How To Treat Powdery Mildew on Cucumbers

When growing cucumbers, a fungus developing and ruining your crop is one of the worst things that can happen. One example of such a fungus is powdery mildew, which starts slowly but can spread quickly and voraciously until you can’t control it. The key is to treat it before it goes out of control.

You can treat powdery mildew on cucumbers using neem oil to eradicate the fungus. You can also use a mix of milk and water as treatment and apply baking soda solution for prevention. Alternatively, you can use chemical fungicides that contain sulfur, potassium bicarbonate, or citric acid.

This article will explore what powdery mildew is and how it develops. I will also discuss the methods you can use to treat the disease if it has already begun to affect your cucumber crop. Lastly, we will explore how to plant and tend your cucumbers to prevent powdery mildew from returning.

Powdery Mildew and Cucumbers

Powdery mildew is a common disease in members of the Cucurbitaceae family caused by the fungus Podosphaera xanthii.

The disease also affects many different types of vegetables and fruits by spreading throughout the plants quickly. The fungi absorb the nutrients of the host plant through their epidermal walls using a specialized feeding tube called the haustoria.

Deprived of nutrients, plants will quickly wither and die. If the plants can survive the mildew, the final yield can still be significantly affected, and the few fruits that can grow will ripen prematurely and be stunted and small. 

Powdery mildew does not only have one culprit but encompasses several strains of parasitic fungi in the Erysiphaceae family. No matter what strain it is, this fungus shows up initially on cucumbers as a few pale yellow or white spots on the leaves of the plants, usually in the mid to late summer when temperatures are highest. 

The mildew will typically attack the plant’s new leaves first since the tissues are softer and easier to penetrate. It can form on the tops and bottoms of the leaves and, if not taken care of quickly, will turn into a fine, thread-like mold that spreads as large powdery patches along your entire crop. 

Causes: Moisture, Humidity, and Planting Density

Several potential factors can lead to powdery mildew forming on your cucumber plants. Still, the common denominator in every scenario is that too much moisture is present on the cucumber leaves.

Just like many other fungi, powdery mildew thrives in humid conditions. Its ideal temperatures are around 68-81 °F (20-27 °C), which is around the same range conducive for cucumber growth.

During the summer, when the air is moist and the temperature is hot during the day and cools down at night, the fungus can start to form on wet cucumber leaves. 

Another reason powdery mildew may appear is if your cucumber plants are growing too close together. If your crop has matured into a dense patch of vines and there is not much air circulation between the plants, fungi can start to develop. 

The worst part about this is because there are so many plants so close together, the mildew will be able to spread much faster and broader. 

Reproduction and Spread Through Fungal Spores

Powdery mildew reproduces both sexually and asexually utilizing spores called conidia. These spores grow in chains upwards from the plant’s surface until the wind breaks them off. The wind disperses these spores as far as hundreds of miles away

These invasive plant destroyers can also spread by splashing rain or water droplets. The disease cycle is pretty aggressive and can develop in as little as 72 hours. However, you will only notice the visible effects a week after infection.

This invasive organism has even adapted a means to survive harsh winters only to reemerge in the summer months. They achieve this by forming protective dome structures called chasmothecia, where they overwinter only to strike again when the weather clears. 

Treatment Options

Dealing with a powdery mildew infestation is essential to ensure that the invasive parasitic fungi don’t destroy your precious plants. Three main concoctions will rescue a plant infected with the mildew, depending on the duration and extent of the infestation.  

Use Neem Oil to Eradicate the Fungus

The best way to treat your cucumber plants affected by powdery mildew is with neem oil. 

Neem oil is popular among gardeners because it works against a lot of common plant pests and fungi. Due to its popularity, you can find it at just about any gardening store. It is also entirely natural and will not harm your cucumber plants while killing off the fungus. 

To use neem oil as a fungicide, follow the steps below:

  1. Dilute one tablespoon of pure neem oil by mixing it with one liter (0.26 gal) of water and one tablespoon of dish soap.
  2. Test-spray the solution on one infected leaf at night and wait 24 hours to see any sign of improvement or worsening. If there’s no damage, you can use the solution as is. Otherwise, dilute it further by adding another liter of water.
  3. Remove severely damaged leaves or those coated entirely with powdery mildew. Place them in a garbage bag and dispose of them properly so the mildew can’t spread further. 
  4. Water the plant deeply a day before the treatment. Alternatively, wait until a day after a rainfall to ensure the oil doesn’t get washed off too soon.
  5. Spray the solution liberally on your plant, especially on visibly affected areas. Do this after sunset to prevent sun damage. The leaves can become sunburned or scalded by the sun if they are covered in oil and exposed for too long. 
  6. Repeat this once a week for 3-4 weeks or until the plant recovers. Once the white covering on the surface of the entire foliage disappears, you can reduce the frequency to once every two weeks to prevent recurrence.

