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How to Identify and Treat Powdery Mildew

Did you wake up one morning to find your plants dusted with white? No, it’s not Christmas in July—it’s just powdery mildew rearing its ugly head in your garden. Don’t panic—powdery mildew isn’t a death threat to your garden and it can be treated, but you will need to act fast.

Powdery mildew is a common fungal disease and unfortunately, there is a strain of the fungus that affects most families of plants. It primarily affects water-stressed plants in hot, humid conditions, so preventative measures like regular watering and pruning excess foliage make a world of difference. Once powdery mildew takes hold in your garden, it spreads easily and can cause significant damage if not treated promptly.

Keep reading for an overview of powdery mildew, plus how to identify and treat it. We’ve even thrown in some of our top recs for mildew-resistant varieties if you want to avoid powdery mildew from the get-go.

What is powdery mildew?

Powdery mildew is a fungal disease that can affect many types of plants. It first appears as a white or gray powdery coating on leaves, stems, and flowers, eventually causing foliage and the plant itself to die. Powdery mildew is caused by several different species of fungi, and it spreads easily from plant to plant.

According to the University of Minnesota, the major strain of powdery mildew is caused by Podosphaera xanthii and prefers humid weather and temperatures in the 68 to 81 degree Fahrenheit range. The disease is transferred by spores, which can be moved between plants by wind, water, animals, and insects.

First signs of powdery mildew

The most common sign of powdery mildew is the tell-tale white powdery coating on leaves, stems, and flowers. Yellow leaf spots might also signify an early powdery mildew infection, as well as curling and wilting leaves. Over time, the affected leaves will continue to turn yellow and fall off.

Powdery mildew doesn’t pose a large problem initially, but as it spreads and covers more and more surface area, it can become very problematic, stressing the plant and forcing it to bolt or produce premature fruit.

Ideal weather conditions for powdery mildew

Powdery mildew thrives in warm, humid conditions, so it is most common in late summer and early fall when the weather is warm and dry. But powdery mildew can still pop up in cooler temperatures, as long as the humidity is high.

Make sure that sun-loving plants are planted in a location where they will receive adequate sunlight since too much shade can help contribute to the formation of the fungus.

Sometimes, powdery mildew is inevitable. As the weather changes from summer to autumn, annuals will naturally die back, and powdery mildew will take hold.

Plants susceptible to powdery mildew

While powdery mildew can affect many types of plants, some are more susceptible than others. No vegetables or flowers are truly safe from powdery mildew, but warm-season annuals seem to always get hit the hardest, including:

  • Cucurbits (cucumbers, squash, and melons)
  • Nightshades (eggplant, peppers, and tomatoes)
  • Asters (zinnias, sunflowers, and black-eyed Susans)
  • Legumes (peas and beans)

Since powdery mildew primarily affects plant foliage, fruit and vegetables from afflicted plants are still safe to eat, but refrain from consuming any leaves. Bear in mind that fruit will likely be of much lower quality, especially if it was sunburned due to fewer healthy leaves. Fruit from stressed plants may not be as sweet and probably won’t store as well, either.

5 mildew-resistant varieties

Mildew-resistant varieties have been bred to better withstand the fungi responsible for powdery mildew. Here are just a few all-star varieties that hold up to powdery mildew in the field (you can shop our complete disease-resistant collection here).

1. Ambrosia Hybrid Cantaloupe Melon

One of our best-tasting and best-selling cantaloupe varieties, Ambrosia Hybrid produces beautiful round fruits with ripe peach-colored flesh in 86 days. Ambrosia is resistant to downy mildew and powdery mildew.

2. Sugar Ann Pea

This snap pea is known for being particularly productive and extra-sweet. An AAS winner, Sugar Ann is extremely early, maturing at only 56 days, and is resistant to both powdery mildew and pea wilt.

3. Provider Bean

This purple-seeded bush bean is a favorite of home gardeners and market growers alike for its early maturity (50 days) and superb disease resistance to powdery mildew and other diseases.

4. Aztec Sunset Zinnia

Powdery mildew won’t spell the end of your cut flower bouquets if you plant Aztec Sunset. The more you cut this branching zinnia variety, the more blooms you’ll have, so don’t hold back. Compact plants start producing bicolored petite blooms in 75 days.

5. Garden Sweet Burpless Hybrid

This seedless cucumber has it all—crisp, delicious bitter-free flavor and excellent resistance to powdery mildew. This versatile variety matures in 55 days and can be eaten fresh or made into pickles.

