Cucumber Leaves Turning Yellow | How to Fix

Many avid gardeners appreciate the presence of cucumber plants in their gardens because their fruit is produced all summer long, can be eaten raw and pickled, and the plants are generally easy to maintain. But cucumber plants can be susceptible to stress, particularly from nutrient deficiencies, pests, and diseases. And if not rectified quickly, cucumber plants can have a difficult time reviving themselves.

Watch for leaf yellowing and stunted growth, which are great first indications of an issue. The following article describes the causes of cucumber leaves turning yellow and how to fix it to still obtain a harvest that season. 

Why Do Cucumber Plant Leaves Turn Yellow

The yellowing of leaves is called chlorosis, and it is a natural process that occurs when environmental factors, such as nutrient deficiencies, overwatering, or disease, have caused an issue with the growth of the plant. Chlorosis disrupts the chloroplasts that keep the leaves green and photosynthesize, and the plant grows and fruits.

Without chloroplasts, the leaves will wither, and the plant will die from a lack of photosynthesis.

The environmental factors that are the most common causes of cucumber leave turning yellow are nutrient deficiencies, watering issues, lack of sunlight, pests, and diseases. 

Diseases in Cucumber Plants

Among the endless number of diseases that can plague a simple garden, cucumber plants are quite susceptible to several serious conditions including Cucumber Vein Yellowing Virus (CVYV), Cucumber mosaic virus (CMV), and Bacterial Wilt.

Cucumber Vein Yellowing Virus (CVYV) and Whiteflies

Cucumber Vein Yellowing Virus (CVYV) is a cucumber-specific virus that is transmitted by the Silverleaf whitefly, Bemisia tabaci, which is known to farmers as being one of the worst agricultural pests from across the globe. While individual whitefly can be carrying the virus, it typically takes a population of 20 whiteflies or more to cause a real bacterial infestation. Use Integrated Pest Management (IPM) practices to prevent these infestations from occurring, especially since whitefly species are becoming increasingly more resistant to insecticides. 

Whitefly populations are much easier to prevent than they are to treat. According to the Integrated Pest Management Guide created by the University of California, removing host plants like herbaceous ornamentals, and adding natural enemies like lady beetles can help limit whitefly populations while using neem oil sprays can help prevent populations and limit harmful effects to beneficial insects and pollinators. 

Cucumber Mosaic Virus (CMV) 

Cucumber mosaic virus (CMV) is a common virus that is often spread by aphids and other garden pests. Warning signs include yellowing, mottling, and overall distortion of the cucumber plant’s foliage and in some cases, the leaves appear almost white. If left untreated, leaves will begin to narrow, and growth will be severely stunted. The only effective treatment is the removal of infected leaves. Prevention techniques include the control of aphid infestations using IPM techniques. 

Cucumber with mosaic virus
Cucumber with mosaic virus

Downy Mildew

Downy mildew causes dark spots, yellow patches, and wilting of the leaves. It can spread quickly, so it’s important to act as soon as you spot any signs of infection. Cucumber plants are particularly susceptible to this disease and tend to be affected during warm, humid weather conditions.

To prevent downy mildew, practice good hygiene by removing infected leaves, and avoiding over-watering or overcrowding your plants.

Fungicidal treatments can be used, but prioritize removing affected foliage and disposing of it in the waste garbage, do not compost the affected material.

Cucumber affected by downy mildew
Cucumber affected by downy mildew

Bacterial Wilt

Bacterial wilt is caused by the bacteria, Erwinia tracheiphila, which is transferred to the cucumber plants during visits from cucumber beetles. Severe wilting and areas of leaf chlorosis are the most common symptoms of this uncurable disease.

The prevention of bacterial wilt involves the control of the cucumber beetle, which can be achieved using trap plants, row covers, or periodic all-natural neem and lemongrass sprays that coat the leaves in unpleasant oils. 

