How to Get Rid of Powdery Mildew: Organic Treatment and Prevention

Come learn how to get rid of powdery mildew in your garden. I live in an area where powdery mildew is incredibly persistent, so I’ll share my top tips to prevent mildew, reduce its spread, and the organic treatment methods I use to keep my garden looking beautiful, healthy and productive.

I’ve also included how to identify powdery mildew, its favored conditions, and what plants are most susceptible (or resistant!) to help you better manage it. Or, skip straight to the prevention and treatment options here.

What is Powdery Mildew?

Powdery mildew (also known as “PM”) is a common fungal disease that can affect a wide variety of edible crops, flowers, and ornamental plants. Though it goes by one collective name, powdery mildew can be caused by several different species of fungi.

Powdery mildew appears as white powdery mold spots on plant leaves. As the fungus feeds and robs nutrients from the plant, it causes stunting and reduced vigor. This stress can also make the plant more vulnerable to other diseases or pest damage. 

If powdery mildew coats a significant portion of leaves it can inhibit photosynthesis – slowly starving the plant. A change in photosynthesis can also negatively impact crop flavor as the plant produces less sugars.

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Ideal conditions for PM and how it spreads

Powdery mildew can be found in virtually every growing zone, though it especially thrives in climates with prolonged periods of warm dry weather with a touch of humidity. The dampness of humid air (or evening fog/dew) encourages the fungi to grow, and the warm dry air during the day helps the spores spread.

Powdery mildew spores primarily live on plants, but can also survive or overwinter in soil, compost, mulch, or other plant debris. The spores spread from plant to plant (or, are initially introduced into your garden) by wind, insects, splashing water or direct contact from infected plants. 

Crowding plants, lack of airflow, and shade also increase the risk for powdery mildew.

Identifying Powdery Mildew on Plants

Powdery mildew appears as light grey or white, dusty-looking spots or blotches. The spots are usually round, fuzzy, sometimes slightly raised, and typically start on leaves and stems before moving to the flowers or fruit themselves. The infected leaves may eventually turn yellow and dry out too. In severe cases, the plant looks like it’s been dusted in powdered sugar or flour.

Powdery mildew is sometimes confused with downy mildew, which causes darker yellow-brown spots on leaves instead.

Keep in mind that natural leaf patterns can sometimes look a lot like mildew too! Some varieties of zucchini, squash, and melon plants have natural white spots or variegation on their leaves. (See example photo below.) If you need help identifying the difference between powdery mildew vs natural white spots on leaves, see this post.

What plants get powdery mildew? 

Powdery mildew can inflict over 1000 different plant species including many common vegetables, flowers, and ornamental plants.

Plants that are especially susceptible to get powdery mildew include squash (both summer squash like zucchini and hard squash such as pumpkins or butternut), melons, cucumbers, tomatoes, eggplant, peppers, kale, peas, beans, lettuce, strawberries, grapes, mustard greens, carrots (the greens), apple trees, and many flowers including zinnia, calendula, roses, sunflowers, peonies and hydrangeas.

I’ve found that cabbage, cauliflower, broccoli, bok choy, and collards may get a little PM, but not nearly as bad as the aforementioned plants.

Some plants that are resistant or do not readily get powdery mildew include avocados, citrus trees, garlic, and many herbs – including basil, sage, lavender, rosemary, thyme, oregano, dill and more.

7 Ways to Prevent Powdery Mildew 

Here are several ways I’ve found help prevent powdery mildew in my garden. In fact, they’re often so effective at keeping it to minimum that I rarely need to treat for it!

1. Choose plants and varieties that are naturally resistant to powdery mildew. Reading plant descriptions or seed packets, you can find many varieties of mildew-resistant zucchini, melon, tomatoes, zinnia and more. They won’t be totally immune but are effected far less. For instance, Dunja is my go-to zucchini variety – it rarely gets mildew, while most squash in our garden does.
   
   
2. Practice good plant spacing. PM thrives in humid, crowded conditions. Prevent powdery mildew from spreading between plants by reducing overcrowding. Follow spacing recommendations for each type of plant, and prune plants as needed to increase airflow and reduce touching between them. Mildew also grows most readily in the shade, so provide susceptible plants plenty of sunshine.
   
   
3. Don’t compost diseased plants. Instead, put them in the trash or municipal green waste bin. Powdery mildew spores can survive in your compost and will spread to other areas of your garden when you use the compost later.
   
   
4. Clean your garden tools! It’s always a good idea to routinely sanitize your garden tools (e.g. pruning shears), especially when you’re working around diseased plants. To prevent powdery mildew from spreading, I always use rubbing alcohol, white vinegar, hydrogen peroxide, dilute bleach, or another sanitizing agent to wipe down tools before moving on to the next plant.
   
