Cucumber Leaves Turning Yellow – 5 Causes and Solutions

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Healthy cucumber leaves should be a bright, solid green. When your cucumber leaves turn yellow, it is a sign of a problem with the plant.

In this article, I’ll share 5 common reasons for cucumber leaves turning yellow. For each of the issues, I’ll include some options for fixing the issue and returning your plants to health.

Yellowing leaves on cucumber plant caused by downy mildew.

1. Nitrogen Deficiency

All plants use nitrogen to produce chlorophyll, giving leaves and foliage their rich green color. Without enough nitrogen available in the soil, cucumber leaves will begin to go pale and yellow. Nitrogen is the most commonly deficient nutrient in soil, since plants use so much of it.

If your plants lack nitrogen, the lower leaves will turn yellow, first. Over time, the yellowing will progress upwards, eventually leading to dying leaves.

Cucumber seedlings in pots
Pale, yellowing cucumber leaves can be caused by low nitrogen.

Solutions:

  • Feed with water-soluble fertilizer immediately
  • Amend soil with compost in spring and/or fall

Another, less common nutrient deficiency is a lack of magnesium. Plants use less magnesium than nitrogen, but the symptoms are similar.

With a nitrogen deficiency, the entire leaf will turn yellow. However, in the case of low magnesium, the veins of leaves typically remain green, while in between turns yellow (interveinal chlorosis).

Either way, the best solution is to apply an all-purpose fertilizer to ensure your cucumber plants have every nutrient they need. We prefer to use organic and slow-release types to avoid over-feeding or burning our plants.


2. Downy Mildew (Fungal Disease)

Here in New England, we often see our cucumber plants get downy mildew. This fungal pathogen is also known as water mold, and it thrives in cool, damp, and humid conditions. Once it appears it can spread quickly throughout a plant’s foliage.

Infected leaves will usually have yellow spots, often with hard edges to them. If you look under the leaves, you will often see dark purple mildew, which is where the spores are produced and spread.

Cucumber foliage with yellowing leaves from downy mildey
Cucumber leaf showing signs of downy mildew.

Solutions:

  • Leave more room between plants for better airflow
  • Avoid watering over the tops of leaves, water around the base of the plants instead
  • Apply a fungicide to reduce the spread

Since this fungus loves damp and stuffy conditions, it is best to avoid watering over the tops of your cucumber leaves. This will only help it spread to other parts of the plant. Instead, always water at the base of the plant.

Also, consider thinning out your cucumbers if they are planted too closely. If they are climbing, I recommend planting cucumbers 12-18″ from each other. When left to sprawl along the ground, give each plant 36″+ (not ideal).

Removing affected leaves and spraying with an appropriate fungicide can help slow the spread. While it is rare to stop it completely, you can prolong the life of your cucumber plant, increasing your overall harvests.


3. Cucumber Mosaic Virus (CMV)

Cucumber mosaic virus is an important plant virus in North America, affecting a wide number of plant species. However, it got its name from a common host: cucumbers.

There are many mosaic viruses and they are named for the mottled discoloration it causes on plant foliage. Cucumber leaves will begin to show spots of yellow and light green, often in a mosaic-like pattern.

Cucumber mosaic virus is spread by aphids. These sap-sucking pests will often pick up virus particles from nearby weeds. If they move on to your cucumber plants, the virus is transmitted into the plant’s tissue.

Solutions:

  • CMV is not curable, so if the plants are infected, it is recommended to remove the plant entirely
  • Monitor for aphids and control populations with insecticidal soap
  • Plant CMV-resistant cucumber varieties

My best advice is to remove the plant from your garden to avoid further spread. Like I said, many other species are susceptible to CMV, so your peppers, spinach, flowers, and many other plants are at risk.

To further reduce the spread, do what you can to reduce aphid populations. Plant more flowers to attract beneficial insects that eat aphids, plant “sacrificial” plants nearby your garden, and always monitor closely for pest activity.


4. Over-Watering

Too much water can lead to poor root development in many vegetable plants, including cucumbers. If your soil does not drain well, you should work on improving it.

Compost is a great amendment for both sandy and clay-heavy soils. It helps improve aeration, drainage, moisture retention, and nutrient retention. Work compost into the first 2-3″ of soil in the early spring after the soil thaws.

If you are growing in containers, make sure they have drainage holes on the bottom. If a drainage hole becomes plugged, next time place a small rock over the hole before filling the pots with soil.

Solutions:

  • Check soil for dryness before watering
  • Improve soil drainage
  • Amend soil with compost each spring

Under-watering can potentially cause yellow cucumber leaves, too. Thankfully, cucumber plants are resilient and bounce back quickly after drying out. If you notice leaves wilting, it is likely due to under-watering, so give the plants a drink!


5. Pest Damage

As I mentioned, aphids serve as a vector for some viruses. However, their feeding habits alone can cause damage to cucumber plants. There are also a wide variety of pests that can cause cucumber leaves to turn yellow.

Cucumber pests:

With mosts pests, you are likely to see holes in your leaves and/or fruits. Around these bite marks, the leaves may turn yellow or brown. Be sure to check for the presence of pests regularly, especially underneath leaves, on new foliage growth, and in flowers.

Cucumber seedling transplanted into raised bed
Cucumber seedling in raised bed.

Solutions:

Planting a variety of flowers near your vegetable garden is an excellent way to improve its overall health. Flowers attract a greater diversity of insect life, including both pollinators and predatory insects.

Closeup of hover fly on alyssum
Sweet alyssum attracts hover flies, a common aphid predator.

Common Cucumber Questions

Finding yellow cucumber leaves can be a bit disheartening, so let’s go through some common questions and answers on the topic:

  • Should I remove yellow leaves? If your plants are diseased, it is best to remove the affected foliage to slow down the spread. This is especially true with fungal pathogens that produce spores to reproduce.
  • Will yellow leaves turn green again? Cucumber leaves that have turned yellow will not turn back to green. However, any new leaves should grow in green if the root cause was addressed.
  • Are my cucumbers still edible? If your cucumbers have visible lesions from disease, I don’t recommend eating them. However, you can likely save fruits from infection if you pick them promptly and slow the spread disease.
  • Read more cucumber plant FAQs
Cucumber growing on vine
Healthy cucumber growing on plant.
  • Which fertilizer is best for cucumbers? Cucumbers are heavy-feeding plants, preferring a well-drained, nutrient rich soil. If you plan to fertilize, use a balanced all-purpose fertilizer like Garden Tone for raised beds, or Miracle Gro Organics for potted plants. For a quicker nutrient uptake, use a water soluble formula like Miracle Gro plant food or slow release Ozmocote.
  • How far apart should I plant cucumbers? Plant cucumbers at least 12-18″ (30-46 cm) from each other, and give the plants something to climb. Cucumber plants grow like a vine, meaning they can be trellised and grown vertically. This allows the plants to be planted closer, while sprawling ground plants need 36″ (91 cm) or more between plants.
  • Should I prune cucumbers? Pruning cucumber sucker shoots can help provide better airflow around the plants, reducing the chances of some diseases. It can also encourage larger fruits and a longer productivity period.
  • What are the best cucumber companion plants? Cucumbers are climbing plants, so they don’t take up much space in the garden bed. Some of our favorite plants to grow near cucumbers are garlic and onions, tomatoes, peppers, carrots, thyme, and a wide variety of flowers.

I hope this article helps you figure out why your cucumber leaves are turning yellow. Growing cucumbers can be tricky with the numerous diseases and pests that can find them. Next time, try planting disease-resistant varieties!

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