Once a cucumber plant is infected, it must be removed and destroyed to prevent further spread. Never compost plants that are infected with bacterial wilt because the wilt will spread and attract more cucumber beetles, perpetuating the cycle. 

Nutrient Deficiencies in Cucumber Plants

Nutrients become unavailable due to factors including poor organic matter content in the soil, nutrient leaching from heavy rains or overwatering, and pH imbalances in the soil that cause the nutrients to bind with the substances around it. 

The most commonly lacking nutrients that a cucumber plant needs to create a successful harvest are nitrogen, potassium, and iron. A lack of these necessary macronutrients in the soil will cause leaf chlorosis, stunted growth, and plant death. Magnesium, sulfur, and calcium deficiencies can also cause cucumber leaves to turn yellow, but they are less commonly unavailable.  

Nitrogen Deficiency 

Nitrogen deficiencies show signs of rapid leaf chlorosis starting in the older, larger leaves first. A lack of nitrogen is often caused by the absence of organic matter in the soil, insufficient fertilizing, and nutrient leaching. These problems can easily be prevented by starting with rich well-draining soil and sticking to a watering and fertilizing routine.  

I suggest adding a nitrogen-rich vegetable fertilizer when flowering begins to ensure the plant has enough nutrients to begin its costly fruit production. 

Potassium Deficiency

Signs of potassium deficiencies include the leaves yellowing along the edges and veins, then slowly progressing to the entire leaf. The older, larger leaves will be impacted first with symptoms typically presenting by mid-summer. 

Start with a rich sandy loam soil that will hold moisture and consider using a mulch because potassium becomes less bioavailable during times of drought. 

Iron Deficiency

Iron deficiencies show distinct signs of yellowing leaves and green veins. While iron is usually already present in the soil, it can often become unavailable due to environmental conditions within the soil. Fixing pH imbalances with specifically designed pH flushes is one of the most common solutions to iron deficiencies. 

Watering Issues

Cucumber plants require at least 1 – 2 inches of water per week during their vegetative growth cycles and 2 – 4 inches per week during the end-of-the-season fruit production. Otherwise, cucumber plants will begin to show symptoms of leaf chlorosis and weakened development. Water-stressed cucumber plants will likely not produce viable cucumbers because the plant will instead use its energy to survive the hardships. 

Overwatering 

Consistently overwatering can cause many problems, including the leaching of valuable soil nutrients, root rot issues, and weakened immune systems. All of which will limit the plant’s ability to create a successful harvest. Checking the soil moisture level with a moisture meter before watering is a good way to prevent these issues. 

What Does an Overwatered Cucumber Plant Look Like

Leaf yellowing is the number one sign of an overwatered cucumber plant. Wilting, which is usually thought of as a sign of underwatering, will also be very noticeable. 

Irregular Watering

General stress from irregular watering can cause chlorosis. I suggest watering your cucumber plants only when needed by checking the soil moisture every few days. The goal is to not let the soil and plants’ roots dry out repeatedly because it can cause too much stress for the plants to produce mature cucumbers. 

Lack of Sunlight

Cucumber plants need around 6 and 10 hours of sunlight each day. If they don’t receive enough sunlight, chlorosis will begin and the cucumber plants will exhibit diminished growth, increased fungal development, and a lower probability of fruit production.

Cucumber plants do not transplant well, so only move them if necessary. Instead, be strategic about planting your cucumbers in an area that will supply sun throughout each stage of the growing season. They need full sun from spring through the end of summer. 

Lowered temperatures, decreased daytime hours, and high nighttime humidity in the fall will also trigger the cucumber plants to senesce, in which the yellowing and dropping of their leaves is the first step. Cucumbers are annuals, so the entire plant will die and not come back in the spring.  

