Here’s Why Your Cucumber Leaves Are Yellowing

Here’s Why Your Cucumber Leaves Are Yellowing

If you’re growing your own cucumbers, there’s one specific color change you should be aware of. When you notice that your cucumber leaves are yellowing, this is a sign that something is wrong with your plants.

Unfortunately, there are a lot of reasons why the leaves of your cucumber plants could be turning yellow, and you’ll need to figure out which one is the cause before implementing a fix. 

We’re going to be outlining all the potential causes of yellow cucumber leaves in this guide to help you work out what’s going on in your garden. 

Reasons For Yellowing Cucumber Leaves

Overwatering

This is one of the most common causes of yellowing cucumber leaves, but luckily, it’s also one of the easiest to remedy. 

Many of us associate unhealthy plants with underwatering, but overwatering can be just as detrimental since it stops the plant’s roots from getting enough oxygen.

This lack of oxygen can lead to wilting and yellowing in the leaves of your cucumber plants. 

If you live somewhere that gets a fair amount of rain regularly, you should only be giving your cucumber plants a couple of inches of water per week. 

If you don’t think you’re overwatering your plants, it’s also possible that an excess of water is being caused by drainage issues in the soil. 

You can test for drainage difficulties by digging a hole somewhere in the soil (make it about 12 inches deep if possible) and fill it up to the top with water.

Leave it to drain for 12 hours and fill it up again. This time, pay attention to the draining time. If it’s longer than 3 hours, your soil isn’t draining properly and you should try adding some sand to make the soil looser. 

You might also want to consider changing your cucumbers’ growing arrangement. Raised beds often drain and absorb water better than flat soil, so it’s worth considering.

You could even grow your cucumbers in pots that have drainage holes in the bottom to ensure water doesn’t accumulate beneath the surface of the soil. 

Insufficient Sunlight

If your cucumbers’ leaves look yellow and generally sickly, especially if they’re starting to droop or wilt, it could be because they’re not getting enough sunlight. 

Obviously, this can be a tricky issue to resolve because you don’t have control over the weather.

However, if you’re currently growing your cucumbers in a shaded area of your garden, you should try moving them to somewhere that gets more sunlight. 

If you simply don’t get enough natural sunlight in your garden, you can also grow cucumbers indoors. You can do this in your home or in a greenhouse as long as you have a suitable full-spectrum fluorescent grow light.

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This will simulate sunlight and should allow your plants to thrive as long as you get the timings right. 

Deficiencies 

Cucumber plants need nutrition from the soil and fertilizer in order to grow healthy and strong.

Yellowing leaves can be caused by a nutrient deficiency, and if that’s the case, you’ll need to add some nutrients to the soil to get your plants healthy again. 

The main nutrients in which cucumber plants can be deficient include zinc, iron, nitrogen and potassium. 

You can tell if your cucumber plant is deficient in iron when new leaves are yellow from the outset while the older leaves remain their normal green color. The veins in the new leaves will be green as well. 

Luckily, you can buy iron supplements for plants in spray or powder form. The powder needs to be sprinkled around the roots and the spray should be directed straight onto the plant. 

On the other hand, if your plants are zinc-deficient, the new leaves won’t be yellow. In fact, you might not see any new leaves at all since a lack of zinc slows down plant growth. However, old leaves will start to yellow.

Using a solution of zinc sulfate is an efficient way to get more zinc into your plants, but organic kelp works too.

A nitrogen deficiency is easy to spot because while new leaves will be green when they start to grow, the old leaves will start to turn yellow, specifically at the tips and the veins in the center of the leaves.

The plant will also start to look sickly, growth will slow down, and if you don’t add more nitrogen to the soil, the cucumber plant will eventually die. 

You can buy fertilizer (6-10-10) from a garden center or simply add compost to the soil around your cucumber plants. You only need to add 2 inches of compost or a couple of tablespoons of fertilizer at most. If you overdo it, you might compromise your cucumber yield.

If the compost or fertilizer seems to be working, add ammonium nitrate (just a tablespoon) to the soil, and do it again in 3 weeks. 

Finally, be on the lookout for signs of a potassium deficiency if your cucumber plants have yellowing leaves. The yellowing will be mainly localized to the edges and tips of the leaves.

Any new leaves that grow will look puckered and will be smaller than usual, and if you get any new fruit, you may notice that the cucumbers are smaller at the stem end and larger at the other. 