Use Milk and Water

There are a couple of homemade treatments you can make to get rid of and prevent powdery mildew, and one of them is a mixture of milk and water. 

This concoction best treats mildew that has just started appearing on your cucumber leaves. You can also use it as a preventive method if you don’t want powdery mildew to come back.

To use this combination, simply: 

  1. Dilute 1 part milk in 10 parts water and mix it. 
  2. Spray the mixture onto your cucumber plants before sunrise, focusing on the infected leaves and those closest to them. 

It is important to spray your milk and water treatment onto the cucumber plants early in the day because they need enough time to dry before nighttime. After sunset, the excess moisture on the leaves can cause more fungus to grow. 

If you use this method, you must measure the milk-to-water ratio precisely as too much milk can invite thrips or mites to your plants. Although they will die after consuming the milk, some may survive to infest nearby plants.

To avoid this, ensure the milk is not too rich or diluted, and only apply it to your cucumbers once every 1-2 weeks.

Use Baking Soda and Water for Prevention

The final remedy to remove powdery mildew from your cucumber plants is a mixture of baking soda and water. 

The best way to use this method is as follows:

  1. Add one tablespoon of baking soda to a gallon (3.8 L) of water. 
  2. Spray the baking soda/water mixture onto your cucumber leaves early in the day. 

This treatment works because of how alkaline baking soda is. Powdery mildew grows and spreads in plants with acidic pH, and the baking soda increases the pH enough to make the leaves of the cucumbers impossible for fungus spores to grow on. 

While pure baking soda will change the pH balance of your soil, diluting it to the ratio recommended above will not result in any negative effects on your plant. You can also alternate this mixture with the milk and water combination, using one every other week.

Like the milk and water mixture, you should spray your baking soda treatment onto the cucumber leaves early in the day. Doing this will dry out the solution before the sun goes down and keep the leaves from inviting more fungus to grow.

Try Chemical Fungicides 

The most commonly used chemical fungicides to treat powdery mildew include low-impact fungicides, such as those containing sulfur and potassium bicarbonate.

Although generally less toxic than synthetic fungicides, these chemical fungicides may have adverse effects when used in the wrong dosages or when applied to vulnerable plants. Be sure to check the product label to confirm if your cucumber species belongs to the list of plants.

You can also use products that contain citric acid. I recommend you use Grower’s Ally Fungicide Spray from Amazon.com. The earth-friendly spray uses concentrated citric acid to remove mildew from your plants and comes in a ready-to-spray bottle. 

Preventive Measures

Although powdery mildew is frustrating to deal with and can be detrimental to your cucumber crop, there are, fortunately, many preventive measures to keep it from growing and spreading:

Order Cucumber Cultivars That Are Mildew-Resistant

The first and easiest step you can take to keep powdery mildew off your cucumber plants is to order cucumber cultivars that are resistant to fungus. 

There are dozens of cucumber plant varieties, many of which have been bred with specific properties so mildew can’t grow on them. If you buy seeds for these kinds of cucumbers, you have a much smaller chance of them becoming infected with powdery mildew. 

Some cucumber cultivars resistant to powdery mildew include the following:

  • Turbo cucumbers
  • Cherokee cucumbers
  • Marketmore cucumbers

Look specifically on the seed packets to see if there is a mention of powdery mildew resistance. If there is, you should be good to go with that breed of cucumber.

Plant Seeds Wide Apart

Powdery mildew has the best chance of growing and spreading quickly when your plants grow close together. The best way to avoid this is to plant your cucumber seeds with plenty of space between them. 

Planting your cucumber seeds approximately 6 inches (15cm) apart and thinning the healthy seedlings to at least three feet (90 cm) apart as they grow will help encourage airflow between the plants as they mature and expand.

You can also grow them 12 inches (30 cm) apart by training them to grow up the trellis for better air circulation. If little to no air can move throughout the plants, they are much more likely to harbor moisture, which is the perfect breeding ground for powdery mildew. 

Plant Seeds in Full Sunlight

Cucumber plants need at least 8 hours of sunlight daily to grow well and keep powdery mildew from growing. The heat from the sun helps evaporate any water leftover on the plants’ leaves and kills off any fungus that may be attempting to grow. 

