Cucumbers are long-standing favorites of home gardeners. They are fairly low-maintenance, and in the right conditions, they produce fruit like crazy! So when cucumber plants turn white, it can be alarming.
If your cucumber plants are turning white, diseases and pests are the most likely causes. Prevention through good growing practices is the best solution, but if you catch the problem in time, you may still be able to save your affected plants.
Let’s look at these common issues and what you can do to not only prevent them but help them recover as well.
Check out Garden In Minutes' super-simple TOOL-FREE, Cypress Raised Garden Bed Kits (link to Garden In Minutes website).
Of the diseases that strike cucumbers and other cucurbits (squash, melons, and pumpkins), the following are the ones most likely to cause white patches on your plants:
- Powdery mildew
- Septoria leaf spot
- Angular leaf spot
Powdery mildew is one of the most common cucurbit diseases. The earliest symptoms are white spots with a powdery texture on the surface of the leaves. As it progresses, powdery mildew covers both sides of the leaves as well as the plant’s stems.
Eventually, the leaves develop a distorted shape and drop from the plant entirely. If your plant has already set fruit, it may ripen before it’s fully mature.
Powdery mildew is caused by a fungus that is most robust in humid, mild weather. The wind carries fungus spores from plant to plant.
If your cucumber plants have powdery mildew, apply a fungicide right away. You can purchase an organic fungicide online (link to Amazon) or ask a local expert about the fungicides available in your area.
You can prevent powdery mildew in the future by planting resistant cucumber varieties, providing ample space between plants, managing weeds, and applying fertilizer as necessary.
Septoria Leaf Spot
Septoria leaf spot is another fungal disease that affects cucumbers and other cucurbits. In its early stages, it looks like dark, wet spots on leaves. These spots eventually become beige or white (source).
The fungus that causes septoria leaf spot spreads from plant to plant in water droplets from splashing rain or irrigation from above.
If you notice signs of septoria leaf spot, apply an approved fungicide as soon as possible. Spores from the fungus can survive for a year or longer, regardless of the plant’s health, so destroy any debris from affected plants.
You can prevent septoria leaf spot by rotating your cucurbits with other plants every two years, allowing plenty of space between plants, and irrigating your plants at their bases instead of from above (source).
Angular Leaf Spot
Angular leaf spot is another disease that begins with small wet-looking spots on leaves. The spots develop into lesions which grow in an angular pattern. The lesions secrete a substance that is milky and turns into a white crust as it dries.
As the disease progresses, the centers of the lesions may drop, leaving holes in the leaves.
Angular leaf spot is bacterial and spreads in a variety of ways: infected seed, insects, water droplets, and even people moving from one plant to another. The bacteria can survive on debris and in the soil for two and a half years.
If you catch it early, you can treat angular leaf spot with copper. Don’t plant another cucurbit on the same site for another two years.
Scab is yet another fungal disease that first presents with small, watery lesions on leaves and stems. The lesions turn white or gray, and the leaves develop a raggedy appearance as their tissue dies.
If your plant has already set fruit, you may notice signs like gummy gray spots on the fruit that turn into little craters. Spores may develop on these spots that look like olive green velvet.
In its early stages, you can treat scab with an approved fungicide. Once the disease has advanced, your best option is to remove and destroy the affected plants and avoid planting cucurbits on the same site next year.
You can prevent scab by planting resistant cucumber varieties and keeping the area free from weeds or other plants that may carry spores.
Diseases are not the only cause of bleaching and white spots. Your cucumber plant may have a pest problem instead.
There are several pests that are attracted to cucumbers, but the two that cause whitening are thrips and spider mites.
Thrips are extremely thin, tiny insects, only about 1.5 millimeters long. They may be pale yellow or tan, and because of their size and color, they can blend in very easily.
If thrips have infested your cucumber plants, you may first notice “rasping” of the leaves. This looks like mottled, silvery patterns on the leaves’ surface. Thrips leave black feces behind, which is one way to tell the difference between thrip damage and disease.
Thrips feed on leaves, and these silvery scars are the result. Thrips don’t just feed on leaves, however. They will also eat flowers, shoots, and fruit; if their numbers are high enough, they will severely stunt the entire plant.
Furthermore, thrips are carriers of tomato spotted wilt virus, so controlling the thrip population will benefit more than just your cucumber plants.
If your cucumber plants have a small thrip infestation but are otherwise healthy, they can probably tolerate the damage. Prune affected leaves and use a reflective mulch to discourage the thrip population from growing.
If the infestation is large, consult a local expert about insecticide options that are available in your area. It may be best to avoid broad-spectrum insecticides, since those could kill thrips’ natural predators.
You can prevent future thrip infestations by planting your cucumbers away from onions, garlic, weeds, and other plants that attract thrips. Applying reflective mulch at the beginning of the growing season can also encourage them to make their home somewhere else.
Spider mites are even smaller than thrips, only 0.5 millimeters in size. These tiny arachnids feed on dozens of flowers and vegetables that are popular in home gardens.
Like thrips, spider mites feed on leaves. They suck out nutrients from the underside of leaves, leaving white spots behind. When the spider mite population is large, leaves may appear completely bleached.
Furthermore, like other arachnids, spider mites spin protective webs. If you notice stippling and bleaching on your plant’s leaves, check for webbing as well, as this is a sure sign of a spider mite infestation.
If your cucumbers are hosting spider mites, use a strong spray of water to wash them and their webbing off of your plants. In some cases, insecticidal soaps may help, too.
The good news is that spider mite populations only flourish on plants that are already stressed. You can prevent an infestation just by keeping your cucumbers well-watered, especially during the hottest days of summer.
You can also prevent infestations by refraining from the use of broad-spectrum insecticides which kill off spider mites’ natural predators like lady beetles (source).
Best Growing Practices
You can prevent many cucurbit diseases and pest problems by doing the following:
- Buy seeds and seedlings from reputable sellers. Many fungal and bacterial diseases start in the seeds themselves.
- Plant your cucumbers when the soil reaches 65℉ and air temperatures are consistently above 70℉. Cucumbers wither in cool temperatures, so wait for springtime temperatures to rise before planting.
- Give your cucumber plants plenty of space to allow air to flow in between them. Plant them eight to fifteen inches away from other plants. Training them to grow upward on a trellis or fence is helpful, too.
- As much as possible, avoid planting your cucumbers near other cucurbits to prevent the spread of shared diseases and pests.
- Use reflective mulch to deter insects from attacking.
- Keep weeds under control. Dense vegetation invites pests to the garden.
- Water and fertilize regularly. Cucumbers need about an inch of water per week. As needed, apply a balanced fertilizer formulated for vegetables.
- Rotate your crops to prevent soil-borne cucurbit diseases. If possible, don’t plant cucumbers in the same site two years in a row (source).
Diseases like powdery mildew and pests like thrips are the most likely causes of your cucumber plants turning white. While fungicides and insecticides may help solve the issue, prevention is the most effective defense. Maintaining the overall health of your cucumber plants, from seed to harvest, is your best course of action.