How to Treat and Prevent Powdery Mildew Effectively

Learn how to identify, prevent and treat powdery mildew disease on plants using time-tested, organic and natural techniques.

Common on many plants and easily recognized, powdery mildew is a fungal disease found throughout the United States.

It is caused by a variety of closely related fungal species, each with a limited host range. Low soil moisture combined with high humidity levels at the plant surface favors this disease.

Symptoms usually appear later in the growing season on outdoor plants. Powdery mildew starts on young leaves as raised blister-like areas that cause leaves to curl, exposing the lower leaf surface.

Infected leaves become covered with a white to gray powdery growth, usually on the upper surface; unopened flower buds may be white with mildew and may never open. Leaves of severely infected plants turn brown and drop.

The disease prefers young, succulent growth; mature leaves are usually not affected. Fungal spores overwinter inside leaf buds and other plant debris.

Wind, water, and insects transmit the spores to other nearby plants. Zucchini, beans, cucumbers, squash, pumpkins, tomatoes, roses, and zinnia are especially susceptible.

If powdery mildew is something you’re concerned about in your home garden, then this guide will teach you everything you need to know to prevent and control it effectively. Also, learn which plants are the most susceptible to this infection.

What is Powdery Mildew?

Powdery mildew is a fungal disease that affects many different types of plants. These white, powdery spots emerge in a circular pattern on the leaves, stems, and even the fruit.

Powdery mildew often grows on the upper surface of the leaves, although it can also develop on the undersides. Young foliage is the most vulnerable to damage. The leaves then turn yellow and start to dry out. Some leaves could twist, break, or change shape because of the fungus.

Unfortunately, this disease spreads and will cause white spots to spread until they cover most of the leaves or affected areas. Additionally, the leaves, buds, and developing tips will become deformed. Most of the time, these signs show up late in the growing season.

What are the Symptoms of Powdery Mildew Infection?

Powdery mildew fungi produce structures that are inserted into plant cells, allowing them to extract nutrients required for development and spore production.

This results in a general loss in the host’s growth and health, with noticeable symptoms appearing. This includes abnormal growth, such as leaf curling, twisting, and yellowing may be observed before the fungus’s white symptoms are visible.

The white symptoms appear as circular white, powdery spots, hence why it’s called powdery mildew. Understanding which plants are susceptible to it the most can help spot it before it spreads and takes over the entire plant.

Which Vegetables and Plants are Affected the Most?

Powdery mildew infection can affect a wide variety of plants, as there are many different species of this fungal disease that attack different plant families.

When it comes to vegetables, the following are the most vulnerable to this disease:

Nightshades: tomatoes, eggplants, peppers, lettuce, potatoes
Cucurbits: cucumbers, melons, squash, pumpkins, zucchini
Legumes: beans, peas.

Other ornamental plants can also suffer from this fungi. These include lilacs, hibiscus, peonies, roses, succulents, dahlias, begonias, and zinnias.

How Does Powdery Mildew Spread?

Powdery mildew spores normally enter your garden with the wind. However, if you’ve experienced powdery mildew outbreaks in the past, subsequent outbreaks may also start from dormant spores in surrounding dead vegetation or weeds.

Unlike many other fungal diseases you might have come across, this one spreads and thrives the most in dry and warm climates. However, it requires high relative humidity to spread. This isn’t the humidity in the air, but more in terms of the humidity around the plant.

It doesn’t spread as well in cooler and rainy areas. Interestingly, it slows down when temperatures are higher than 90°F. This means that the ideal temperature for powdery mildew to thrive is around 60 to 80°F. It also affects plants that are in shady spots more than those that get direct sun.

How to Prevent Powdery Mildew Infection

Plant resistant cultivars in sunny locations whenever possible.
Prune or stake plants to improve air circulation. Make sure to disinfect your pruning tools (one part bleach to 4 parts water) after each cut.
Remove diseased foliage from the plant and clean up fallen debris on the ground.
Use a thick layer of mulch or organic compost to cover the soil after you have raked and cleaned it well. Mulch will prevent the disease spores from splashing back up onto the leaves.
Water in the morning, so plants have a chance to dry during the day. Drip irrigation and soaker hoses will help keep the foliage dry.
Use a slow-release, organic fertilizer on crops and avoid excess nitrogen. Soft, leafy, new growth is most susceptible.

How to Treat Powdery Mildew Effectively to Get Rid of It

Milk Spray

Milk sprays, made with 40% milk and 60% water, are an effective home remedy for use on a wide range of plants. For best results, spray plant leaves as a preventative measure every 10-14 days.

Baking Soda

Baking soda is one of the best home remedies for treating powdery mildew. For this method, 1ix 1 tablespoon baking soda and ½ teaspoon liquid soap in 1 gallon of water. Transfer it into a spray bottle and spray the tops and underside of leaves and any other affected areas.

Although this works best as a preventative measure, it’s also known to be effective on existing powdery mildew infections as well.

Wash Foliage

Wash foliage occasionally to disrupt the daily spore-releasing cycle. Neem oil and PM Wash, used on a 7-day schedule, will prevent a fungal attack on plants grown indoors.

Sulfur or Copper-Based Fungicide

Apply sulfur or copper-based fungicides to prevent infection of susceptible plants. For best results, apply early or at the first sign of disease. Spray all plant parts thoroughly and repeat at 7-10 day intervals up to the day of harvest.

Potassium Bicarbonate

Green Cure Fungicide contains a patented formula of potassium bicarbonate — commonly used in food products — that kills many plant diseases on contact and provides up to 2 weeks of residual protection.

At the first sign of disease, mix 1-2 Tbsp/ gallon of water and apply to all exposed surfaces of the plant. Monterey® BI-CARB is a similar product containing micro-encapsulated potassium bicarbonate as the active ingredient. Mix 4 teaspoons in 2 gallons of water to thoroughly cover foliage.

Broad-Spectrum Fungicide

Effectively treat fungal diseases with SERENADE Garden. This broad-spectrum bio-fungicide uses a patented strain of Bacillus subtilis that is approved for organic gardening. Best of all, it’s safe to use — you can treat and pick crops on the same day!

Sulfur Burner for Indoor Plants

Indoor growers may want to consider using a Sulfur Burner/ Vaporizer which turns sulfur prills into fine dust and changes the pH of leaf surfaces.

Fungal spores and mold can’t get established on this plant coating. As an added benefit, studies have shown that this dust will eliminate spider mites.

Author

Eric Vinje

Eric Vinje founded Planet Natural with his father Wayne in 1991, originally running it as a grasshopper bait mail-order business out of a garage.

Eric is now retired, but is still a renowned gardener known for his expertise in composting, organic gardening and pest control, utilizing pesticide-free options, such as beneficial insects.

Eric believes when you do something good for the environment, the effects will benefit generations to come.