How to Get Rid of Powdery Mildew: A Complete Guide

Last Updated on March 18, 2022 by Zachary Smith

Have you noticed a strange white fungus on the leaves of your trees or garden plants?

If so, it could be powdery mildew.

Powdery mildew is a fungal disease that affects many common species of trees, shrubs, flowers, and grasses.

If left untreated, it can slow the growth of your trees, reduce fruit yield and quality, and spread to other areas of your garden or landscaping.

If you’re wondering how to get rid of powdery mildew, you’ve come to the right place. The team at Zachary Smith Arboriculture treats powdery mildew in the San Francisco Bay Area and Central Coast each day, so we know how to get rid of the fungus for good.

In this blog, we’ll share our top powdery mildew treatment tips.

Let’s dive in.

What is Powdery Mildew?

Powdery mildew is one of the most common plant diseases in the US. It’s also one of the easiest to recognize.

Characterized by patches of white or gray growth on a plant’s leaves (they look like dustings of flour), powdery mildew is caused by the fungus Podosphaera xanthii. While powdery mildew occurs throughout the country, it’s most common in warm, dry climates like here in California.

Powdery mildew spores spread on the wind and can easily be carried to plants hundreds of miles away from the source.

Once a spore lands on a host plant, it germinates quickly and starts a new infection. Unlike other leaf fungi, powdery mildew doesn’t need moisture to thrive and can easily spread on dry leaves.

Because powdery mildew is so common, some plants have developed natural resistance to the fungus.

Still, it affects at least 10,000 different species of plants, including vegetables like squash and tomato plants, ornamental garden plants like peonies, weeds, shrubs, fruit trees, grasses, shade and forest trees, bee balm plants, and even cannabis plants.

Fortunately, treating powdery mildew is relatively easy. Cultural controls are effective on this fungus, especially when combined with targeted chemical treatments.

How Does Powdery Mildew Damage Plants & Trees?

As powdery mildew germinates on a plant leaf, it creates a mat of fungal growth on the leaf’s surface. Specialized structures within the fungus penetrate the plant tissue and suck up nutrients.

While powdery mildew usually doesn’t kill plants, it does weaken them.

Over time, affected plants may display the following symptoms:

Yellowing or brown leaves.
Decreasing fruit of flower production.
Drying, disfigured, or curling leaves.
Slower growth.

The symptoms of powdery mildew usually become evident late in the growing season and can appear on leaves, fruit, and stems.

How Do You Get Rid of Powdery Mildew? 3 Steps to Take

treating powdery mildew with fungicide spray

As powdery mildew proliferates, it produces long chains of spores that grow in a vertical column. These spores break off easily and can be carried great distances by the wind.

Because of this, removing the fungus from your garden requires getting rid of all signs of powdery mildew growth.

Here’s how:

1. Trim infected plants

removing branch of apple tree with powdery mildew

Remove infected plants from your garden by digging them up entirely or using plant clippers to cut back parts of the plant with visible powdery mildew.

Instead of composting or burning these infected plants, seal them in a plastic bag and throw them in the trash.

After pruning, wash your hands thoroughly and clean your clippers or other garden tools with alcohol wipes to kill remaining fungal spores.

2. Apply a fungicide

Fungicide application is one of the easiest ways to kill powdery mildew. Conventional products containing active ingredients like chlorothalonil can be effective on powdery mildew. There are also many natural treatment options available.

Here are a few home remedies that will kill powdery mildew quickly:

Vinegar

Vinegar contains acetic acid, which can control powdery mildew.

For best results, mix 2-3 tablespoons of apple cider vinegar into a gallon of water and spray the mixture onto powdery mildew.

If you’re short on vinegar, you can also use mouthwash since the ethanol it contains will act like acetic acid and kill powdery mildew.

For best results, we recommend mixing one part mouthwash with three parts water and spraying it directly onto the fungus.

Pros: Effective, affordable, natural.

Cons: Too much vinegar or mouthwash can burn plants, so be sure to stick to the formulas above.

Baking Soda

Baking soda, or sodium bicarbonate, is one of the best-known organic treatments for powdery mildew.