Preventative measures for powdery mildew

You know the saying, an ounce of prevention is worth a pound of cure? That old adage has never been more true than with pesky powdery mildew.

A powdery mildew infection can appear quickly and without warning, covering an entire plant in a few days and spreading to infect a whole garden within weeks. Prioritize preventative measures by keeping plants healthy and well-ventilated, and you’ll never have to spray.

Weekly garden walk

To stay ahead of a potential powdery mildew infection, get in the habit of walking through your garden at least once a week, taking the time to inspect multiple plants for foliar damage or other issues. Take a close look at the leaves, and the moment you see signs of powdery mildew start a treatment program, beginning with removing the foliage and plants with the worst damage.

Plant at the correct spacing

Be sure to plant and seed plants at the proper spacing, and thin crowded seedlings so that mature plants have enough room to breathe.

Stake vining plants

Consider staking plants that would normally sprawl (like cucumbers and squash) to maximize sunlight to the leaves and encourage better airflow to the vines.

Keep rows clear of weeds

While you may not want to hear it, it’s important to keep garden rows weeded! Weeds compete for water and nutrients your plants need to perform at their best. The easiest way to correct overcrowded plants is by removing weeds first.

Water consistently and fertilize minimally

Plants are more likely to succumb to powdery mildew if stressed, so give your garden adequate water and stay on top of pests to keep your plants’ natural immunity at its peak. Avoid over-fertilizing plants and avoid nitrogen-rich fertilizers that promote excessive vegetative growth. Keep the garden beds free of weeds, and thin plants to the proper spacing and prune bottom leaves to promote better airflow.

Prune foliage

Removing lower leaves (even if they’re healthy) will open up the plant to better airflow, minimizing the risk of powdery mildew.

Once foliage does show signs of powdery mildew, removing as much of the infected leaves as possible will help slow the spread to other plants. Leave at least two-thirds of the original plant foliage, but throw out the rest. Make sure to transfer foliage infected with powdery mildew directly to the garbage—don’t add it to your compost pile, or you’ll create an environment that will harbor the fungus for future seasons.

How to treat powdery mildew

The only way to treat powdery mildew after it sets in is to regularly spray. You’ll want to spray both affected and susceptible plants, ideally twice a week. Use caution when diluting a commercial fungicide or mixing up a homemade spray, both for your safety and the safety of the plants. If possible, spray early in the morning on an overcast day without rain in the forecast, as foliage sprays can damage plants if not applied correctly.

Use a commercial fungicide

There are many commercial fungicides available to treat powdery mildew. If using a concentrated fungicide solution, be sure to follow the manufacturer's instructions carefully and dilute accordingly.

Unfortunately, the powdery mildew fungus does acquire resistance to certain fungicides after time, so check this site to make sure that you’re using the most effective fungicide for the current year.

Make a homemade spray

A mixture of baking soda and water or milk and water can be used as a natural fungicide. Spray the affected parts of the plant regularly.

While it’s nearly impossible to eradicate powdery mildew from the garden completely, there is a recipe that can help control the spread of powdery mildew in your garden: One tablespoon of baking soda and one teaspoon of liquid Castile soap per one gallon of water.

Prioritize your own safety by using gloves, safety glasses, and a mask any time you handle or spray chemicals, even if they are household items.

  1. Fill a backpack sprayer or a spray bottle with an inch of water.
  2. Add baking soda and liquid Castile soap to the sprayer.
  3. Fill the sprayer the rest of the way with water.
  4. Mix the solution together by shaking the sprayer.
  5. Spray the solution on the affected plants, making sure to cover all surfaces thoroughly.

Generously coat all parts of the affected plants, excluding flowers and fruit, and don’t neglect to spray nearby plants that could be susceptible. Spraying leaves without visible mildew won't harm plants.

Optimal times to spray are in the early morning or evening or on an overcast day, as spraying in the heat of the day may burn leaves and cause wilt.

Repeat the process at least once a week for the remainder of the season just to keep the mildew under control. If the infection is particularly bad, you can spray twice a week until the powdery mildew noticeably subsides.

Powdery mildew is a common fungal disease that can cause significant damage to plants, but it doesn’t have to spell disaster for your garden. By identifying the signs of powdery mildew and taking prompt action, you can reduce the risk of damage and keep your plants healthy.

Remember to start with quality seeds from disease-resistant cultivars to give your garden the best possible start. And should you see signs of powdery mildew, begin a treatment program early to prevent it from spreading to other areas of the garden.

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