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All Things Garden,  Beginner Basics,  Pests

How to Prevent or Treat Powdery Mildew Organically

What is the white stuff on my plant leaves, you ask? Well, it could be powdery mildew! Powdery mildew is one of the most common garden diseases around. While it can be an unsightly nuisance, and you certainly don’t want it to run a muck in your garden, it isn’t anything to stress over! Despite the fact that mildew is present and persistent in our garden, we still grow an abundance of beautiful, healthy plants and crops. 

Read along to learn how to organically control powdery mildew in your garden, with tips for prevention, ways to reduce its spread, or treat it when necessary.  Let’s start with a quick overview of the disease in general. Understanding how to identify PM, its favored conditions, and what plants are most susceptible (or resistant!) will help shape your response to manage the disease in your garden. As with all garden pests or disease, you’ll have the best success by checking your plants often and intervening early.



What is Powdery Mildew?


Powdery mildew is a fungal disease that can affect a wide variety of edible crops, flowers, and ornamental plants. It is quite common and can be caused by several different species of fungi. True to its name, powdery mildew (often shortened to “PM”) looks like white powdery mold spots on plant leaves. A few spots aren’t usually a big deal, though advanced infections can cause stress and damage to plants. Yet the good news is: it is possible to prevent, control, or get rid of powdery mildew organically!


Ideal conditions for PM and how it spreads


Powdery mildew can be found in virtually every growing zone, though it thrives in climates with prolonged periods of warm dry weather. Just like here on the temperate Central Coast of California, where PM can be fairly rampant in our garden if left unchecked! However, just like any good fungus would, powdery mildew loves a touch of humidity too. The dampness of humid air (or evening 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. Crowded conditions, lack of airflow, and shade also increase the risk for powdery mildew.


A watering can that isn't visible is watering the soil below the canopy of collard greens so the water isn't getting on the plant's leaves. The water resembles that of a rain shower spray.
Providing good airflow between plants and avoiding wetting plant leaves helps to reduce the spread of PM.


Identifying Powdery Mildew on Plants


Powdery mildew appears as light grey or white, dusty-looking spots or blotches. They’re usually round, fuzzy, and sometimes slightly raised. You’ll first notice the tell-tale spots on the tops of plant leaves. Upon closer inspection (or as the infection progresses), you may find mildew on stems, the undersides of leaves, on flowers, and sometimes on the fruit or vegetables themselves. A severe powdery mildew case makes the plant look like it’s been dusted in powdered sugar or flour. The infected leaves may eventually turn yellow and dry out too.

Once you’re familiar with powdery mildew, it’s pretty easy to spot and identify. However, it may be confused with other types of fungal diseases like downy mildew (which causes darker spots on leaves instead). Even more tricky, natural patterns on plant leaves can sometimes look a lot like mildew too! Check out the photo of our zucchini plant below for example. Depending on the variety, some zucchini, squash, and melon plants have natural white spots or variegation on their leaves. 

The key difference is that a natural leaf pattern will be flat (not fuzzy or dusty) and appear more uniform, likely “mirrored” across both sides of leaf veins. On the other hand, mildew spots are far more irregularly dispersed, and usually on both the top and bottom sides of leaves. PM can also be wiped off (or at least appear to temporarily disappear) with a wet cloth or paper towel, whereas the natural leaf pattern would stay as-is. 


A small cucumber seedling is showing small splotches of powdery mildew on its leaves. Two small yellow flowers are emanating from the main stem below the leaves. Powdery mildew favors plants in the cucurbit family.
Powdery mildew spots on cucumber leaves
A two part image collage, the first image shows the top leaf of a PM resistant variety of squash. There are natural gray variegation on the leaves that are fairly symmetrical across the whole leaf. The second image shows DeannaCat turning the leaf over so you can see the underside which looks green and healthy where as plants inflicted with PM will usually have spots on the undersides of their leaves as well.
This is a perfectly healthy zucchini leaf, believe it or not. It just happens to have a natural variegated pattern that resembles PM at first glance. This is quite common among many varieties of squash and melons. Upon further inspection, you’ll see the spots are uniform across most or all of the leaves, not present on the underside, not growing, raised, powdery, or fuzzy – telltale signs it’s not mildew.