   
5. Wet the soil, not your plants! As much as possible, avoid watering plants overhead. Constantly damp leaves or splashing water from an already-infected plant encourages the growth and spread of powdery mildew. Instead, water the soil below the plant. Get my guide on how to install drip irrigation for raised garden beds here.
   
   
6. Use companion planting and crop rotation. If you know you struggle with powdery mildew, don’t plant a bunch of plants that are highly-prone to PM all together in one bed. Buffer and interplant them with resistant species. Learn more about garden companion planting here – printable chart included! It’s also important to practice crop rotation, especially after a bought of PM.
   
   
7. Promote overall healthy plants and immune systems. Healthy, well-nourished plants are more resilient to fend off disease – including mildew. To keep my garden happy and healthy, I routinely amend our garden soil with organic fertilizers and well-aged compost, and water with homemade compost tea and aloe vera extract. Develop a consistent watering schedule and avoid drought stress too.
   

5 Ways to Get Rid of Powdery Mildew 

Now, here are 5 organic ways to treat or get rid of powdery mildew. There are all sorts of other “hacks” out there, but these are the most effective options in my experience. (I’ve tried the suggestion of spraying plants with milk… and all it did was make my plants smell like rotten milk.)

1. Cut and remove infected leaves. If caught early enough (and as long as the plant can survive without the infected part) the simple act of pruning off a few infected leaves can often halt or drastically slow the spread of powdery mildew to the rest of the plant. Just be sure to clean your pruning shears well before and after!
   
   
2. Spray the plant with baking soda. Sodium bicarbonate (baking soda) changes the pH of the leaves, making them more alkaline and less hospitable for the fungus to thrive. Thoroughly mix 1 to 2 tablespoons of baking soda per 1 gallon of water plus 1 tablespoon of liquid castile soap or dawn dish soap (which helps the baking soda spread and stick to the leaves better) and then spray the plant down, saturating both the tops and bottoms of leaves.*
   
   
3. Use potassium bicarbonate. This is the most effective organic treatment for powdery mildew I’ve found! Potassium bicarbonate has a similar method of action to baking soda (raises pH to inhibit spore growth), but is even stronger and longer-lasting. Once I treated young seedlings that developed mildew very early on, and after just one application they stayed PM-free for the rest of growing season! Mix 1 Tbsp of potassium bicarbonate powder and 1 Tbsp of liquid soap in 1 gallon of water. Add it to your favorite sprayer, shake it all up, then spray everything down – coating all parts of the plant.*
   
   
4. Neem oil spray – In my experience, neem oil slows down the spread of powdery mildew, but doesn’t usually cure an established infection. I personally don’t love to use neem oil directly on tender foliage that I’m going to eat (like kale leaves) or veggies with a lot of nooks ‘n crannies (such as broccoli) because it can leave an oily residue that isn’t always easy to wash off. However, it’s great for for the leaves of squash plants! Neem oil must be diluted and applied carefully*. Learn exactly how to properly emulsify, mix, and use neem oil spray here.
   
   
5. Trash infected plants. If all else fails, it is best to completely remove significantly infected plants from your garden to stop the spread of disease. Use caution and common sense during removal: avoid shaking the plants about, dragging them over other plants or prime soil, or anything else that would cause the fungi spores spread even more. Remember, don’t add these to your compost heap.

*For any type of foliar spray treatment, I always recommend waiting to apply it until the evening hours – once the plant is out of direct sunlight. Bright sun on wet leaves can cause scorching. Also, beneficial insects are typically less active during the evening to dusk hours. Though all of these treatment options are considered safe and organic, it’s best not to directly spray beneficial insects with them.

Can you eat vegetables that have powdery mildew? 

In general, powdery mildew isn’t harmful to people. Meaning, it is not toxic or poisonous. Although, some folks are sensitive or even allergic to mildews and mold, so it’s wise to proceed with caution.

Since mildew is quite common in my garden, I’ve definitely eaten plants with a few spots of mildew! (I hate the idea of wasting food). I simply wash the produce well before eating it, but avoid consuming severely infected parts.

And that concludes this lesson on organic powdery mildew management.

All in all, don’t worry if your plants get a few spots of mildew. It’s almost to be expected! We’ve learned to accept and peacefully co-exist with the relentless fungi in our garden, yet do try to be proactive about keeping it to a minimum. I hope this article gave you plenty of ideas of how to do just that. Feel free to ask any questions in the comments below.

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