7 Reasons Why Your Cucumbers Are Turning Yellow

Cucumbers are one of the most prolific garden vegetables when they’re healthy. They can quickly climb over our heads and send out pounds of crunchy cukes each week. And what a treat this is!

However, cucumber plants are also susceptible to various environmental problems, diseases, and pests. Even if you don’t know what’s plaguing your cucumber plants, watching the green leaves turn yellow is an obvious way to tell that something has gone wrong. But what, exactly?

Yellow leaves can indicate a variety of problems, including issues with water, light, and nutrients. I’ll cover seven reasons why your cucumbers may be turning yellow so you can remedy the cause of the yellow leaves.

Too Much Water

As gardeners, we often love our plants to death. One way this happens is by watering our plants too much. While cucumber plants need water to grow and remain healthy, too much water can prevent the roots from taking up nutrients and completing gas exchange. It can also increase the odds that the plants develop fungal diseases.

If you overwater your cucumber plant, the roots will remain sitting in water. This stresses the plant and limits its ability to complete crucial processes like photosynthesis. If the roots become rotten, they’re unable to take up the necessary nutrients and water. This stress eventually leads to yellow leaves.

So, how much water is too much? Generally, the soil your cucumbers are growing in should remain moist but not soaking wet. When you’re determining how often to water your cucumber plants, you should look at and think about the following factors:

Rainfall

It probably seems obvious, but if the soil is wet from rain, you probably don’t have to water your cucumber plant. I say probably because the amount of rainfall determines whether or not your plants receive enough moisture. A short and gentle rain can make the soil surface appear moist, but the soil may still be dry four or six inches underground. Therefore, don’t assume a wet soil surface means you can skip watering.

A half inch or more of rain usually means you can skip your planned watering. If you’re unsure if your plants received enough water, you can dig into a portion of the ground a foot away from your cucumber. Soil that’s moist six inches under the surface is wet enough to skip watering.

Temperature

You know how we need to drink more water when it’s hot outside? Well, cucumber plants are the same way. As temperatures rise, cucumber plants cool themselves through a process called transpiration. During transportation, plants open leaf pores called stomata to release water vapor. The conversion of water from liquid to gaseous form cools the plant.

Low temperatures coincide with reduced rates of transpiration. Additionally, high temperatures lead water to evaporate from the soil more quickly. Therefore, plants require less water during cooler times of the year and more water during hot times.

Humidity

Humidity also plays a big part in transpiration rates; plants lose less water through their leaves if there’s already a lot of water vapor in the air. However, dry air speeds up transpiration. Therefore, plan to water your plants more during dry periods than during humid periods.

Soil Type

Just think about a sandy beach and a clay pot to understand how soil type impacts water movement. As waves wash over a beach, the water quickly infiltrates the surface and percolates into the ground. The same thing happens if you happen to spill a drink. However, if you spill water on an unglazed clay pot, the water will form a puddle before slowly moving through the clay. The only difference between sand and clay is the size of the soil particles.

Since sand consists of larger particles, there’s more air space that water can travel through. Clay’s small particles can pack closely together, leading to small air spaces and slowed water movement. Since soil contains a combination of clay, silt, sand, and organic matter, the amount of each material impacts water movement.

Generally, sand will increase soil drainage and encourage you to water your plants more frequently. Clay and organic matter increase the water-holding capacity of the soil, so you won’t have to water your soil as often.

Mulch

Adding mulch to the surface of the soil limits evaporation. Therefore, you can plan on watering mulched soil less frequently than you would water soil without mulch. The type of mulch doesn’t matter, but a thick layer of mulch will keep the soil moister than a thin layer of mulch.

Not Enough Water

Just like too much water can cause a problem for cucumber plants, so can too little water. Cucumber plants have a large leaf surface, so they lose a lot of water through transpiration. Plus, since the fruits have a high water content and grow quickly, cucumber plants need a lot of water to produce healthy and crunchy cukes.

