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How to Prevent and Treat Powdery Mildew

Learn how to prevent and treat powdery mildew before it causes problems with the fruits and vegetables growing in your garden. Powdery mildew can spread quickly and, if left untreated, can cause significant damage to infected plants.

Fortunately, there are several methods available to help control and prevent this fungal disease from spreading in your garden. All gardeners should be aware of the signs of powdery mildew and how to treat it quickly and effectively.

What is powdery mildew?

When trying to prevent or control powdery mildew, start by understanding what it is and where it comes from. Powdery mildew is a disease caused by several types of fungi. It first appears as a whitish-gray powdery coating on the surfaces of plants, eventually turning the leaves yellow or brown.

Left untreated, powdery mildew quickly spreads by spores on wind or physical contact. The disease weakens the plant and causes it to become stunted in growth. If not treated promptly, it can sometimes lead to leaf drop or plant death.

What does powdery mildew look like?

Learn what powdery mildew looks like so you can begin to combat it when it first appears. Powdery mildew is a fungal disease that can infect many plants, including vegetables. It appears as white powder-like spots on the leaves and other parts of the plant.

  • Roses: New flower buds are covered in white powder. Powdery white or gray coating on the leaves and stems. The infected leaves may curl or become distorted, and the plant may produce fewer blooms.
  • Raspberries: Undersides and tips of leaves become deformed.
  • Cantaloupe: Leaves and stems turn brown; poor fruit production.
  • Peas: White or grayish coating on the leaves can then spread to the stems and pods. The entire plant may be covered in a powdery coating in severe cases.
  • Zinnias: Mildew covers leaves, buds, and stems. Infected plants may produce fewer blooms or stop blooming altogether.
  • Squash: White or grayish patches on the leaves, which can then spread to the stems, fruit, and flowers. Infected foliage may turn yellow or brown and fall off the plant. (Tip: Know what you are growing. Many squash varieties have variegated leaves that may look like powdery mildew.)
  • Apples: White patches on the leaves, fruit, and stems.
  • Cucumbers: Powdery mildew appears as white or grayish spots on the leaves, stems, and fruit. As the infection progresses, the spots may merge and cover larger areas of the plant.
  • Phlox: Powdery mildew appears as white or grayish spots on the leaves and stems. Infected foliage may turn yellow or brown and fall off the plant.
  • Lilacs: Powdery mildew appears as a white or grayish powder on the leaves, which can then spread to the stems and flowers. The infected foliage may become distorted or stunted.
  • Grapes: White or grayish-white powder or coating on the leaves, stems, flowers, and fruit. The infected leaves may become distorted, yellow, or fall off the plant. If left untreated, powdery mildew can reduce grape yields and quality.

Do these zucchini leaves have powdery mildew?

No. Some types of squash leaves have variegated leaves that, at first glance, may look like powdery mildew. However, it’s merely a color variation. Look closely at the leaves, and you’ll be able to distinguish between the two. The color variation will be consistent on all the leaves, but powdery mildew will appear in patches.

Variegated leaves on a zucchini plant (not powdery mildew)

If you think your leaves may have powdery mildew, check for other signs, such as yellow or brown spots on the leaf surface and white, dusty-looking growth on the top and bottom of the leaves. It’s important to take action if you suspect powdery mildew since it can spread quickly throughout your plant.

Light-colored leaves on a summer squash plant (not powdery mildew)

7 Tips for How to Prevent and Treat Powdery Mildew

1. Plant powdery mildew-resistant varieties.

To mitigate the occurrence of powdery mildew, opt for disease-resistant varieties when planting crops that are vulnerable to this fungal disease. Examples of such susceptible crops include cantaloupe, zinnias, roses, squash, and peas.

By selecting resistant strains, you can safeguard your plants and minimize the impact of powdery mildew. Powdery mildew resistance is marked on seed packages with the designation PMR. Be sure to rotate crops each year, as the same crop planted in the same spot can lead to soil-borne disease problems.

2. Prune or stake plants to improve air circulation.

An abundance of foliage or vines left sprawling on the ground invites pests and diseases. Proper air circulation helps prevent fungal diseases. Prune and trellis plants to allow as much air and sunlight as possible to reach them.

3. Water correctly.

I use these garden grids in all of my raised beds.

Space your plants with enough room for air circulation so that the leaves have a chance to dry out after rain or watering. Also, avoid wetting the foliage when you water, and water in the morning rather than late afternoon or evening.

  • Water in the morning. Evening watering creates a cool, damp climate that promotes powdery mildew and other fungal diseases.
  • Using drip irrigation (rather than overhead) puts the water on the soil (where it is needed) rather than on the leaves. 
  • Do not over-water. Let the top inch or two of soil dry out between waterings. Damp soil promotes fungal diseases. 
  • For more information, read this article about correct watering

4. Keep plant debris cleaned up to prevent powdery mildew.

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Leaves and trimmings from plants can harbor fungal spores that cause powdery mildew.

Rake up and remove debris from around plants. Prune away affected branches to minimize spread. Good air circulation around the plants also helps reduce mildew growth.

5. Use organic options to treat powdery mildew

Choose one (or more) of the following methods. To prevent further infections, continue treatment while warm, humid conditions persist.  

  • Treat the remaining leaves with a baking soda / castile soap solution every 7 days. Use 5 milliliters (1 teaspoon) of baking soda plus 5 milliliters (1 teaspoon) of liquid castile soap per 1 liter (quart) of water in a spray bottle. Apply as a preventative measure or to halt early outbreaks. Apply after the sun goes down and spray off afterward to prevent leaf burn.
  • Treat powdery mildew with a milk solution every 7-10 days. The protein in milk may create an antiseptic effect when exposed to sunlight. Use a 40% milk to 60% water solution. Spray on the leaves of affected and nearby plants, lightly coating all surfaces while the sun shines for the best effect. 
  • Mix 15 ml (1 tablespoon) of potassium bicarbonate powder and 15 ml (1 tablespoon) of liquid castile soap in 3.8 liters (1 gallon) of water and spray all parts of the plant well. To treat an outbreak, spray plants every 2-3 days. Apply after the sun goes down and spray off afterward to prevent leaf burn.

Any diseased plant parts should be removed and discarded away from other plants. These steps will help reduce the chance of a fungal disease outbreak in your garden.

6. Remove affected leaves or plants, and destroy them.

An effective method for preventing further powdery mildew is to remove and destroy affected leaves as you see them. Sanitize hands, gloves, and tools that come in contact with the diseased leaves. This will help to prevent the spread of the infection.

7. If a plant becomes overwhelmed, consider removing the entire plant.

If all methods fail and you haven’t been able to prevent or treat powdery mildew successfully, consider removing the entire plant. 

  • An unhealthy plant is more susceptible to pests and other diseases
  • A diseased plant can easily spread powdery mildew to other plants in the garden. 

Remove the entire plant, and do not compost it. Plant another fruit or vegetable in that spot that isn’t as susceptible to powdery mildew. 

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