7 Reasons Why Your Cucumber Leaves Have Yellow Spots and How to Treat Them

One of the joys of growing cucurbits like cucumbers is searching amongst the massive foliage for the delectable vegetables hiding underneath.

But sometimes, your search turns up something more unpleasant: yellow spots on the leaves of your cucumbers.

Yellow spots on cucumber leaves typically suggest potential issues. These can range from fungal infections like downy mildew to viral diseases like cucumber mosaic virus. Pests that is feeding on your plants such as mites, aphids, and whiteflies might also be the culprits. Additionally, magnesium deficiency or Alternaria (late blight) can also lead to these yellow spots.

At the onset, they might not seem like much at first, these issues can retard growth, reduce yield, or even kill your plant if left untended.

While fungal diseases and viruses are incurable, insects and magnesium deficiency can be treated and cured.

Even so, with careful management, these issues can be prevented in the first place so the only surprise you will get this growing season is how big your harvest is.

Let’s take a look at how to identify what is causing yellow spots on your cucumber’s leaves, and how to naturally manage these problems.

Why Yellow Spots on Cucumber Leaves Are a Cause for Concern?

So why are yellow spots a bad thing? Sometimes a yellow spot can simply be a damaged leaf, or a wandering bug took a bit as he was passing by, but sometimes the yellow spots can be indicative of something far more problematic.

Yellow Spots On Cucumber Leaves? Here’s How To Identify The 7 Most Common Causes And Fix Them

Yellow spots could be caused by:

A disease, either fungal or viral, that can kill your cucumbers, spread to other plants, and wreak havoc in the garden.
The start of an infestation of bad bugs that can do a lot of damage to your crop if they get out of control.
Nutrients that are out of balance, or missing completely, from your soil.

Plants need nice green leave to properly photosynthesis sunlight into plant food. Yellow spots reduce the plant’s ability to make this food which will inhibit the plant’s growth, reduce its yield, or lead to its premature demise.

7 Reasons Why Your Cucumber Leaves Have Yellow Spots and How to Remedy Them

So, when you see yellow spots on the leaves, you want to quickly identify what is causing them so you can rectify the problem before it gets serious. Yellow spots on cucumber leaves can be caused by:

1: Downy Mildew

Cucumber leaves infected by downy mildew (Pseudoperonospora cubensis) in the garden. Cucurbits disease.

Downy mildew can cause spots of several different colours depending on the variety of the pathogen and which type of plant is infected. On cucumbers, however, the pathogen will cause chlorotic spots. The symptoms of downy mildew are:

Yellow or light green spots on the upper side of the leaves. The spots will be interveinal, or between the veins, of the leaves. The spots will slowly spread to cover the entire leaf.
The spots will dry and turn brown as they age and the leaves can die and fall off. Severely infected plants look as if they were frost-killed.
A soft, down-like mould on the underside of the leaves. The mould is usually gray but can range from white, brown, black, or purple-ish.

Downy mildew is an Oomycete, or a water mould, which is a fungal-like disease that affects the foliage of the cucumbers. It is a widespread fungus that infects many plants, including everything in the cucurbit family.

However, it is particularly devastating to cucumbers. Downy mildew will spread through the air, splash on the plants from contaminated soil, or can transfer mechanically (by hands, tools, or clothing).

Downy mildew requires living green plant matter to survive on, so in cold climates, it will die after frosts or extreme cold. However, it can produce oospores, which are resting spores that can survive extreme temperatures for up to ten years.

Downy mildew requires moisture (at least 85% relative humidity) to attach itself to the leaves and to breed and spread, so it is most prevalent during wet summers. New spores are produced on the underside of the leaves where they then travel to other leaves and plants.

It is viable through a wide range of temperatures, from 5°C to 30°C (41-86°F), but it mostly flourishes between 15°C and 20°C (59-68°F).

