Cucumbers are among the easiest low-maintenance vegetables to grow. They long for little, but even a little of something they need can make them behave strangely.
When you get white leaves on cucumber plants, your initial thought might be that it needs a nitrogen fix.
That is the nutrient it needs for the leaves to be green. Without green leaves, it cannot produce food (photosynthesize).
While true, several things can hinder photosynthesis causing the leaves to pale.
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What causes white leaves on cucumbers?
The advanced stage of powdery mildew covers leaves in white spores. Blanching can occur by overhead leaves blocking sunlight. Overwatering can leach nutrients from the soil. The most damaging pest as far as cucumbers go is the leafhopper; its toxic saliva causing white speckling on the parts of leaves it feeds on.
Advanced stages of powdery mildew
Powdery mildew is a fungal infection that causes white spots on cucumber leaves. It starts at the leaf edges and spreads inward toward the stem.
It can be treated early with an insecticidal soap or neem oil. If it goes untreated, in its advanced stages, the fungal spores spread from the leaf edges and merge to coat the entire leaf.
It can look like your cucumber leaves are coated entirely in white. It is not white leaves, but rather a blanket of tiny white fungal spores.
Caught early, homemade organic fungicides such as neem oil or horticultural oil can kill the spores, preventing spread.
In the advanced stages, once it has spread across entire leaves, a stronger chemical fungicide may be needed.
If you do decide chemical fungicides are the way to go, do not rely on one solution. Powdery mildew (like most plant diseases) can develop a chemical resistance.
To prevent a recurrence of powdery mildew disease, alternate treatments between any of 11 strobilurin fungicides with SuffOil-X.
Overwatering will leach the soil of nutrients
Overwatering any plant is a recipe for disaster. It is often the primary contributing factor that ends a plant’s life by early fungal diseases in the soil or root rot.
The cucumber needs a lot of phosphorous and potassium for it to grow fruit. And cucumbers are a fruit. They are classed as a vegetable in the culinary sense because they get served with vegetable salads, not in a fruit salad.
Nitrogen only promotes foliage growth. Phosphorous and potassium are what help these flourish.
The best fertilizer for these vegetables is a low nitrogen, high phosphorous, and high potassium plant food. On the labels, the NPK numbers are in order of Nitrogen, Phosphorous, Potassium.
Use a fertilizer that has the first number lower, and the other two higher and equal. As an example, a 5-10-10 fertilizer, or a 10-20-20 fertilizer are the types to use for cucumbers.
Watering too much will dilute the nutrients in the soil. Essentially, the water runoff leaches crucial nutrients the plant needs.
Since it’s already being fed a low amount of nitrogen (the nutrient it needs for green leaves), it will instead turn pale yellow. When there’s an extreme lack of nitrogen, they’ll turn more white than yellow.
Fix leaching by adding a little fertilizer, water lightly, and cut back on how much water you add to the soil. Too much is just wastage. It hurts the plant more than it helps it.
Accidental blanching by letting surrounding plants grow too big
Plenty of vegetables benefit from blanching. It is the process of blocking sunlight to deliberately hinder photosynthesis.
Why would you do that?
To improve the taste. Vegetables that can taste bitter (such as celery) benefit from blanching. Other vegetables like cabbage take care of this themselves.
Any vegetable that has green leaves on the outermost layer that turns white on the inside is because of blanching. You see this on cabbage, celery, spring onions, and leeks.
You do not want to see it on the leaves of cucumbers.
When you have some healthy green foliage mixed with partially white leaves, the problem may be blanching caused by overhead plants blocking sunlight.
The fix is to prune the surrounding plants that are blocking sunlight from overhead. Failing that, relocate the plants to another patch where sunlight can reach all the leaves will restore their color.
Leafhoppers poison plants causing the leaves to whiten
The majority of insects are problematic for most plants. The ones most likely to cause whitening on cucumber leaves are leafhoppers.
Similar to most leaf piercing insects, leafhoppers suck the sap from the leaves. However, the whitening caused by the leafhopper is because its saliva is toxic.
Other insects that feed similarly cause yellow discoloration due to the fluid drainage. The toxicity of the saliva from leafhoppers causes the whitening on plant leaves.
Where it pierces the leaves on cucumber plants, the toxins from its saliva cause the holes to turn white.
The simplest control method is to use a floating row cover to prevent adult leafhoppers from reaching the plant.
Unless you’re growing a self-pollinator, the row cover will need to be removed once the plant matures for beneficial insects to move the pollen between plants.
If they have already gotten at your plants, then it will be too late for prevention. These lay eggs in the plants they feed on. You need to break the cycle by killing the larvae.
To treat the larval stage of leafhopper infestations, use horticultural oil, or neem oil to kill insect larvae before they mature and damage more of your crops.
Frequently asked questions related to white leaves on cucumbers
How far apart should cucumbers be planted to prevent powdery mildew?
When growing cucumbers from seed, plant them at least 6-inches apart. Vining varieties of cucumber need a trellis to support upward growth. Preferably, a 6-foot trellis. For bush varieties, rows should have three to four feet of spacing between the rows to prevent the leaves crowding the garden bed.
How to water cucumber plants?
Much of the problems that lead to white leaves on cucumbers are watering-related. Wet leaves encourage fungal spores and insects. When watering, water the roots, not the leaves. Avoid overhead irrigation as wet leaves are how problems start.
Daniel has been a plant enthusiast for over 20 years. He owns hundreds of houseplants and prepares for the chili growing seasons yearly with great anticipation. His favorite plants are plant species in the Araceae family, such as Monstera, Philodendron, and Anthurium. He also loves gardening and is growing hot peppers, tomatoes, and many more vegetables.