Wahoo! You Have Powdery Mildew

July 17, 2014 | By | Comments (3)
Powdery mildew

Powdery mildew on summer phlox (Phlox paniculata). Photo: signaturegardens.blogspot.com

It’s a wonderful morning in a heretofore wonderful world. You walk out to your garden, only to discover to your shame and horror that some miscreant has confused it with a baby’s bottom and showered Johnson’s Baby Powder all over your plants.

Well, it looks like baby powder. There is white stuff are over the leaves and flower buds. How did it get there? More importantly, how can you get rid of it? Once again, the benevolent Grump is here to help.

Powdery mildew

Good for babies’ bums, not bee balms. Photo: Johnson & Johnson

First of all, it isn’t powder. It’s a lookalike fungus that’s very common around this time of year called powdery mildew. There isn’t just one organism that causes powdery mildew. There are hundreds. So the one that gets on crepe myrtle is different from the one that gets on roses is different from the one that attacks phlox is different than the one that plasters lilacs.

But the result is the same. Small, white spots form on the upper surface of the leaves and coalesce into a powdery film. The film causes flowers and flower buds to wither and die.

How Did the Mildew Get There?
Powdery mildew spreads by spores. The spores are everywhere. They blow in on the wind and splash from plant to plant when it rains. When they land on a leaf and conditions are right, they germinate, grow, and make more spores.

What are the right conditions? Warm days, cooler nights, high humidity, still air, and some shade. Although powdery mildew is often associated with wet foliage, the funny thing is that spores can only germinate on dry foliage. If foliage stays wet too long, the spores die.

What Are Some of the Most Common Garden Plants Affected?
1. Bee balm (Monarda didyma)

2. Crepe myrtle

3. Euonymus

4. Flowering dogwood

5. Hydrangea

6. Lilac

7. Rose

8. Squash

9. Summer phlox (Phlox paniculata)

10. Zinnia

How Can I Prevent Powdery Mildew?
The best way is to plant mildew-resistant selections if they’re available. For bee balm, try ‘Gardenview Scarlet’ (red), ‘Marshall’s Delight’ (purplish-pink), and ‘Violet Queen’ (purple). For crepe myrtle, choose any selection with a Native American name, like ‘Natchez,’ ‘Sioux,’ or ‘Arapaho.’ Most newer crepe myrtles, such as ‘Red Rocket,’ ‘Dynamite,’ ‘Pink Velour,’ and ‘Burgundy Cotton,’ also resist mildew. For summer phlox, try ‘David’ (white), ‘Delta Snow’ (white), ‘John Fanick’ (lavender-blue), ‘Robert Poore’ (pink), and ‘Rosalinde’ (pink). For dogwoods, try ‘Cherokee Brave’ (red), ‘Cherokee Chief’ (red), ‘Cherokee Daybreak’ (white), and ‘Springtime’ (white).

Powdery mildew

A beautiful collection of mildew-resistant summer phlox. Photo: perennialpleasures.net

Control method #2 — Deny powdery mildew the conditions it likes. Reduce shade around susceptible plants. Don’t crowd plants — leave space between them so that air circulates freely. Don’t splash water onto the leaves.

Control method #3 — Quickly remove any infected leaves, flowers, and flower buds and either burn them or throw them out with the trash. This controls the spread of spores. On crepe myrtles, cut off side branches affected with mildew and get rid of them too.

Control method #4 — Spray the foliage according to label directions with an appropriate fungicide. Two natural products that work are Natria Disease Control and neem oil. The first suppresses powdery mildew and many other diseases using a beneficial bacterium, Bacillus subtilis, that is safe for people and wildlife. The second employs an extract from the tropical neem tree. It controls both insects and diseases. For the longest lasting control between sprayings, use a systemic fungicide called Immunox. It’s absorbed into plant tissues and works for weeks.



  1. Gary

    Powedery Mildew is the worst! I did try a few at home remedies but didnt get very good results. I ended up using a product call Green Cleaner… http://www.greenerhydroponics.com/Old-Stage-_bymfg_780-0-1.html

    Worked great and didnt have any reoccuring problems.

    October 14, 2015 at 10:40 am
  2. Outdoor Mama

    The only things that got powdery mildew in my garden this year were new plants from a big box nursery (all flowers). I have been so disappointed with having to nurse my new plants back to health, I’m going to make a better effort to find local growers. I think all the forced early growth has been very detrimental. My veronica has about bit the dust by now, fortunately my coneflowers have bounced back.

    July 23, 2014 at 1:52 pm
  3. Diane

    I’m surprised you didn’t include diluted non-fat milk. I tried it at a 10% solution with much skepticism, & was pleasantly surprised at how fast & how well it worked. The progression was halted & the plants put out new healthy leaves & fruit. I use powdered non-fat milk (NONfat, so it doesn’t smell like spoiled milk!) diluted extra, & it’s quite cost-effective. I also don’t put foreign chemicals in my garden, a factor as it’s primarily squash & cucumber plants that are most effective. For some reason the bee balm hasn’t been touched at all.

    July 17, 2014 at 8:44 pm

Leave a Comment

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s