How To Prevent

With a bit of careful planning, you can keep downy mildew from taking hold of your cucumbers by:

Grow varieties that are resistant to whichever disease is prevalent in your area, whether you are growing from seeds or buying nursery stock. If you are buying transplants, be very careful that they come from a reliable disease-free greenhouse.
Let in air and sunlight by spreading out your cucumbers, spacing your plants at least 30 cm (1 foot) apart in rows that are 1 meter (3 feet) wide, or even wider if your area is particularly damp.
Rotate your crops so you do not grow cucumbers (or any other related cucurbits) in an area more than once every 3 to 4 years to give them pathogens time to die off.
Trellising is another great way to get air circulation and sunlight to dry the area around your cucumbers.
Use drip irrigation or some other method of applying water directly to the soil and keeping it off the foliage, and avoid overhead watering at all costs.
Water early in the day so any water that does splash on the plants will have time to dry during the day.
Prune out any foliage that shows signs of the disease.
Remove entire plants if they have become too diseased to prune since it is better to lose one plant than risk the disease spreading to another.
Weed thoroughly around your plants as certain weeds can also harbour the disease and transfer it to cucumbers.
Sterilize all of your equipment and wash your hands after handing diseases cucumber plants.
Mulch around your plants to keep them from contacting contaminated soil.
Homemade DIY fungicides can easily be made from household items such as vinegar, mouthwash, garlic, cinnamon, baking soda, or neem oil. Begin application early in the season to help protect your plants from infection.

How To Treat Downy Mildew

Downy mildew cannot be cured once it takes hold of your plants, so prevention is the best line of defence. There are a few fungicides available for downy mildew, but these chemicals can cause serious environmental damage. Not to mention that downy mildew is becoming resistant due to the excessive use of these fungicides.

2: Alternaria (Early Blight)

Alternaria does not produce true yellow spots. Instead, it develops brown dead spots on the leaves that are surrounded by a chlorotic halo. You can tell Alternaria apart from other fungal diseases by:

Brown or tan spots shrouded in a yellow halo. Older leaves are often the first to show symptoms.
Dark brown cankers on the plant’s stems.
The cucumbers, if infected, can develop dark, water-soaked areas.

Alternaria, or early blight, is a common disease found in many gardens. Cucumbers are infected by the Alternaria cucumerina variety which is sometimes called cucumber blight.

Unlike many other fungal pathogens, Alternaria prefers warm temperatures. It is active from 15°C (59°F), but it grows and spreads most easily between 27°C to 30°C (82-86°F).

Alternaria spores can enter your garden through infected seeds or transplants, but once there is can spread by wind, insects, splashing water, or on your tools. It can also survive for up to two years on infected plant matter.

Alternaria can affect all parts of the plant, including the leaves, stems, and fruits. While Alternaria will stunt the growth of the plant and reduce its yield, it rarely becomes severe enough to kill the plant.

How To Prevent Alternaria

The best way to protect your cucumbers from Alternaria is to keep your plants from getting it in the first place.

Check out the list of preventative measures above to keep all fungal diseases away from your cucumbers.

How To Treat Alternaria

Like downy mildew, there is no way to cure Alternaria. Since fungal diseases are essentially untreatable, prevention is your best option.

3: Cucumber Mosaic Virus

Cucumber mosaic virus (CMV) was so named because it was first identified on a cucumber plant, yet it can infect lots of different plants in the garden. The most common symptoms include:

Yellow patches, or yellow mottling, on leaves give the foliage a mosaic-like appearance.
Distorted leaves will be stunted and curl downwards.
The plant’s overall growth will be stunted due to shortened internodes of the stems.
White steaks through flowers.
Spots or streaking on the cucumbers themselves.

Cucumber mosaic virus won’t often kill the plants. However, the entire plant’s growth will be stunted and your yield will be reduced with distorted cucumbers.

Viruses, like cucumber mosaic virus, cannot spread on their own. You can spread cucumber mosaic virus on your hands or tools, but it is most commonly spread via aphids (see all the other issues aphids can cause and how to control them below).

Once the virus has entered the plant’s cells, it will spread and quickly infect the entire plant. In many cases, once symptoms are first seen, the entire plant is already infected with the disease.

How To Prevent Cucumber Mosaic Virus

CMV can devastate the garden once it takes hold. Here are the best ways to protect your plants from becoming infected in the first place:

Focus on plant health: As with people, a healthy plant has a stronger immune system to fight the virus. Keeping our plants healthy by creating a rich, natural growing environment is the best way to prevent many problems from arising.
Control aphids. We will discuss how to keep aphids in check in more detail below.
Choose disease resistant varieties. Most reputable seed companies will work to improve the disease resistance of the varieties they sell. The websites, or seed packets, should list if the variety is resistant to cucumber mosaic virus or not.
Grow from seeds. Viruses rarely transfer through seeds, so cultivating your cucumbers from seeds is a safer option than purchasing transplants.
Weed Your Garden. Groundsel and chickweed are weeds common to many gardens all over the world and both can harbour CMV and transfer it to your cucumbers.