How does mildew harm plants?


The good news is, powdery mildew is rarely lethal to plants! But just because it isn’t fatal, doesn’t mean you should let it go unchecked. At first, a few minor spots aren’t going to do much harm to the host plant, but it can spread spores to other plants or linger in your garden soil. As the disease progresses, the fungus feeds and robs nutrients from the plant, making them stunted or less productive

When powdery mildew coats a significant portion of leaves it can inhibit photosynthesis. This is essentially like a slow starvation for the plant. A change in photosynthesis can also impact crop flavor since the plant will be producing less sugars. Last but not least, powdery mildew infections cause stress to the plant, and a stressed plant is more susceptible to other diseases or pest damage. 


A close up image of a squash plant with a heavy infestation of powdery mildew. The leaves are caked with a  whitish silver coating that resemble spray paint.
A very severe case of powdery mildew on zucchini or squash. This is a close up of the plant in the feature photo at the beginning of this article. I saw this plant in our neighborhood last year and snapped photos in preparation for this article… I’ve never seem PM so bad before! Do NOT let an infection get this advanced in your garden. Treat or remove the plant before it gets like this.


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 our garden, we’ve definitely eaten our fair share of crops with a few spots of mildew! (I hate the idea of wasting food). We simply wash the produce well before eating it, but do avoid consuming severely infected parts.


What plants get powdery mildew? 


Unfortunately, many plants are prone to powdery mildew – including a plethora of common vegetables, flowers, and ornamental plants you may grow in your garden. Plants that are especially susceptible to get powdery mildew include squash (both summer squash like zucchini and hard squash such as pumpkins or butternut), melon, cucumber, tomatoes, eggplant, peppers, kale, peas, beans, lettuce, mustard greens, cannabis, carrots (the greens), apple trees, zinnia, calendula, roses, sunflowers, begonias, bee balm, peonies, phlox, and hydrangeas. Cabbage, cauliflower, broccoli, and collards may get a little PM, but not nearly as bad as the aforementioned plants.


DeannaCat is holding a leaf of lacinato kale that has be inflicted with powdery mildew. The raised white and fuzzy growth is stretching across the length of the leaf.
Here is a moderate case of powdery mildew on kale leaves. Also note the smaller white spots on the leaves in the background. I would personally not eat leaves with this much mildew on it.


Plants resistant to powdery mildew


Powdery mildew can inflict over 1000 different plant species, though many types do not develop it as readily or severely as the susceptible plants listed above. Despite having plenty of PM in our garden, we’ve never seen powdery mildew on our citrus, figs, avocados, passionfruit vines, and rarely on herbs – including basil, sage, lavender, rosemary, thyme, oregano, dill and more. Other plant species that are less likely to get powdery mildew include arugula, bok choy, swiss chard, onions, radishes, garlic, fava beans, turmeric, ginger, guava, strawberries, salvia species, and succulents or cacti. Thank goodness, because those are all some of my low-fuss favorites to grow! (Click on any of the highlighted crops to visit our corresponding grow guide)

Additionally, humans have selectively bred plants that are otherwise usually prone to powdery mildew to develop hybrid resistant varieties. For instance, you can find varieties of powdery mildew-resistant zucchini, melon, tomatoes, zinnia and more. Look for powdery mildew resistant varieties in the descriptions on seed packages, nursery plant label, online or in catalogs. Choosing resistant cultivars is an excellent way to prevent powdery mildew in your garden! 


A large Dunja zucchini variety plant is featured. It's leaves are more than twice the width of a large hand. Many green fruit are growing amongst its base with a few flowers mixed in as well. Surrounding the squash are many calendula and borage with yellow and purple flowers. Beyond in other raised beds lie onions, collard greens, an array of calendula flowers, pole beans and zinnia. A wall of flowering salvia make up the background with pink flowers dotted amongst the green foliage.
Another perfectly healthy squash plant with natural leaf patterns that look similar to PM. In fact, this Dunja zucchini variety is resistant to powdery mildew – a prized cultivar in our summer garden.