The simplest way to ensure your cucumber plants are getting enough sun is to plant them in an area with little to no shade covering them. The only exception to this rule is if you live in a hot and dry climate. 

If you expose your cucumber plants to intense sunlight for too long, they can become sunburned and eventually wither and die. In this instance, you will want to plant them in an area that gets shade during the day’s hottest hours or install shade covers that you can spread at midday. 

Apply Fertilizer Sparingly

Powdery mildew thrives on plants with an acidic pH, which is primarily influenced by the type of nutrients present in the soil they are growing in. Although giving your growing seedlings enough nutrients is essential, too much fertilizer in the soil can encourage powdery mildew to infect the cucumber leaves. 

Additionally, the point of adding fertilizer to your soil is to encourage lots of foliage to grow. If your plants are already susceptible to the mildew and they start to develop a lot of new leaves, those leaves can quickly become infected with the fungus.

Cucumbers thrive at pH levels ranging from 6.0-6.8. Aiming for a pH of around 6.5 to make nutrients still available for your plants while reducing the risk of powdery mildew. This will give your plant room for error when using the alkaline milk or baking soda solution as a treatment.

Mulch Your Plants

If you are concerned about your cucumbers not taking in enough nutrients, spread organic mulch, such as compost, which also serves as a slow-release organic fertilizer. You can also use straw mulch for cucumber plants.

Adding mulch to the mix has a few benefits. It can help to keep the temperature of the soil at a comfortable level when the climate is very hot or cold. Mulch can also help to keep weeds from sprouting near your cucumber plants and stealing valuable nutrients and water from them. 

And perhaps most importantly, it can help keep the cucumber plants’ foliage and vines from directly touching the soil. 

Preventing the leaves and vines from touching the ground is essential, especially after you water the plants. The soil will likely be somewhat waterlogged, and if you expose the cucumbers to the water for too long, it can create a breeding ground for powdery mildew to grow. 

Water at the Base to Keep the Leaves From Becoming Too Wet

If your main goal is to keep your cucumber plants’ leaves from becoming too wet, it is best to water them at their base instead of from overhead. 

The best way to accomplish this is by using a soaker hose or drip irrigation system to water the plants as opposed to a regular hose or watering can. If you pour water directly onto the leaves, they need time to dry before the sun sets. Otherwise, the leaves will remain wet overnight, and powdery mildew will have a much easier time developing. 

When using a narrow-spouted watering can, water your cucumbers at soil level so the moisture goes directly into the ground. If you water them deeply and infrequently, the roots will be able to grow deeper and take in much more of the water. 

When you have strong and sturdy roots that have plenty of water and nutrients, your cucumbers will be less likely to be stressed, making them more resistant to diseases like powdery mildew. 

Prune the Dead Leaves

The older leaves on your cucumber plants starting to die are the most vulnerable to powdery mildew attacks. To prevent them from endangering the rest of the plants, prune off the dead or dying leaves and remove them from the area altogether. 

After you prune any plant that you may have exposed to powdery mildew, you must clean your pruning shears or scissors with rubbing alcohol or a 10% bleach solution. Doing so will keep any potential fungi or diseases from developing and spreading to the other plants in your garden. 

Rotate the Planting Location

The last measure to prevent powdery mildew from growing on your cucumbers is to rotate where you grow them each season. Wait 2-3 years before growing your cucumbers in the same area, especially if there’s a history of cucumber-specific pests and diseases.

If you plant them in the same space in the garden every time, a buildup of fungi and diseases can occur and affect the plants year after year.

By moving them to a new area in the garden, you are providing them with fresh, new soil that does not have any lingering pests, mold, or bacteria to affect them negatively. 

Final Thoughts

It is common for powdery mildew to appear on your cucumber plants, making it crucial to treat and prevent it from returning. As long as you keep the leaves free of moisture and don’t plant them too close together, your cucumber plants should be healthy. 

And if they do develop powdery mildew, a mixture of milk and water, baking soda and water, or neem oil spray will do the trick!

Dr. Moritz Picot

Dr. Moritz Picot is a horticulture enthusiast and the founder of TheGrowingLeaf.com, where he serves as the lead content writer. He established the website in 2022 as a valuable resource for both gardening aficionados and beginners, compiling all the gardening tips he has accumulated over the past 25 years. Alex has a passion for nurturing plants, transforming backyards into inviting spaces, and sharing his knowledge with the world.

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