Last Updated on May 6, 2023 by Griselda M.
Cucumbers are easy to grow but leaves turning yellow can be a problem. When asking “Why are my cucumber leaves turning yellow?” there are a few simple answers as to why, and how to fix this problem. In this article, we will cover the 8 most common reasons I have encountered and how to correct them.
A Quick Overview of Cucumbers
Cucumbers are squash vines that generally produce male and female flowers. The fruits can be either oblong or round. Some people divide them into three categories – slicing, pickling (including gherkins), and burpless varieties. The burpless varieties are seedless, and also really complex things to grow as a home producer as they require the exclusion of pollinators which is difficult in a garden.
If you are going to grow cucumbers, start with Ashely Cucumbers. This heirloom cultivar is incredibly productive and difficult to kill by mistake. It will take over your garden, and produce buckets and buckets of cucumbers. The small cucumbers can be pickled whole and are a bit like Gherkins, the larger ones can be peeled and used in salads, and if you skip a day of picking and end up with a three-pound monster, you can peel it and juice it in smoothies, or use it instead of water in soup and stew. Ashley cucumbers are great, with the one drawback being that the skin tastes a bit bitter and should be peeled off before eating.
If you do not like bitter skin, you can grow a slicing cultivar. I find that these fancy cultivars are a bit more fiddly and tend to give some trouble. If you do not mind having a “funny looking” cucumber, you can grow Lemon Cucumbers which can be eaten with the skins on. I just grow these as a snack to eat while gardening. You can eat them instead of drinking water while you garden – they are delicious and make you feel very healthy.
The surplus can be pickled (pickle baby ones) and the rest can be used in smoothies and whatever is left over can be fed to the chickens. I have sometimes had situations where every time I would go to the garden I got two 5-gallon buckets of Ashley and Lemon cucumbers. It gets silly after a while as these plants are overproductive. Your friends will stop liking you because you keep giving them so many cucumbers.
8 Reasons Why My Cucumber Leaves Are Turning Yellow
1) Nitrogen deficiency – Cucumber plant leaves turning pale green or yellow
Cucumbers are fast-growing heavy feeder plants. In the wild, they ramble around on the ground and put down roots wherever the stems touch and these roots can feed the plant. We tend to grow them on vines, so everything they need has to come from the one rooted area.
When cucumbers have a Nitrogen deficiency the entire plant looks a bit pale. The older leaves especially get pale, and you will find you get a shallow fruit set. I hardly ever get this problem because I plant my cucumbers in soil that is made to be rich in compost, manure, and wood ash. However, if I see yellow, I tend to mulch the plants with a layer of composted manure about 5-6 inches thick.
Using a fertilizer that is incorporated into your manure, compost, and pearlite mix will help. If you are correcting the problem you can add the same fertilizer, as per the instructions, to the surface of the soil and water it in.
2) Potassium deficiency – Older cucumber plant leaves yellowing
When cucumbers become Potassium deficient, the older leaves turn yellow as the plant moves potassium to the younger leaves. You will also note that the veins stay green while the spaces between veins get yellower the further away it is from the vein.
Potassium is a very water-soluble salt, and if your soil is well-drained, as it should be for cucumbers, it is easy for the soil to be Potassium deficient due to leaching. I always incorporate wood ash into my soil mix when I plant my cucumbers.
Wood ash is a rich source of Potassium (and Magnesium). I add a cup per square foot of soil and dig this into the soil. Once the plants reach a height of two to three feet, you can mix more wood ash into manure and top-dress the soil around the cucumber. Potassium moves from the surface downwards in soil, hence placing it on the surface allows it to wash slowly downwards. As the plants get established their roots get deeper and they tend to be able to find more Potassium. If you wanted to use a natural fertilizer.
3) Magnesium deficiency – Yellowing of older leaves
Magnesium tends to wash out of soils quite easily, and if there is a deficiency, the older leaves take on a yellow color. This is a relatively simple deficiency to treat. If you find you have older yellow leaves, take a level teaspoon of Epsom salts (Magnesium sulfate) in a gallon of water and sprinkle this over your plants. Repeat a month later.
There is no need to go overboard with Epsom salts as your plants do not need too much of this fertilizer. A healthy well prepared soil can produce quite a bit of Magnesium, hence as the soil matures it should do this. If you have included slow-release organic fertilizers, compost, and manure in your soil mix, it normally fixes itself over time, and a little dash of Epsom salt water helps.
Epsom Salts are great for your own health in bath water. I tend just to use these for a bath, and then I harvest the bath water in buckets and use it to treat my citrus, squash, banana, and avocado trees. These plants all seem to develop magnesium shortages in summer when rain is high and the soil leaches. In this way, you get two cycles of health benefits from the Epsom salts.