8 Ways to Prevent Powdery Mildew 



One of the best ways to get rid of powdery mildew is to prevent it from occurring in the first place! Ha. If only it were that simple, eh? It’s a never-ending battle in our garden; there is always a low-grade PM infection somewhere. Yet these 8 tips to prevent powdery mildew on plants definitely help to keep it under control! So much so, that we rarely need to resort to treatment.


  1. Choose plant species and varieties that are naturally resistant to powdery mildew, especially if PM is a known issue in your garden. For instance, we’ve found that Dunja zucchini rarely gets mildew, while many other types of zucchini that we grow will. While they may not be completely immune, it should help! Also refer to the list of less susceptible plants above.

  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, use rubbing alcohol, white vinegar, hydrogen peroxide, dilute bleach, or another sanitizing agent to wipe down your 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. Drip irrigation is ideal! We have soaker hoses below an inch of mulch in all of our raised garden beds.

  6. Use wise companion planting and polyculture. 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 companion planting here – printable chart included!

  7. Promote overall healthy plants and immune systems. By providing plants the nutrients and conditions they need to grow big and strong, the more resilient they are to fend off disease – including mildew. To keep our garden feelin’ funky fresh, we 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.

  8. Preemptively treat plants with neem oil. As described more below, neem oil is one common treatment option for powdery mildew. In addition, you could use neem spray as a preventative measure where mildew development seems inevitable. PM is a big issue for our cannabis, and we have very little tolerance for it there. While I don’t mind eating a tiny bit of mildew, I definitely do not want to smoke it. So, Aaron sprays our young cannabis plants with neem oil on a weekly basis as part of their organic pest control routine – but only up until they begin to form flowers, because we don’t want to inhale neem either.


Raised garden beds are set against a greenish blue house, the beds are lush with a variety of plants growing in them from chard, to calendula, to pole beans growing up a metal trellis. Zinnia and sunflower plants are growing up towards the setting sun while more calendula and borage is growing along the ground in front of the beds leading to a dazzling array of flowers that are purple, pink, orange, and yellow. Prevent powdery mildew by maximizing companion planting.
An example of wise companion planting: Swiss chard (usually resistant to powdery mildew, at least in our garden!) sandwiched between pole beans and calendula in back and front – two crops that are very prone to PM. Planting other susceptible crops like bush beans, kale, collard greens, or eggplant in the place of the Swiss chard would have been asking for trouble.


5 Ways to Get Rid of Powdery Mildew 


There are all sorts of tricks to get rid of powdery mildew, ranging from hippie-dippie to toxic fungicides. Case in point: have you heard that you can treat the infected plant by spraying it with milk? The rumor is that mixing 1 part milk to 10 parts water creates an organic fungicide. Maybe that works for some folks (good for you!) but when we tried it, we just ended up with very, very stinky plants. 

Your choice of treatment method depends on the severity of the issue, what resources are most available to you, and personal preference. Perhaps you’ll need minimal intervention at all. Either way, the following 5 ways to treat powdery mildew are considered safe for organic gardening – and work!


  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 on the plant leaves, making conditions more alkaline and less hospitable for the fungus to thrive. Mildew and blight like a more neutral pH (around 7). Thoroughly mix 1 to 2 tablespoons of baking soda per 1 gallon of water. Also add 1 tablespoon of liquid castile soap or other dish soap, which will help the baking soda spread and stick to the leaves better. Spray the plant thoroughly, saturating both the tops and bottoms of leaves.*

  3. Use potassium bicarbonate. This is the most effective treatment for powdery mildew we’ve found! Potassium bicarbonate has a very similar method of action to baking soda (described above), but is stronger and longer-lasting. One time we treated young seedlings that developed mildew very early on, and they stayed PM-free for the entire growing season after only one application! We used to rely on a product called Green Cure, but they recently stopped production. Now we switched to this similar product. Mix 1 Tbsp of 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 completely eradicate it. That is, unless you start treating your plants early and often throughout the season (see prevention section above).  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! Like the bicarbonate treatment options, neem oil must also be diluted before application. However, because it is an oil, it is even more tricky to mix with water (heeeey middle school science!) 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 as you do this. Avoid shaking the plants about, dragging them over other plants or prime soil, or anything else that would cause the fungi spore to go flying! Remember, don’t add these to your compost heap. After removing severely infected plants from your garden, it’s wise to replace (trash) the mulch in that immediate area. (If you’re using mulch… I sure hope you are!). You could also spray down the soil surface with baking soda, potassium bicarbonate, or neem.