Pests and Diseases in Cucumber Plants

There are numerous pests that prefer cucumber plants as their hosts. Some, like leafhoppers, can do extensive damage as a few individuals, while others, like spider mites, do the most damage as a huge infestation. Many times, these pests bring along a host of other problems, like viruses, bacteria, and other insects that will cause more problems for an already weakened plant. 

Cucumber worms and beetles

Cucumber worms are the larvae stage of the cucumber beetle. They live in the soil and feed on the root system, causing the plant’s leaves to turn yellow and the plant to slowly die. The beetles can easily devastate an entire garden of cucumber plants, so it’s best to prevent these pests. 

I suggest sprinkling diatomaceous earth on the soil around the cucumber plants, especially near plants that touch the ground. This will dry out the larvae and limit the number of individuals that make it to maturity. Be careful not to get the diatomaceous earth on the plant leaves because, while it’s harmless to mammals, it can dry out leaves very quickly. 

Aphids

Aphids by themselves are not an issue, but they quickly gather in large numbers that overwhelm a cucumber plant. They will pierce the leaves and suck the moisture out, leaving the foliage yellow and dry. Aphids also deposit a sticky substance that encourages the growth of sooty mold, which spreads quickly and can infect the fruit. 

Leafhoppers and Leaf Miners

Leafhopper and leaf miner species are piercing and sucking insects that drain the nutrients from the foliage. These pests also have the potential of depositing bacteria, fungal spores, and viruses on the plant, which can cause diminished fruiting abilities or plant death. 

If large infestations are left unmanaged, then the cucumber plants are not likely to produce viable flowers or mature fruit. Introduce lady beetles, praying mantises, and other beneficial predators to the garden to keep the aphid, leafhopper, and leaf miner populations in check. 

Spider Mites

Spider mites are common and naturally occurring tiny insects that will leave small yellow dots all over the surface of the leaves as they feed. As the infestation worsens, the spider mites will cover the entire plant in a fine webbing, starting on the underside of the leaves. If the infestation is not managed, the cucumber plants will die from the spider mite invasion.

An infestation of spider mites can easily be prevented and treated by spraying the leaves with mite-specific insecticidal soap, or an all-natural mixture that includes essential oils such as lemongrass and neem. Make sure to use all products as directed for the best results. 

Cucumber leaf affected by spider mites
Cucumber leaf affected by spider mites

Cucumber Plant Leaves Turning Yellow Final Thoughts 

The most common causes of cucumber leave turning yellow are nutrient deficiencies, watering issues, lack of sunlight, pests, and diseases. Nitrogen deficiencies show signs of rapid leaf chlorosis starting in the older, larger leaves first. Signs of potassium deficiencies begin with the leaves yellowing along the edges and veins. These nutrient deficiencies are often caused by the absence of organic matter in the soil, insufficient fertilizing, and nutrient leaching. 

Cucumber plants require at least 1 – 4 inches of water per week and between 6 – 10 hours of sunlight each day. Otherwise, cucumber plants will begin to show signs of leaf chlorosis, weakened development, increased fungal development, and a lower probability of fruit production.

Damage from cucumber worms, cucumber beetles, aphids, leafhoppers, leaf miners, and spider mites can all cause a cucumber’s leaves to turn yellow. Among the endless number of diseases that can plague a simple garden, cucumber plants are quite susceptible to several serious conditions including Cucumber Vein Yellowing Virus (CVYV), Cucumber mosaic virus (CMV), and Bacterial Wilt. 

Integrated Pest Management (IPM) techniques like the removal of host plants and the addition of natural enemies like lady beetles can help limit the populations of the pests that cause these serious diseases while using neem oil sprays can help prevent pest populations and limit harmful effects to beneficial insects and pollinators.

FAQ Yellow Leaves on Cucumber Plants

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Ben's horticultural interest grew when graduating from Hertfordshire University in 1997. Having contributed to numerous publications including Better Homes & Gardens, Garden Design Magazine, and The English Garden. He is also the author of Propagating Houseplants Made Easy.