Cucumber leaves turning yellow usually means one thing:
A sickly plant in need of attention.
In this guide, I’ll show you how to identify the cause and what you can do about it.
Let’s dive in.
What Is Happening?
The loss of the green coloration of the leaves, caused by a lack of chlorophyll, is known as chlorosis.
As chlorophyll is essential for converting sunlight into chemical energy, its reduction threatens the health of your cucumber plant.
But to find out exactly what’s causing it you need to take a closer look.
Cucumber Leaves Turning Yellow: The Causes
The causes of chlorosis usually fall into one of 5 categories: plant pests, diseases, nutritional deficiencies, light problems, and water problems.
Let’s take a closer look at how you can identify your problem, and importantly, what you can do about it:
Aphids (Aphidoidea) on a leaf
You’ve probably come across a variety of different insects on your plants. But did you know that some of them suck the sap out of the leaves?
This is a common cause of leaf discoloration and can cause them to turn yellow.
These are some of the most common:
Aphids – these small oval-shaped insects hang out on the underside of leaves. They are often green or yellow and leave behind a sticky black substance called honeydew.
Spider Mites – suck on the cucumber leaves causing yellow stippling. They are another insect that’s usually found on leaf undersides where they leave behind a silvery, fine webbing.
Whiteflies – if you shake the leaves and see a cloud of tiny, white winged insects rise up, then your plant is infested with whiteflies. Like aphids, they also leave behind honeydew.
You can spray insecticidal soap on the leaves of the plant to get rid of these unwanted bugs.
The Potato Leafhopper
Potato Leafhopper (Empoasca fabae) Source: gailhampshire
These are bigger than the pests mentioned above, but they also enjoy feeding on your plants by sucking the sap from the leaves.
While feeding, they inject a toxin that causes yellowing. And as the damage accumulates, the leaves will eventually fall from the plant.
You can use insecticides to kill potato leafhoppers. But for obvious reasons, that’s not always desirable on plants that you’re using for food.
So another method is to keep the area free from weeds and use row covers over the plants to keep the insects away.
Cucumber mosaic virus
Diseases are often responsible for the appearance of yellow spots or streaks on cucumber leaves.
Check to see if your plants are infected with any of these:
Cucumber Mosaic Virus
Are the leaves wrinkled and downward-curving with yellow spots?
Then they might be infected by cucumber mosaic virus.
Unfortunately, the only solution to this problem is to remove the infected plants because there’s no treatment available. If left, the disease will often spread as it’s carried by aphids and leafhoppers.
Be aware that it can live in the soil and infect new plants the following year.
The same also applies to our next disease…
Take a close look at the older leaves. Are they turning yellow, starting from the edges and spreading inwards?
Cucumber beetle larvae are responsible for its spread as they feed on the plant’s roots.
The same as mosaic virus, the only way to deal with it is to get rid of the infected plants. There are a number of options for cultural and biological control of this fungus.
Another similar fungus is Verticillium wilt. This is common if your cucumber plants are planted in an area that was previously used for vegetables such as peppers, tomatoes, potatoes, or eggplants.
The telltale signs of downy mildew are yellow spots on the upper surface of the leaves which turn to brown later on.
These spots are angularly bounded by veins in the leaves, giving an almost patchwork appearance.
And in high humidity, a grey fuzz can be found on the underside of the leaves.
To prevent and manage the disease there are several options:
- Planting cucumber varieties that have a high level of resistance.
- Improve air circulation around the cucumber plants and keep leaves dry by using wide row spacing and drip irrigation.
- Remove infected plants immediately to prevent spread.
- Contact and systemic fungicides are available that are effective against downy mildew when applied early in the disease progression.
Cucumbers need a number of different nutrients to grow properly, and if these are lacking chlorosis can occur.
To accurately assess whether your plant is lacking in specific nutrients it’s a good idea to have the soil in your vegetable garden tested.
The following are the most common deficiencies that can result in your cucumber plant leaves turning yellow:
Not only will nitrogen deficiency turn cuke leaves yellow, but it also stunts their growth. With the plant eventually dying if the situation is severe.
Look out for the older leaves on the plant turning yellow along central veins and at the tips. With newer leaves continuing to look green.
Fortunately, this problem is also very easy to fix. You can increase the nitrogen content of the soil by adding a 2-inch layer of compost.
Or, you can add 1-2 tablespoons of 6-10-10 fertilizer if you start to notice a problem.
Another thing you can do is use a tablespoon of ammonium nitrate when you see the flowers of the plant start to bloom. Add it to the soil again 3 weeks later.
But, be careful not to put too much fertilizer in the soil as it will prevent the plant from producing its fruit, the cucumbers themselves!
If new leaves are yellow with green veins, but the older leaves are still green, then iron deficiency is a possible cause.
To treat this deficiency you can add granular or powdered chelated iron to the soil around the root zone. Or you can spray liquid iron on the plant.
Cucumbers have a high potassium requirement, and are one of the only crops that need more potassium than nitrogen
The symptoms of potassium deficiency are leaves that turn yellow at their tips and edges. Young leaves tend to be small and dull looking, and are cupped or puckered. The cucumber fruit often have a club-shaped appearance due to being narrow close to the stem.
Using a well-balanced fertilizer can solve this problem. And treat the soil for acidity or alkalinity if needed.
To identify cucumber plants suffering from a zinc deficiency, look for older leaves turning yellow between the vein. Leaf size is usually small and growth of the plant is restricted.
Spraying with a zinc sulfate solution is an easy way to correct this problem. You can also use organic kelp.
Water Related Yellowing
Be careful not to overwater your cukes. Overwatering can lead to oxygen-deprived roots, a sign of which is the leaves turning yellow and wilting. Including rainfall, cucumber plants only need about 1-2 inches of water each week.
If your soil has drainage problems, consider loosening it with sand. Another solution is to grow your cukes in raised beds instead.
Lack Of Sunlight
Sometimes, the reason for yellow droopy leaves on your cucumber plant is simply that it’s not getting enough sunlight. Try moving them to a location in your garden where they can get at least 6 hours per day.
Featured image source: Cristina Sanvito