Learn how to recognize, treat, and prevent powdery mildew in your garden without losing your mind (or your zinnias).
Picture this: it’s a sunny day with blue skies, the zucchini are plump and thriving, the zinnias dressed in layers of petals, and then all of a sudden… everything looks like a tub of baby powder exploded on your plants.
When you try to brush it off, it doesn’t budge. You hose the plants down with a jet stream of water, and they still look powdery, but not the good kind.
Welcome, friend, to the club no gardener wanted to join: The Powdery Mildew Society.
By the way, if you want to keep your garden low-stress and low-maintenance, read this post next.
What Exactly Is Powdery Mildew?
Powdery mildew is a fungal disease that looks like a white, powdery coating on plants’ leaves and stems.
Think of powdery mildew as a clingy party crasher (let’s call her Mildred) who was never invited but somehow knows your address. No one is safe from Powdery Mildred.
For me in the middle of Indiana, powdery mildew reliably shows up at the end of September. Typically it hasn’t rained for a while, the plants have matured and gotten large (so less airflow), and while the days are sunny and warm, the nights are cool and damp.
This is the type of party that Powdery Mildred loves to crash.
Powdery mildew blocks sunlight, messes with photosynthesis, stunts or distorts growth, and will lower your yield. So if you had your fingers crossed for a bumper crop of butternut squash, powdery mildew could turn all your hopes and dreams to dust.
Literally.
Powdery mildew is not instantly deadly, so don’t panic when you first see her. But just like an uninvited guest who overstays their welcome, she will suck the life out of your plants over time as the disease spreads.
The Most Susceptible Victims
Powdery mildew is an equal-opportunity party crasher. No one is safe, and especially not:
- zucchini
- squash
- cucumbers
- grapes
- peas
- roses
- azaleas
- zinnias
Yes, even those incredible zinnias you grew from seed will eventually get powdery mildew.
It’s basically the fungal version of glitter. Once it spills, it’s everywhere.
How Plants Get Infected
Powdery mildew comes from fungal spores that overwinter in plant debris or dormant plant buds. It’s easily spread by the wind, insects, or water splashing from infected debris on the ground.
Weather conditions like I mentioned earlier (warm, dry days and cool nights) create the perfect atmosphere for powdery mildew to thrive.
Additionally if plants are packed tighter than Swifties at the Eras tour or there’s not enough air circulation, then you’re just rolling out the red carpet for a fungal outbreak.
Once powdery mildew gets comfy, it’ll spread rapidly and can produce new spores every few days.
That’s right, powdery mildew can outgrow even these fast-growing vegetables for impatient gardeners.
How to Evict Powdery Mildew from the Party
Once Powdery Mildred has learned your address, she’ll always come back. However there are ways to manage powdery mildew attacks and keep it from killing all your plants.
Tip #1: Give your plants some space.
Don’t plant too close together on all sides. Prune plants throughout the season to improve airflow. Keep susceptible plants out in the open away from walls or fences where air may not circulate as well.
Tip #2: Water wisely.
Water the soil with minimal splashback by using drip irrigation. If you must use a sprinkler or hose, avoid watering early in the morning or in the evening. Instead water in mid-morning so that leaves can dry quickly with sunshine and point the hose away from leaves as much as possible.
Tip #3: Cut it out.
As soon as you notice signs of infection, remove affected leaves or, if you must, whole plants. Bag up the infected leaves and toss them in the garbage can. Don’t leave them on the ground! Destroying infected leaves now will reduce the problem next year.
Tip #4: Mix up a DIY spray.
In a spray bottle combine 1 part milk to 2 parts water. Shake it up and spray it on the leaves in full sunlight. Continue this routine once a week until the fungus is gone. You can also do this as a preventative measure before powdery mildew even shows up.
Tip #5: Grow resistant varieties and keep plants healthy.
Some cultivars of both vegetables and flowers are bred to be powdery mildew-resistant. Seek these out and plant them if you really struggle with this disease. And if you can’t find PM-resistant varieties, just aim to keep plants as healthy as possible. Don’t overfertilize or overwater. Don’t underwater, either! Maintain a Goldilocks garden and make sure everything is juuuust right.
Do you have a plant that seems more naturally resistant to powdery mildew even when others are going down like flies? Save its seeds! You may have stumbled on your own variety of PMR plants. I have this post about saving zinnia seeds to help you get started.
Powdery Mildew Resistant Varieties
As you’re browsing seed catalogues or scouring seed sellers’ websites, look for the PMR designation (powdery mildew-resistant) in the description. While this won’t guarantee you won’t get the fungus, it will give you a stronger plant from the start.
A few notable PMR varieties include:
- Winter squash: Autumn Delight Acorn Squash, Sugar Dumpling Acorn Squash, Autumn Frost Butternut Squash, Honeynut Squash, Bush Delicata Squash, Harvest Moon Hubbard Squash
- Zucchini: Mutabile, Nigro, General Patton, Dunja
- Pumpkin: Cronus F1, Gladiator F1, Jack of Hearts, Renegade, Duchess, Millionaire
- Zinnias: Profusion series, Zahara, Oklahoma series
- Grapes: Regent, Solaris, Feurtai, Queen of Esther
- Cucumbers: Spacemaster, Burpless Hybrid, Carmen, Adam Gherkin, Alibi, Beit Alpha
- Peas: Dakota, PLS 141, PLS595, Super Sugar Snap
- Roses: Zephirine Drouhin, New Dawn, Tournament of Roses, Electron, Hansa, Knock Outs
Some perennials are naturally more resistant to powdery mildew as well. Check out this list of powdery mildew resistant perennials from Nature Hills.
The Takeaway
Even the best gardeners get powdery mildew. It’s an uninvited guest that continues to show up even if you’re doing everything right.
That’s just the nature of nature!
Powdery mildew is annoying, yes, but it doesn’t have to signal the doom of your garden. Being proactive and making a few easy, smart moves now will help you have a better garden later.
Remember:
- Give your plants some space.
- Water wisely and avoid splashing leaves.
- Cut out and dispose of infected leaves.
- Spray leaves with a DIY milk spray.
- Grow PM-resistant varieties and focus on overall plant health.
Every garden has a little drama and no gardener is safe from the fungus among us.
At least now, you know exactly what to do when you see those telltale white patches forming.
You’ll say, “Ah, I’ve been expecting you, Powdery Mildred,” and greet her with a skim milk spray that’ll send her to the curb.
Do you have any additional tips for dealing with powdery mildew in your garden? Share your comments down below!