4) Spider mites – Yellow spots on cucumber leaves
Spider mites make a web-like mess under your leaves and if you look closely you will see lots of little red dots – the mites. Each place they bite makes a little yellow dot. The top of the leaf takes on a spotty yellow-dotted look. The mites make the leaf susceptible to all sorts of diseases as it is now full of holes.
You can use neem oil to spray on the underside of your plant leaves. Neem oil inhibits the mites from being able to molt, so they stop growing, and in a while, they vanish. I also have a population of Persimilis predatory mites in my garden that eat spider mites. These can be sourced in your area from a provider – ask the local agricultural extension officer. They are not normally available on Amazon.
5) Whitefly – Yellow spots on cucumber leaves
Whiteflies are an irritating pest that appears during warm dry low wind patches. You will notice when you lift a leaf that the flies fall like white confetti and that the undersides of the leaves are covered in developing flies and eggs. The top of the leaves will have yellow spots, and then dead patches, and then die completely.
I find that I usually control this with neem oil spray. You need to be able to spray the underside of the leaves quite forcefully with neem oil, and I find that three applications, three days apart normally get the infestation under control.
I have a resident population of Encarsia formosa parasitic wasps in my garden. These wasps are quite effective parasites of the whitefly and you will find that the whitefly appears, the wasp numbers increase, and suddenly the whitefly population crashes and vanishes. In my case, there is a cucumber farm across the valley from me and their biocontrols escape and get blown in the wind to my garden. As do their pests.
6) Powdery mildew – Cucumber diseases yellowing of leaves
Normally after you have had spider mite or whitefly damage, the plant leaves are weakened. If you get a bit of wetter weather, powdery mildew will become entrenched. This starts as little white fluffy patches on the top of the leaves, and all the leaves die within days.
Luckily, the neem oil you spray to control Spider mites and Whitefly also controls powdery mildew. Some people suggest using vinegar or bicarb sprays, but I find neem oil works very well.
7) Overwatered cucumber plant turning yellow
If you have rich organic matter-filled soil and it is too wet, the soil will begin to rot, and in so doing, bacteria use up all the oxygen in the soil. The roots will then be oxygen starved and die and in this way, the plant will not have enough nutrients and water, and its leaves will begin to turn yellow.
An overwatered plant looks a bit like a dry plant – its leaves hang limply and look wilted. If you see this, and the soil is very wet, you know the plant has been overwatered. You can immediately stop all watering, and if you take a gardening fork and poke some holes into the soil near the plant to aerate the soil, you may be able to save the plant.
8) Underwatered tomato plant turning yellow
Cucumbers need heavy watering once a week, and a top-up on dry days. If they receive too little water, the plants can develop yellow leaves and become stunted. They are quite a drought-tolerant plant type but will produce weird-looking cucumbers if they receive erratic watering.
I have found that giving them a weekly deep watering causes their roots to grow quite deep into the soil and this in turn makes them resistant to drought, and mineral deficiencies. If you are uncertain about the amount of water to give them, buy a simple moisture meter such as this one and water to the point of being wet – not the top of the wet scale. Just wet. Then allow the soil to dry out to between normal and the start of dry and water again.
Summary: What Causes Cucumber Leaves to Turn Yellow
We have looked at 8 more common reasons to answer your question “Why are my cucumber leaves turning yellow?” Numerous other nutrient deficiencies can also cause cucumbers to turn yellow – more often than not. If you add mulch, manure, and a slow-release organic natural fertilizer of some other sort, there will be enough of all the required nutrients to solve the problems of both the major nutrient and micronutrient deficiencies.
In some cases, if you plant a cucumber in a shaded place, it will become a disease magnet and perform poorly. I find they do very well if they get 7 or 8 hours and at least 6 hours of direct sunlight. When this happens they produce like crazy.
In Conclusion: Why Are My Cucumber Leaves Turning Yellow?
One of the most common reasons for yellowing leaves is that you have a watering problem with too much or too little water causing the problem. Another reason could be nutrient deficiency such as a shortage of Nitrogen, Potassium, or Magnesium. Diseases and pests that affect cucumber plants include spider mites, white flies, and powdery mildew. Make sure the plants receive the right amount of sunlight so that they are more resistant to such pests. Your cucumber plants should receive at least six hours of sunlight daily. If your cucumbers aren’t getting enough sunlight yellowing will appear in the leaves.
Dr. Garth A. Cambray is a Canadian/South African entrepreneur and beekeeper with 28 years of experience in apiculture and specializes in adding value to honey. His Ph.D. research developed a new advanced continuous fermentation method for making mead that has resulted in a number of companies globally being able to access markets for mead. His company, Makana Meadery, exports honey mead to the USA where it is available to discerning connoisseurs. He has also developed technologies to commercially manufacture organic honey vinegar in Zambia for export globally. He holds a few patents globally in the ethanol industry and believes in technology and knowledge transfer for human development and environmental sustainability. One of his proudest achievements is the fact that the wind farm he started at one of his old apiary sites has essentially made his hometown carbon neutral.