Why Cucumber Plants Turn Yellow (And What To Do About It)

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Cucumbers are widely believed to be one of the easiest vegetables to grow in a home garden. Don’t let their reputation fool you, though–while cucumbers can be low maintenance, they can also develop frustrating problems. One such problem is yellowing leaves. 

If your cucumber plants are turning yellow, the most likely causes are disease or nutrient deficiency. Other possible causes are pests, insufficient space between plants, or low temperatures.

Some of these problems don’t have easy short-term solutions, but it’s important to understand the underlying cause so that you can prevent long-term issues.

Diseases

Cucumbers, along with pumpkins, squash, and melons, belong to a family of vegetables called cucurbits. Cucurbits are susceptible to many of the same diseases. If you have previously lost a cucurbit to disease, avoid planting another cucurbit on the same site.

The following diseases can cause cucumber plants to yellow:

• Cucumber mosaic virus (CMV)
• Aster yellows
• Bacterial leaf spot
• Downy mildew
• Cucurbit yellow stunt disorder virus (CYSDV)

Cucumber Mosaic Virus (CMV)

Cucumber mosaic virus is spread from plant to plant by aphids, garden tools, or other mechanisms. It first appears as yellow mottling on the leaves, and symptoms progress to the following:

• Stunted growth
• Leaf curl
• Discolored flowers
• Small, deformed, and discolored fruit

Unfortunately, once your cucumber plants have been infected with CMV, your best option is to uproot and destroy them. Since it is a viral disease, no amount of fertilizer or chemical controls will save your plants.

You can prevent future outbreaks of CMV by planting resistant varieties like “Pacer” and “Slicemaster,” and by controlling aphid populations. Reflective mulches, insecticidal soaps, and mineral oils offer your plants some protection against large numbers of aphids (source).

Aster Yellows

Caused by a phytoplasma, the primary symptom of aster yellows is exactly that:  yellowing foliage. Other symptoms include:

• Rigid upright growth
• New leaves are small and distorted
• Disfigured flowers
• Small, pale fruits

Like CMV, there is nothing you can do once aster yellows sets in. Remove and destroy the infected plants to limit the disease’s spread.

The phytoplasma that causes aster yellows is spread by leafhoppers. You can control the leafhopper population by controlling weeds and protecting your cucumber plants with row covers if necessary (source).

Bacterial Leaf Spot

The first sign of bacterial leaf spot is the appearance of lesions on cucumber leaves’ undersides. Before long, the lesions develop into yellow patches on the leaves’ surfaces. 

Other symptoms of bacterial leaf spot include:

• Translucent patches near the center of the lesions
• Yellow “halos” surrounding the lesions

Bacterial leaf spot is caused by the bacteria Xanthomonas campestris, which infects seeds before they are even planted. The bacteria also lingers in the soil where infected seeds were planted.

Again, if your cucumber plants are affected by bacterial leaf spot, it will be best to remove them. Replant in a different site using disease-free seeds. Avoid planting any other cucurbits in the same location for at least two years, since they are susceptible to this disease as well.

Downy Mildew

A fungal disease, downy mildew, first appears as yellow spots on the surface of leaves. Don’t confuse downy mildew with powdery mildew–powdery mildew appears as white spots on the leaves instead of yellow.

You will also notice the following symptoms if your plant has a downy mildew problem:

• Fluffy, purple-gray mildew on the leaves’ undersides
• Extensive yellowing of foliage
• Eventual browning of leaves, which resembles frost damage

The fungus that causes downy mildew thrives in cool, humid environments. You can prevent it by waiting to plant your cucumbers until outdoor temperatures are consistently warm. Allow plenty of space between plants for the air to circulate.

You may be able to apply fungicide to kill downy mildew before it takes over your cucumber plants. Consult a local expert to learn which fungicides are available in your area or use a natural fungicide spray such as Grower’s Ally Fungicide (link to Amazon) which is OMRI listed for use in organic gardening and uses food-grade citric acid.

Cucurbit Yellow Stunt Disorder Virus (CYSDV)

Cucurbit Yellow Stunt Disorder Virus was discovered recently and affects all plants in the cucurbit family. So far, the virus has been detected in several states as well as Mexico and countries that border the Mediterranean Sea. 