*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.



A pump sprayer is sitting on the corner of a raised garden bed, Aaron's hand is reaching into the image holding the wand of the sprayer as he applies a foliar spray to prevent or treat powdery mildew onto bush beans that are growing in the raised bed. There are other plants in the bed including more bush beans and kale. In another garden bed next to it there are tomatoes, peppers, and onions growing. The evening sun is just about setting below the horizon as a yellow orange glow recedes amongst the shadows.
Spraying down mildewy bush beans with potassium bicarbonate.


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. Or, share you experience with controlling powdery mildew in your garden – especially if I missed a helpful tip! Please also spread the love by pinning or sharing this article if you found it useful. Thank you for reading, and best of luck on your battles against PM!



DeannaCat signature, keep on growing

11 Comments

  • Jenessa

    Hi Deanna and Aaron,

    I bought some potassium bicarbonate to treat powdery mildew on my cantaloupe and cucumber plants. How often should I spray them with the potassium mixture?

    Thanks so much,

    Jenessa

    • Aaron (Mr. DeannaCat)

      Hi Jenessa, spraying once every week or two is usually sufficient for preventing mildew but if you currently have an outbreak I would spray at least twice a week until you get the mildew under control. Hope that helps and good luck!

  • Kathleen Jackson

    I would very much like to know the name of the gray/silver plant on the cover of your book about powdery mildew. I know there are many types of this plant but the leaves on this plant are the same as the plant in my garden except that I cannot find it anywhere in plant stores.
    Do you have its name? Thank you very much.

    • Aaron (Mr. DeannaCat)

      Hello Kathleen, that is an unknown variety of summer squash that our neighbor was growing, likely a zucchini of some sort.

  • Eunice Olson

    This is not about powdery mildew! I live on the Central Oregon Coast…chilly, damp, NEVER gets hot. We grow cannabis inside a greenhouse to combat mildew (as best we can). We were told that growing tomatoes in the same greenhouse as pot is a big no-no. Not sure why or which plant has problems when they are combined. We do grow tomatoes (outside, in front of a large shop where the sun radiates heat) but they don’t get enough heat to make them really sweet and wonderful like they are in CA valley. Any info about growing them in the greenhouse with pot?

    I love your blog, your little farm is fantastic. Thanks so much for taking the time to do this, it is a gem!

    • Aaron (Mr. DeannaCat)

      Hello Eunice, I am not familiar with growing tomatoes and cannabis together and if it is advisable or not to have both in a greenhouse together. I am sure most people say to not put the two together because there is more of a chance for pest or disease to inflict the cannabis which is most likely a monetary investment. When growing tomatoes in climates with more cool and temperate weather, it is best to choose varieties that do well in those conditions. Adaptive Seeds is a company in the PNW with many varieties of tomatoes that do well in those conditions. Check out our Growing Organic Tomatoes: How to Plant, Feed, Prune & Grow Tomato Plants article for more information. Thanks for reading and good luck on the upcoming grow season!

  • C.E. Corngold

    Hey DeannaCat,

    Love your website/blog!

    About PM, might you have come across any research regarding how long PM spores survive in soil?
    I’m going to spray down my soil w/potassium bicarbonate, as you suggest, but I don’t want to lower the pH too much.
    I wonder how long the spores are viable?

    Many thanks and thanks for your research and reliable advice (and recipes!)
    Cara in Altadena/Pasadena

    • Aaron (Mr. DeannaCat)

      Hello C.E., I don’t think the potassium bicarbonate will lower you pH too much, especially if you have soil that is alive and rich in organic matter as it buffers pH quite well. I think a couple of weeks is enough time for the PM to dissipate if there are no infected plants remaining as the mildew survives on living plant tissue. Thanks for reading, hope that helps and good luck!

    • Aaron (Mr. DeannaCat)

      Hi Barbara, check with the brand of grow bag you have and see what their washing instructions are as you may be able to wash them, you could add vinegar to the water when you wash and or treat it with the recommended treatments listed in the article. Hope that helps and good luck!

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