Why Your Cucumber Leaves Are Turning Yellow and How to Fix Them

Homegrown cucumbers are amazing; growing cucumbers is a challenge. Most gardeners struggle with cucumber leaves turning yellow at some point. Figuring out why your cucumber leaves turned yellow isn’t easy, but if you can solve the mystery, you can often fix yellow cucumber leaves and save your harvest.

Why Do Cucumber Leaves Turn Yellow?

Cucumber-plant

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“Chlorosis” is the technical term for what’s happening. Chlorosis occurs when chlorophyll, an essential ingredient for photosynthesis, is in short supply. Because the plant’s ability to photosynthesize is affected, it struggles to produce enough sugar to feed itself.

Yellowing leaves can be caused by: 

  • Pests
  • Improper watering 
  • Insufficient lighting 
  • Disease
  • Poor nutrition 

Ultimately, a cucumber plant with yellow leaves is stressed and needs attention. 

So how do you figure out what is causing the problem? 

A Diagnosis Guide for Cucumber Leaves That Are Turning Yellow

Here are five factors to consider:

1. Are There Any Insects on the Leaves?

Perform a close inspection. Be sure to check the bottoms of leaves, especially those that are curling.

Pests that suck the sap from the leaf can cause leaves to yellow. Aphids, potato leafhoppers, spider mites, and whiteflies are all common pests that do this. 

A visual inspection will often reveal if these insects are the source of your problem. Aphid colonies can be quite significant, making them easy to spot. 

2. Are You Over or Under Watering?

When it comes to watering, cucumbers are a bit like Goldilocks. They love that sweet spot in the middle. When cucumbers receive too much water, leaves will yellow, become limp, and may fall off. Too little water and they’ll quickly crisp up and die.

To test moisture, stick your finger into the soil up to the first knuckle. The soil should be moist, but not waterlogged

Ideally, a cucumber plant should receive one to two inches of water per week. 

3. How Many Hours of Sunlight Does Your Cucumber Get?

Cucumbers are sun worshippers. They need a minimum of six hours of direct sunlight per day. If you are growing your cucumbers in pots, moving your plants to a brighter location may be an option.

If you’re growing under artificial lights, you’ll need between twelve and sixteen hours as grow lights aren’t nearly as powerful as the sun. 

4. How to Identify Diseased Cucumber Plants

A number of diseases can also cause cucumber leaves to yellow. The most likely diseases to affect cucumbers are: 

Mosaic Virus 

Cucumber mosaic, caused by Cucumber mosaic virus (CMV)“Cucumber mosaic, caused by Cucumber mosaic virus (CMV)” by Plant pests and diseases is licensed under CC0 1.0

  • Symptoms: Leaves wrinkle, curve down, have yellow spots, and are brittle.  
  • Causes: Fungus lives in the soil and is spread by insects like leafhoppers and aphids.

Verticillium Wilt 

File:Verticillium wilt on hops.jpg“File:Verticillium wilt on hops.jpg” by David Gent is licensed under CC BY-SA 4.0

  • Symptoms: Stems streak brown or tan. Lower leaves wilt, die, and dry up. Often only one side of the plant is affected. 
  • Causes: The pathogen lives in the soil, often in beds where nightshades were grown previously. It can live in the soil for up to a decade.

Fusarium Wilt 

Tomatoes with Fusarium Wilt 2013“Tomatoes with Fusarium Wilt 2013” by F. D. Richards is licensed under CC BY-SA 2.0

  • Symptoms: Some leaves will yellow while others will become a dull, gray-green. Dark brown spots on the lower portion of the stem. 
  • Causes: Fungus lives in the soil and is spread by cucumber beetles.

In the case of all three of the above diseases, it is recommended to remove affected plants to prevent the spread of the disease.

Powdery Mildew 

powdery-mildew-cucumber

  • Symptoms: White, powdery spots that form on the leaves and stems of cucumbers. 
  • Causes: High humidity, prolonged leaf wetness, cool temperatures, and overcrowding.

 A plant can often survive a mild case of powdery mildew without fruit yields being heavily affected. 

Downy Mildew 

Downy mildew of collard (Brassica oleracea) caused by Peronospora parasitica“Downy mildew of collard (Brassica oleracea) caused by Peronospora parasitica” by Plant pests and diseases is licensed under CC0 1.0

  • Symptoms: Leaves have yellow spots which later turn brown. Sometimes, gray fuzz is present on the undersides of leaves. 
  • Causes: High humidity, prolonged leaf wetness, cool temperatures, and overcrowding.

Treatment for mildew-based problems includes improving air circulation by cutting away damaged leaves and avoiding overhead watering. 

5. How to Identify Mineral Deficiencies in Cucumbers 

Cucumbers are heavy feeders, and they will be the first to complain if they aren’t receiving the minerals they need. 

Often, you can tell if a plant is nutrient deficient because the entire plant will be pale green. Bottom leaves turn yellow. Growth is slow, and plants are stunted. 

Cucumbers require higher levels of nitrogen and potassium than many other plants. Adding a couple of inches of high-quality compost around the bases of plants is an excellent solution.  Store-bought fertilizers can also be used to give N-P-K levels a boost.

If you know exactly which minerals your plants are missing, you can administer very specific treatments to increase the levels of iron, zinc, potassium, magnesium, and so on.

For example, one tablespoon of Epsom salts in a gallon-sized watering can will replenish soil with magnesium and sulfur — two essential minerals.  It can be applied as a foliar spray or to the base of the plant. Many gardeners swear that this concoction is the key to their cucumber success.

To find out which minerals are lacking, usually a soil test is needed. These can be performed in a lab for a reasonable price, and they can be a valuable diagnostic tool for the home gardener to access.

What to Do With Yellow Cucumber Leaves

If you suspect the cause of the yellowing is disease, cutting off yellow cucumber leaves will help prevent the disease from spreading. Pruning also improves air circulation, which helps combat fungal illnesses. 

If all of your leaves are showing signs of yellowing, consider removing only the worst affected leaves and leaving the better-looking ones behind. Leaves are the solar panels of your plant, and your plant needs them to turn sunlight into the sugars that feed them.

Figuring out why your cucumber leaves have turned yellow can be an adventure full of visual inspections, research, and no small amount of guess-and-test experimentation. It can be a finicky process.

Yet often the most difficult problems are our greatest teachers. It all seems worth the effort when you are enjoying the bountiful harvest of your healthy cucumber plants. 

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