Symptoms of Cucurbit Yellow Stunt Disorder Virus in cucumber plants include the following:

• Yellowing leaves with green veins
• Leaf roll
• Stunted growth
• Reduced yields

These symptoms greatly resemble nutrient deficiencies. If your plant has a true nutrient deficiency, it will respond to fertilizers; plants infected with CYSDV will not (source).

If you suspect your cucumbers have been infected with CYSDV, remove and destroy them. Do not plant another cucurbit in the same location. 

CYSDV is spread by whiteflies, which exist all across North America, so controlling the whitefly population is your best defense. Ask a local expert about the insecticides available in your area that may be effective against whiteflies (source).

Nutrient Deficiencies

If disease doesn’t seem to be the problem, your cucumber plants may be turning yellow due to a nutrient deficiency. Fortunately, this is a very easy problem to correct.

Nitrogen Deficiency

The most common sign of nitrogen deficiency is extensive yellowing of leaves followed by an overall yellowing of the entire plant.

You can apply a liquid fertilizer to the soil near the base of your plants. Make sure to buy a fertilizer that is specifically formulated for vegetables. Follow the manufacturer’s recommendations regarding how often to use it. 

At the end of the growing season and again at the beginning of the next growing season, mix organic matter like compost into your soil to increase your soil’s nitrogen levels. Every three years or so, test your soil’s nutrient levels and adjust your fertilizers accordingly (source).

Iron

Iron deficiencies also cause yellowing, but the leaves’ veins will remain green. Iron deficiencies are most common in areas where the pH level of the soil is above 7.0 (alkaline). 

Before you add iron to the soil, have your soil tested. If your soil’s pH level is high, look for fertilizers that are designated for use in alkaline soils.

Again, combining compost with your soil at the beginning and end of the growing season is a good way to improve the overall quality and nutrient content of your soil.

Potassium

Unlike other nutrient deficiencies, potassium deficiencies cause yellowing around the edges of the cucumber leaves, as well as leaf puckering and poor fruit development.

Apply a balanced liquid fertilizer to the base of your plants, following the manufacturer’s instructions. Again, compost and soil testing are necessary to improve the long-term quality of your soil.

Test your soil and get specific recommendations for addressing deficiencies. Click here to learn more (link to SoilKit by AgriTech).

Aphids

Only one cucumber pest causes yellowing:  aphids. Since aphids can also spread disease, controlling their populations is important.

Aphids live on the undersides of leaves and have small, soft bodies. They are often green or yellow, and when the population is small, cucumber plants can tolerate them.

However, in large numbers, aphids’ feeding causes yellowing leaves and necrotic spots on leaves and shoots. They excrete a substance called honeydew which creates problems with sooty mold.

If your aphid infestation is fairly small, you can prune the affected leaves and stems or remove them by spraying the affected leaves with a strong hose. Larger populations may require an insecticidal soap; neem and canola oil both work well. 

Prevent future aphid infestations with reflective mulches. Check for infestations before transplanting seedlings.

Cultural Problems

If disease, deficiency, and aphids aren’t the culprits, your plants may have cultural problems that are causing them to turn yellow.

Insufficient Space

If you have multiple cucumber plants in close proximity to each other, they compete with each other for nutrients. Your soil test may indicate that there are enough nutrients in the soil, but when plants compete, neither one gets enough.

Crowding can also decrease the amount of sunlight each plant gets. If this is the case, the leaves near the bottom of your plants will turn yellow due to a lack of sunlight. If the yellowing is limited to the bottom layers of foliage, this is not cause for major concern.

Cucumber transplants need between eight and fifteen inches of space. This will allow enough space for their root systems as well as enough airflow between plants.

Temperature

Cucumbers need warm weather to thrive. They prefer temperatures above 70℉, even at night. 

If your cucumber plants are struggling, and the temperatures are not yet consistently above 70℉, they may continue to struggle. Adding mulch or landscape fabric can help regulate soil temperatures. Beyond that, be patient.

In the future, wait to plant your cucumbers until the soil temperature is above 65℉ and the air temperatures are above 70℉ (source).

Conclusion

While cucumbers are generally low-maintenance garden vegetables, they are not immune to problems. If your plants are turning yellow, investigate further. Yellowing is always a sign that your plant needs some extra attention and care.