If your cucumber plants don’t receive enough water, they’ll wilt. A lack of water also causes nutrient deficiencies since plants rely on water to move nutrients from the soil throughout the plant. If plants don’t receive enough water, they may begin to show signs of nutrient deficiencies even if enough nutrients are present in the soil. Yellow leaves are one of the signs.

Aim for deep, infrequent watering rather than frequent, shallow watering when you water your cucumber plants. This type of irrigation will encourage the plants to develop deep and strong root systems, and it will also help ensure all parts of the roots receive water.

The ideal watering schedule depends on the factors I listed above: temperature, humidity, soil type, rainfall, etc. However, most cucumber plants require deep watering three to four times a week. If you find it difficult to remember to water your plants, you can set up a drip irrigation system and place this system on a timer. This is also a great irrigation method if you travel frequently. If you opt to hand water your plants, make sure to water at the base to avoid getting the leaves wet.

Not Enough Nutrients

Cucumber plants require 16 different nutrients to thrive. Since they obtain three of these nutrients (oxygen, hydrogen, and carbon) through water uptake and gas exchange, you only need to supply the remaining 13. Deficiencies in any of these nutrients can lead to issues with nitrogen fixation, chlorophyll production, disease resistance, and more.

While cucumbers can lack any of the 13 nutrients, some nutrient deficiencies are more common than others. A lack of the following nutrients often shows up as yellow leaves.

Nitrogen

Nitrogen (N) is one of the primary plant nutrients, along with potassium (K) and phosphorus (P). Plants use nitrogen to form proteins, build the genetic material RNA and DNA, take up nutrients and water, and form chlorophyll. Nitrogen is a mobile nutrient, which means it can readily move throughout the plant. Therefore, you’ll notice deficiency symptoms in older leaves first.

Cucumber plants that are nitrogen deficient begin to develop light green to yellow leaves. Over time, these leaves become more and more yellow. While the veins may remain green in the early stages of nitrogen deficiency, they eventually turn yellow. Other signs of nitrogen deficiency include stunted growth and short stems.

Some organic nitrogen sources include blood meal, feather meal, alfalfa meal, and high-nitrogen compost. If your plants need nitrogen ASAP, you can foliar feed your cucumber plants with a liquid fertilizer like fish emulsion.

Potassium

Another primary nutrient, potassium (K), acts as a communicator within plants. Although it doesn’t make up molecules like proteins and carbohydrates, it helps move water and nutrients, activate enzymes, and regulate the production of ATP, an energy transfer molecule. Potassium uptake is highly dependent on soil moisture, so this nutrient deficiency often coincides with underwatering, compacted soil, and other factors inhibiting the plant’s water uptake.

Signs of potassium deficiency include yellow older leaves with brown edges and/or brown spots. Other signs include short stems and an overall weak plant.

If you suspect potassium-deficient soil, add an organic source of potassium, like sunflower hull ash or potassium sulfate. You should also ensure your plants are growing in loose soil and that you’re adding the correct amount of water.

Magnesium

Magnesium (Mg) is a secondary plant nutrient that helps with cell division, cell respiration, and enzyme activation. It’s also a key component of chlorophyll molecules. Therefore, plants that don’t have enough magnesium often develop yellow leaves.

The yellowing starts out in older leaves and takes on a patchy appearance. The spots between leaf veins become dotted with yellow, brown, and even red hues. Over time, this discoloration spreads across leaves and to younger leaves.

Some organic sources of magnesium include rock dusts like azomite and dolomite.

Not Enough Light

Cucumbers thrive in full sun and will suffer if they don’t receive enough light. While the plants can grow with only six hours of direct light, they prefer eight or twelve hours of light. If they don’t receive enough light, they won’t have the energy they need to grow, fight off disease, produce flowers, and more.