How To Treat Cucumber Mosaic Virus

There is no way to cure a plant from CMV. The only course of action to eliminate the virus is to completely cull any infected plants that you identify.

Do not add culled plants to your compost, as the virus can survive through the composting process to reinfect your garden.

Pruning is not an effective method of control since the rest of the plant will probably be infected by the virus even if those parts do not have symptoms.

Studies have found that the only part of the plant that did not contain the virus were cells in new growth points at the tip of the plant.

Make sure to clean any equipment and wash your hands after working with infected plants.

4: Mites

Cucumber plants are one of mites’ favorite sources of food. You can tell you have mites if you see:

Pale yellow spots, or blotches, on the leaves. These spots will dry and turn brown and the whole leaf can be affected if the infestation is bad.
Small red, brown, or black insects crawling around, usually on the underside of the leaves.
A very fine web on the foliage. Again, it is often on the underside of the leaves but the webs can even stretch from stem to stem.

Mites have eight legs and two distinct body parts so these tiny arachnids are often called ‘spider’ mites, and they can be red, brown, or black. They feed on plants by biting the leaves and sucking out the juices,

which is what causes the incision spots to turn yellow and dry. They can also inject a toxin into the plant which causes further discolouration and stunted growth.

Mites can be tiny (0.5-1mm long)], making them very difficult to see on the leaf. In our area, most mites are red, making them a little easier to see, but if you suspect mites

but cannot see them, try tapping the leaves onto a piece of white paper to make it easier to see the small insects. Also, it might be time to bring out the magnifying glass.

They can reproduce quickly since a single female can lay up to 20 eggs per day for her several-week-long adult life. Furthermore, the eggs can still produce male mites without the female mating. They will overwinter in cold gardens as eggs or mated females.

Mites prefer dry and hot conditions so they can be a real pain in the summer. However, this can also be used to our advantage.

How To Prevent Mites

Before mites even arrive, there are some things you can do to make your garden less appealing to these critters.

Attract predatory insects by growing companion plants such as this beneficial insect blend.
Keep your plants hydrated as well-hydrated plants are healthier and less susceptible to mites.
Use floating row covers to keep mites from landing on your plants. These are fine, lightweight mesh that you put over your plants to keep bad bugs from getting through.

How To Treat Mites

Once the mites have arrived, there are a few things you can do.

Spray the leaves with a jet of water from the garden hose to blast the mites off the leaves. This also creates a moist environment that mites do not like. Continue to spray every second day until you think they are all gone. Make sure to get the underside of the leaves.
Wash off the mites by washing the leaves with a damp cloth.
Cull any severely infected plant to keep the mites from crawling to another plant.
Treat infected leaves with a homemade, natural insecticide to repel any mites. Here are some great options that are also good for the environment.

5: Aphids

Swarms of aphids may seem harmless, but they can actually cause a lot of problems. Aphids can be identified from other bugs by:

Yellow spots on leaves where they suck the sap from the plant.
Swarms of aphids, generally on the underside of the leaves.
A gooey, sticky substance on the leaves left behind as the aphids feed (called honeydew).
Sooty mould and cucumber mosaic virus are often secondary infections caused by aphids.
Colonies of certain ants can be drawn to the same plants as they form a symbiotic relationship with the aphids.

Most people see aphids as green, but they can be a range of colours. Each species can be winged or wingless, and all species can produce asexually, so a single aphid can soon turn into a big problem. They usually cause the most damage in late spring, but they can infect your cucumbers all year long.

Aphids will secrete a sticky substance called honeydew that sticks to the leaves. Honeydew can inhibit photosynthesis, and sooty mould fungus can stick to it causing further damage to your cucumbers. And, of course, aphids are carriers of CMV (see above).

Several species of ants will become aphid herders. They will protect the aphids from predators and move them around to the healthiest parts of a leaf.

The ants will even carry the aphids to their ant hills at nighttime and overwinter them there. In return, the ants will rub the aphids with their antennae to “milk” them of their honeydew which they then consume.

How To Prevent Aphids

There are a number of ways to keep aphids from your cucumbers:

Floating row covers will keep the aphids from landing on your cucumbers. There are a number of floating row covers available.
Attract predatory insects as many of them will consume aphids. For example, plant yarrow near your cucumbers as these plants will attract hoverflies who love eating aphids.
Alliums, such as onions and garlic, will repel aphids so plant some of these near your cucumbers. Chives have the added advantage that they flower quickly and will attract more predatory insects. They also do not like fennel, dill, and other strong-scented plants.
Lay off the fertilizer since aphids prefer plants that have become juicy and lush from excessive nitrogen.