If you think potassium deficiency is the problem here, you can buy a fertilizer with an even balance of nutrients and apply it. It might also be worth testing the pH of the soil and buying some treatments for either alkalinity or acidity if the results show an imbalance. 

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Diseases

Just like humans, plants can catch and suffer from diseases. Yellow leaves are a crucial sign that your plant may have come down with something nasty. 

  • Downy Mildew is a disease that causes yellow patches on the tops of cucumber leaves. Eventually, the yellow spots will become brown. While several things can cause yellow spots in cucumber plants, including pests and nutrient deficiencies, one way to tell if it’s  Downy Mildew is to see where the spots are in relation to the veins. Usually, they will grow between the veins. If your growing area is humid, you’ll also see a gray, fuzzy substance that looks like mold under the leaves, hence the ‘downy’ part of the disease’s name. Downy Mildew can be kept at bay using fungicides (if you catch it early), getting rid of infected plants to stop the disease from spreading, or increasing airflow around the plants.
  • Cucumber Mosaic Virus is another plant disease that causes yellow-spotted cucumber leaves, but you can distinguish this disease from Downy Mildew because the leaves will also be curving downwards. There is no effective way to treat plants infected with this virus, so you will need to remove sick plants to minimize the spread since pests can carry the virus from plant to plant. Unfortunately, you may also need to treat or change out your soil since the illness can stick around here and affect next year’s crops.
  • Fusarium Wilt, a type of fungus, is identifiable by yellow coloring that spreads from the edges of cucumber leaves towards their centers. It’s caused by cucumber beetle larvae and there’s no treatment as of yet, which means you need to remove infected plants immediately. Various chemicals can be applied to control the presence and spread of Fusarium Wilt, but all of the cultural controls are preventative since, again, there’s no treatment. Be aware that this is another disease that can live in the soil for a long time. 

Insects

Insects are something all gardeners have to deal with at some point, but some species can be very detrimental to the health of your cucumber plants. 

  • Potato leafhoppers like to feed on the leaves of various plants, including cucumber plants. Part of the feeding process for these pests includes injecting toxins into the leaves while they suck out the sap, and the toxins are what can cause the yellowing. If your plant’s leaves are turning yellow and then falling to the ground, potato leafhoppers might be to blame. Since you don’t want to spray insecticides all over your edible plants, we recommend putting up some row covers to stop the insects from getting to the leaves. De-weeding your garden might also be helpful since potato leafhoppers tend to prefer weedy areas. 
  • Aphids also love to feed on cucumber plants, and they can be difficult to spot since they hide under the leaves rather than on top. They also blend in due to their yellow or green color, but you’ll be able to spot the honeydew they secrete because it’s black and sticky to the touch. 
  • Whiteflies are very small and white, so they’re another garden pest that can fly under the radar. However, you’ll be able to see them fly into the air if you shake some leaves. This is another insect species that produces honeydew, so you can potentially identify an infestation this way, too. 
  • Spider mites will cause yellow spots on your cucumber leaves rather than an overall yellowing. Check the undersides of your cucumber leaves if you want to be sure whether these pests are responsible for the yellowing since this is where they typically hide. If the insects themselves have departed, you might still be able to see their telltale webs, which are thin and silver-colored. 
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All of the three insects we just mentioned can be dealt with by applying insecticidal soap. Just make sure you apply it directly to the leaves rather than the fruit since you don’t want to ingest chemicals when it comes time to eat your cucumbers. 

Yellowing Cucumber Leaves: The Science

It can often help to understand the science behind the color changes that happen in your plants, especially if you’re serious about gardening. 

The green color of cucumber leaves is the result of a chemical called chlorophyll. You might also see this referred to as chlorosis. 

This chemical is a key part of the process of photosynthesis, so when you see that your cucumber leaves are losing their green color and turning yellow, it’s a sign that the levels of chlorophyll are decreasing, and this could impact your plants’ ability to get energy from sunlight, compromising their health. 

Final Thoughts

We hope that our guide has helped you to identify the cause of your yellowing cucumber leaves. 

Please remember that harsh chemical treatments should be a last resort when it comes to salvaging plants you’re planning to eat.

If you do need to apply a chemical product to deal with pests, try to get it directly on the leaves and avoid getting any on the fruit. 

Prevention is more effective than a cure in all gardening matters, so it’s always best to ensure you have healthy, well-draining soil, protective barriers, and good natural light levels before you plant cucumbers.