When combined with horticultural-grade dormant oil and liquid soap, baking soda is excellent for preventing and controlling fungal growth.

Here’s how to use it:

Mix one tablespoon of baking soda with a teaspoon each of dormant oil and liquid soap. Add the mixture to a gallon of water. Shake well and spray the solution on affected plants every 1-2 weeks.

Pros: Effective, affordable, safe for kids and pets, natural, won’t harm neighboring plants.

Cons: Requires regular reapplication.

Potassium Bicarbonate

Like baking soda, potassium bicarbonate effectively kills existing powdery mildew infestations.

Approved for use in organic gardens, potassium bicarbonate kills the fungus on contact and is an excellent preventative method.

Here’s how to use it:

Mix half a teaspoon of liquid dish soap into a gallon of water. Stir in 1 tablespoon of potassium bicarbonate (which you can order online or find at your local garden store). Spray the mixture on the leaves of affected plants.

Pros: Effective, affordable, ideal as a long-term solution, suitable for treating and preventing infections.

Cons: Requires regular reapplication.

Milk

Milk contains naturally-occurring compounds that kill powdery mildew while strengthening and fortifying plants.

Since studies have shown that high concentrations of milk work better than low concentrations, we recommend mixing one part milk with two parts water and spraying it onto plants weekly.

Pros: Affordable, easy to mix, non-toxic, ideal for organic gardening.

Cons: Requires reapplication after rain or watering.

Sulfur/Lime Sulfur/Copper Sulfate

When sulfur is applied directly to powdery mildew, it prevents the infection from spreading. When mixed with hydrated lime, the solution penetrates existing fungal growth to kill infections.

You can find pre-mixed lime sulfur solutions at your local garden store.

Copper sulfate is another solution that will kill and control powdery mildew and other fungal infections. Find premixed sprays and formulas at your local garden store.

Pros: Effective, fast-acting.

Cons: Too much of any sulfur solution can burn plant leaves. Sulfur mixtures can also damage microorganisms in the soil, may kill beneficial insects, and are considered moderately toxic to humans, pets, and other animals.

Soapy Water

A mixture of insecticidal soap and water can kill powdery mildew. For best results, mix 1 quart of water with insecticidal soap, according to the manufacturer’s directions. Spray onto affected areas.

Pros: Easy to apply.

Cons: More expensive than other options, is more effective on plants than trees.

Neem Oil

Neem oil is a popular organic option for treating powdery mildew. Extracted from the neem tree, neem oil is a broad-spectrum insecticide and disease control agent. It targets pest species without harming beneficial insects or mammals.

For best results, mix 2.5 tablespoons of pure neem oil into a gallon of water. Spray powdery mildew with the mixture every 7-14 days, until the infection has cleared.

NOTE: If you’re also using sulfur to treat powdery mildew, wait at least two weeks after your last sulfur spray to apply neem oil.

Also, avoid applying oils when temperatures are above 90 degrees Fahrenheit or when plants are drought-stressed.

Pros: Affordable, eco-friendly, organic, won’t harm kids, pets, or wildlife.

Cons: Requires regular reapplication, especially after rain or watering; applying oils too soon after a sulfur spray or during hot or dry periods can burn plants.

3. Learn how to Manage Your Garden

drip irrigation system to prevent powdery mildew formation

When it comes to powdery mildew, it’s easier to prevent infections than cure them. With this in mind, take proactive steps to fortify your garden against powdery mildew.

Here’s what we recommend:

Focus on airflow. Powdery mildew grows faster in dense, close thatches of plant growth. To prevent this, prune and thin your plants to promote airflow through and around them. This will decrease the risk of powdery mildew and make existing infections easier to identify.
Avoid fertilizing affected plants. If a plant has an active powdery mildew infection, don’t fertilize it. Instead, wait until the outbreak is under control. Since spores favor young, tender growth, fertilizing infected plants will only cause the fungus to spread faster.
Install drip irrigation. Avoid watering plants from above since this can cause spores to splash onto nearby plants. Instead, install drip irrigation systems to ensure slow, deep watering around the root spread of your plants.

How to Prevent Powdery Mildew

Once you’ve killed any existing powdery mildew infection, it’s time to prevent the fungus from coming back.