How To Treat Aphid Infestations

Spray the leaves with a jet of water, since the stream will wash off the soft little bugs.
Use a homemade fungicide as mentioned above.
Prune off infected leaves. If the infestation is great, it might be beneficial to pull the entire plant.

6: Whitefly

Whiteflies are closely related to aphids, but they can be identified by:

Yellow spots on the leaves where they suck sap. They generally prefer young, new leaves and growth.
Stunted growth and development of the plant.
Sticky honeydew on the leaves.
Whiteflies and their eggs may be visible on the underside of the leaves.
Whiteflies are active during the day, so the easiest way to tell if they are the problem is to shake the plant. Once you do, you will soon be surrounded by a cloud of white insects.

Whiteflies are not actually flies, and they are most active during the day. They are most active in mid to late summer and like hot, humid weather.

Like their aphid cousins, they feed on sap from the leaves and cause chlorosis, reduce photosynthesis, and inhibit growth.

How To Prevent Whiteflies

Whiteflies can be prevented in much the same way as aphids and mites:

Attract predatory insects so they can eat the whiteflies.
Repel them with ‘stinky’ plants like aromatic herbs and onions.
Floating row covers can reduce or eliminate whitefly populations from landing in your cucumber patch.

Hot To Treat Whiteflies

Whiteflies can also be gotten rid of in similar ways as aphids.

Spray them with water from the garden hose. Whiteflies are easily disturbed so the adults will fly away with a slight disturbance, and the spray of water can knock off the eggs and larvae.
Homemade insecticides as mentioned above can be both safe and effective. Also, a mix of a squirt of dish soap with 4 litres of water can be an excellent spray to coat the leaves and keep the whiteflies at bay.
Vacuum your plants with a light-powered vacuum to suck off all the whiteflies.

7: Magnesium deficiency

Lack of magnesium can also cause yellow spotting on cucumber leaves.

Magnesium deficiency starts as small spots between veins, then spreads to complete interveinal chlorosis.
Magnesium is an essential component of chlorophyll so if this nutrient is missing it will cause the leaves to form yellow spots or irregularly shaped splotches.
Magnesium chlorosis will usually affect older leaves first, and forms between the veins. As the damage becomes severe, the yellow splotches will dry and leaves can die.
A severe case of magnesium deficiency can inhibit the growth of the plant and can severely reduce your yield of cucumbers.

How To Prevent Magnesium Chlorosis

Making sure your soil is nicely balanced with all the essential nutrients is important. Here are some ways to prevent your cucumbers from becoming deficient in magnesium.

Reduce fertilizer use as too much potassium can enhance magnesium deficiency.
Add compost to your garden as this will help your soil become rich and healthy and nutrient-rich. Compost will also balance the pH of your soil and a pH around 6.5 is ideal for magnesium absorption.
Test your soil before planting to see if your soil has a decent amount of magnesium. Most DIY home soil test kits only test for nitrogen, phosphorus, and potassium so you will probably have to send a soil sample to a lab to determine the magnesium content. Too much calcium or ammonium can also inhibit the absorption of magnesium.

How To Treat Magnesium Deficiencies

If your cucumbers are developing yellow spots from too little magnesium, then you will need to act quickly to rectify the problem before it goes too far.

Add an organic fertilizer that is high in magnesium. This is the fastest way to get needed magnesium to your cucumbers.
Kelp and alfalfa meal are a natural way to add magnesium to the soil plus a lot of trace minerals.
Dolomite lime is high in magnesium, but keep in mind that it will also raise the pH of the soil so it should be applied according to the package rates. Dolomite lime can take a few weeks before it affects the soil.

Don’t Let Yellow Spots Ruin Your Plants

Yellow spots can be a sign of a major problem lurking in your garden. But when these yellow spots show up, don’t worry.

The symptoms are easy to tell apart and once you know what you are dealing with, then you can act quickly and naturally so you will be rewarded with a bountiful crop of nice, crispy cucumbers.

Written By

Amber Noyes

Amber Noyes was born and raised in a suburban California town, San Mateo. She holds a master’s degree in horticulture from the University of California as well as a BS in Biology from the University of San Francisco. With experience working on an organic farm, water conservation research, farmers’ markets, and plant nursery, she understands what makes plants thrive and how we can better understand the connection between microclimate and plant health. When she’s not on the land, Amber loves informing people of new ideas/things related to gardening, especially organic gardening, houseplants, and growing plants in a small space.