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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.

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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, and can sometimes lead to leaf drop or plant death if not treated promptly.

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 is inviting to 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 the plants.

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.

Arizona Garden in January#gardening #garden #arizonagarden #Januarygarden #gardeninginarizona #desertgarden

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. Continue treatment while warm, humid conditions remain to prevent further infections.  

  • 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.
  • 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. Spray plants every 7-14 days to treat an outbreak.

If an outbreak is severe, increase the amount of potassium bicarbonate and castile soap used in the spray. Any diseased plant parts should be removed and discarded away from other plants. Taking these steps will help reduce the chance of a fungal disease outbreak in your garden.

Copper fungicide can be an effective treatment for powdery mildew, but it’s important to use it correctly and take necessary precautions. Here are some general guidelines for using copper fungicide to treat powdery mildew:

  • Apply copper fungicide as a preventative measure or at the first sign of disease. For best control, begin treatment two weeks before the disease normally appears or when weather forecasts predict a long period of wet weather.
  • Make sure the weather is dry and calm when applying copper fungicide. Avoid spraying when the wind blows, or rain is expected within 24 hours.
  • When applying copper fungicide, wear protective clothing, such as gloves, long sleeves, pants, and a mask. Follow the manufacturer’s instructions for safe handling and use.
  • Mix the copper fungicide according to the manufacturer’s instructions. Overusing or misusing copper fungicide can lead to plant damage, so following the recommended rates and application frequency is important.
  • Spray all plant surfaces thoroughly with copper fungicide, covering both the top and bottom of the leaves, stems, and fruit.
  • Reapply copper fungicide at regular intervals, as directed by the manufacturer, to maintain control of powdery mildew.

It’s important to note that copper fungicide should not be used on certain plants, such as apricots, cherries, raspberries, and walnuts, as it can cause phytotoxicity (plant damage). Additionally, overuse of copper fungicide can lead to copper accumulation in the soil, which can be harmful to beneficial organisms and affect soil fertility. As with any pesticide, it’s important to read and follow the label instructions carefully and use copper fungicide only when necessary.

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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