Plants that don’t receive enough light have difficulty producing chlorophyll, the pigment that turns plants green. If plants have less chlorophyll than they need, they will begin to yellow. This type of yellowing often occurs throughout the entire leaf rather than just at leaf margins or in between leaf veins.

Cucumber plants that need more light often appear stunted, and their leaves are also smaller than average. If your plant shows either or both of these signs, there’s a good chance it needs more light.  While it’s hard to move your cucumber plant once it’s in the ground, keep this information in mind for future plantings.

Disease

Cucumber plants are susceptible to a variety of diseases, many of which can lead to yellow leaves. Here are a few common cucumber diseases that can cause yellow leaves.

Downy Mildew

Downy mildew impacts a variety of plants, including cucumbers. It’s caused by a water mold and is most likely to occur during cool, humid conditions. Therefore, it often appears in the spring or early summer.

The first signs of downy mildew are yellow, angular spots between the leaf margins. These spots eventually spread and turn brown, and eventually, the entire leaf turns brown and dies.

Since downy mildew is difficult to treat, prevention is the best strategy. Proper plant spacing and bottom watering can help keep leaves dry, and selecting downy mildew-resistant varieties decreases the chances of plant infection.

Cucumber Mosaic Virus

If you notice your cucumber leaves look mottled with patches of yellow, they may have become infected by the cucumber mosaic virus. This virus causes stunted growth and leaves that become patterned with shades of yellow, light green, and dark green. Unfortunately, this disease cannot be cured once it strikes your plant.

Since it overwinters in plant material, make sure to remove any infected plants from your garden. You can also look for disease-resistant varieties.

Angular Leaf Spot

This bacterial disease causes cucumber leaves to develop dry brown spots surrounded by a yellow halo. It’s most likely to occur during hot and humid summers. You can prevent this disease by using drip irrigation and choosing resistant varieties. 

Pest Damage

Watch out! You’re not the only one who finds cucumbers tasty. Numerous tiny pests feed on insect leaves, leading to holes and discoloration. Typically, pests will only cause yellowing on the sections of cucumber leaves they’ve attacked. Keep an eye out for the following cucumber pests.

Squash Bugs

Although squash bugs are best known for attacking zucchini and butternut squash leaves, they’ll feed on any member of the cucurbit family. And that includes cucumbers. These brown bugs use their piercing mouthparts to stab cucumber leaves and drink their sap. This results in light green or white spots where the bugs ate. If the bugs attack enough of the plant, leaves may turn brown and die.

If you see squash bugs, remove them by hand. You should also look out for clusters of light yellow, oblong eggs on the undersides of the leaves. Another option is to spray the bugs with neem oil or insecticidal soap.

Spider Mites

These tiny mites leave behind spiderweb-like structures, hence their common name. Although spider mites are less than an eighth of an inch long, they can quickly multiply and damage plants. They use their piercing-sucking mouthparts to poke small holes in cucumber leaves and then drink the sap. This feeding eventually leads to small yellow or light green dots known as stippling.

One way to remove spider mites is to spray them off the plants with a steady stream of water. You can also spray the pests with insecticidal soap or neem oil. As long as you treat the pests shortly after they appear, your cucumber plant will recover.

Compacted Soil

If you’re watering your plants the proper amount, providing them with the right nutrients and light, and keeping them disease and pest-free, it’s easy to assume everything is good. But if cucumber leaves still look yellow, you may be dealing with compacted soil. When the soil is compacted, water has a hard time flowing, and the plants’ roots have difficulty completing gas exchange and growing.

When cucumber plants have weak root systems, they have difficulty taking up water and nutrients. This can lead to yellow leaves, stunted plants, and other problems. It’s hard to remedy compacted soil once your plants are growing, so try to choose a well-aerated and well-draining location before planting. If you need to loosen your soil, you can do so by gently lifting it with a digging fork or shovel. Mixing in finished compost also helps improve soil structure and plant health.