Cucumber leaves turning white is a common problem for vegetable gardeners.
But fortunately, it’s easy to identify and do something about.
Disease, pests, and inadequate growing conditions are usually responsible.
So in this article, we’ll take a closer look at the causes, how you can identify them on your cucumber plant, and what you can do to prevent and treat the problem.
Cucumber Leaves Turning White: The Causes
There are 4 common causes of cucumber leaves turning white: powdery mildew, leafhopper infestation, iron deficiency, or sunscald.
It’s important to recognize the problem quickly so you can take the appropriate action. A quick response will often prevent the loss of leaves and save the plant from further harm, as it may die if left.
Photo by Rasbak, CC BY-SA 3.0
Powdery mildew is a common fungal infection that affects plants. In the case of cucumbers and other cucurbits such as squash and zucchini, the main fungus responsible is called Podosphaera xanthii.
The telltale signs are powdery white spots that grow on both the upper and lower sides of the leaves. They rapidly expand into larger patches, and often cover a large percentage of the leaf surface. Spores can also appear on the fruit and stems of the plant.
You’ll often see infected white leaves wilting in the heat. And if the disease progresses, you’ll see the cucumber leaves turning brown, starting to dry up, curl, and eventually falling off.
If you leave it until most of the foliage is infected, then the plant becomes weakened, with premature ripening of fruit that’s often under-sized and malformed.
As it spreads, you might also see cucumbers turning white as the fungus starts to grow on the fruit.
The good news is that it’s not fatal for the plant if you detect it early. They can recover.
Natural fungicides such as neem oil can be useful to treat plants when you first notice them developing a white coating.
And if the infection is mild or moderate, then you can use a horticultural oil such as Sunspray Ultra-Fine Spray Oil or Saf-T-Side Spray Oil to eradicate the infection.
But be careful not to use oil-based sprays when temperatures get over 90°F or on plants with drought stress.
If you know that the area you’re planting in is susceptible to powdery mildew then prevention is the best strategy.
Sulfur based fungicides are a good choice. They should be applied to both sides of the cucumber plant’s leaves every 7-10 days while the conditions for powdery mildew growth persist.
You can also use baking powder as a homemade remedy. It’s not effective on its own, but when you combine it with liquid soap and water it works as a preventative.
Add 1 tablespoon of baking soda and a half-teaspoon of non-detergent soap to a gallon of water. Mix them together to make a spray and make sure your cucumber plants are well covered.
With a bit of planning and care, you can also remove the conditions that give rise to the problem.
Powdery mildew tends to thrive in places that receive a low amount of sunlight, with moderate temperatures. So try to place your plants in a location where they’ll get 6 hours or more of sunlight per day.
It’s also a good idea to space your plants out, allowing good air circulation around them, taking care to stake the plants around 3ft apart, and practice regular weed control.
Another thing to try is removing excess foliage. This allows sunlight to get through to the lower leaves, which are susceptible to developing powdery mildew if they’re left in the shade.
Choosing cucumber varieties that are mildew-resistant is recommended if you’re planting in an area where powdery mildew has occurred before.
Popular choices include Eureka, Jackson Supreme, Sweet Success, and Straight Eight.
There are many insects that pose a threat to cucumber plants. But the leafhopper is the most likely culprit for turning leaves white.
They feed on the underside of the leaves. Sucking out the inner contents, leading to discoloration of the leaf. In doing so, their toxic saliva causes visible white specks.
Leafhoppers are easy to identify. The adults are about 1/4 of an inch long and are quick to fly away if disturbed. Slender and wedge-shaped, they come in brown, yellow, or green, with markings.
Once you know that they’re causing the cucumber plant leaf problems, how do you stop them?
Try floating row covers to prevent them from getting at the plants.
And thoroughly covering the upper and lower surfaces of the leaves with insecticidal soap can get rid of an infestation.
Remember to clean up garden debris after harvest to minimize the number of locations they can use to over-winter.
Chlorosis of bird of paradise leaf
Iron chlorosis is the loss of color in plant leaves due to nitrogen deficiency. To begin with, the cucumber leaves start turning yellow as the color drains from them.
Eventually, as the deficiency becomes severe, you’ll see the pale cucumber leaves turning white. With the outer edges starting to look scorched and brown as the leaf starts to die.
The dark green veins are highly visible against this loss of color and will help you easily identify the problem.
Applying iron chelates to the soil can help as a treatment and prevent your plant from dying. As can foliar sprays that contain chelated iron or ferrous sulfate.
Sunscald on the leaf of Fraxinus americana Robert H. Mohlenbrock. Courtesy of USDA NRCS Wetland Science Institute., Public domain, via Wikimedia Commons
Too much exposure to direct sunlight can cause sunscald on cucumber plant leaves.
Minor cases will see the leaves turn pale and blister. But the affected leaves increasingly appear white with further sun exposure and will eventually die and fall off.
Cucumber seedlings and young plants are particularly susceptible to sunscald after transplant as they move from indoor conditions to outdoor or greenhouse conditions.
To prevent it, remember to give your plants slower exposure to direct sunlight, a process known as hardening off. Try using a sunshade to protect them from sunburn for a couple of weeks until they toughen up.