plants, and make sure to thoroughly clean any tools you use in the process.

To prevent mosaic virus, keep your cucumber patch free of weeds.

Also, keep an eye out for aphids. If you notice a few of these little guys, apply insecticidal soap.

Bacterial Wilt

In the early stages, bacterial wilt causes your cucumber leaves to wilt down during the day but perk up at night. It’s all too easy to mistake this disease as a simple response to heat. 

After a time, though, the leaves will start to take on a yellow color at the leaf perimeter. Eventually, the leaves will turn brown and the plant will die. 

Spotted cucumber beetles are the prime spreader of this bacteria.

Solution: Pull affected plants. No chemical treatments are effective against bacterial wilt. You have no choice but to pull and destroy your infected plants. 

The best way to prevent bacterial spread is to keep your numbers of cucumber beetles as low as possible. 

Downy Mildew

When downy mildew takes hold, your cucumber leaves will take on a mottled yellow appearance. Infected leaves will eventually stunt your plant’s growth and fruit production.

A closeup shot of a patch of downy mildew on the back of a green plant leaf.

This disease is commonly considered a fungus, but it actually belongs to a family of organisms called oomycetes. (There’s a random piece of fun gardening trivia to wow your friends with!)

Oomycetes thrive in damp conditions, and downy mildew often strikes in areas where leaves are wet or areas with high humidity.

Solution: Keep moisture off leaves and don’t overcrowd your plants. Don’t overwater your cucumbers, and avoid splashing leaves with water.

Also, taking steps to encourage good airflow can help keep damp conditions at bay. Leave plenty of space between your cucumber plants, and make sure to keep your cucumber patch weed-free. 

Fusarium Wilt

This disease is a fungal growth that slows plant growth and also turns leaves yellow-brown. 

Holding some leaves that have Fusarium Wilt on them

Certain insects, mainly spotted cucumber beetles, carry Fusarium wilt spores from plant to plant. The spores also live in the soil, where they can remain dormant for up to two years.

Solution: Destroy affected plants. Once a plant becomes infected with Fusarium wilt, you can’t save it. As soon as you identify Fusarium wilt, pull the plant out before the disease can spread.

Also, be careful in how you dispose of the infected plant. Do not add it to the compost pile or allow it to sit.

If you can, burn the plant. Your next best option is throwing it out in the garbage.

Note: Even experts can sometimes have a hard time determining which disease a plant may have. So taking your time in doing thorough research is a wise move.

Bonus Tip: Use a Trellis. Growing your cucumbers on a trellis can help reduce your plant’s risk for disease in several ways:

  • Keeps leaves off the ground
  • Promotes greater airflow
  • Can help you spot harmful insects
  • Gives you a better view of your plants so you can identify issues early on
  • Makes it easier to water appropriately

Pre-made gardening trellises are widely available in a variety of sizes. The downside is that they can be pricey, especially if you want a large size or a heavy-duty material.

Not to worry: Make your own! Check out this awesome video from CaliKim for step-by-step instructions.

Frequently Asked Questions about Yellow Cucumber Leaves

If your cucumber plant has only a few yellow leaves and you’re confident that a communicable plant disease isn’t behind the discoloration, you can safely prune the affected leaves off. 

Especially if the yellowing is due to sunlight, watering or nutrient deficiency, the leaves will fall off on their own.

Cucumbers need a total of about 1 to 2 inches of water every week, whethe it’s you or Mother Nature doing the watering. 

Since that can be hard to measure, use how the soil feels as your guide for watering frequency. Give water when the soil feels dry about 1 inch below the surface. 

Eating a yellow cucumber likely won’t hurt you, but you probably don’t want to try it.

Cucumber fruits turn yellow when they’re lacking the proper growing conditions or haven’t ripened appropriately. This leaves them with a very bitter and unappetizing taste. 

Final Thoughts

Yellow leaves could be due to a number of causes. Determining the specific one could help save your harvest or at least plan ahead for nest year. 

To recap:

  • Cucumbers love the sun, so make sure your plants get plenty of it. 
  • Keep a sharp lookout for pests and take action immediately. 
  • Meet your cucumbers’ nutritional needs with plenty of phosphorous and potassium. 
  • Ensure your cucumbers get about 1 to 2 inches of water weekly. 
  • Observe yellowed leaves to figure out if a plant disease might be to blame, and consider growing your cukes on a trellis. 

It’s no fun when the cucumber plants you’ve worked so hard to nurture start taking a downturn. But with a little diligence and know-how, there is hope!

Did you find this information helpful? Do you have any experience or thoughts you’d like to share?

Post them in the comments!

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