How to Identify and Treat Powdery Mildew

In the right conditions, powdery mildew can cause huge issues for growers indoors and out.

Find out how to identify powdery mildew on your plants and what you can do about it if your plants have it!

Table of Contents

How to Identify Powdery Mildew

Powdery mildew is a fungal disease that both indoor and outdoor gardeners deal with regularly.

It causes white, powdery build-up on leaves and can eventually damage the plant beyond repair.

It often begins as small patches and then progresses to cover the entire leaf and plant if not dealt with.

Below you can see the white, powdery spots on the leaf.

Powdery mildew
Powdery mildew spots on leaves, Photo by: David B. Langston, University of Georgia, Bugwood.org

It can also appear a bit less obviously, lightly covering the surface of the leaf, like in the photo below.

Powdery mildew on squash
Powdery mildew dusting the leaves, Photo by: Julie Beale, University of Kentucky, Bugwood.org

Powdery mildew loves humid, cool, stale conditions.

These conditions can be accidentally created in our homes by placing plants very close together, which creates pockets of stale air and humidity.

For plants that prefer to stay moist, this can be particularly troubling.

Powdery mildew can sit dormant in a plant’s soil, waiting for the right conditions to emerge and wreak havoc.

How to Treat Powdery Mildew

Move affected plants away from other plants (when possible)

Powdery mildew can release spores that spread through the air and onto other plants.

It is best, when identified, to move the affected plants away from others to try to control spread.

Remove severely affected leaves, stems, buds, possible compost the entire plant if it is mostly covered.

Any plant parts or whole plants that are nearly covered in powdery mildew should be removed and composted.

Spray the plants to control and eliminate powdery mildew

#1 Milk spray

It sounds strange and kind of gross, but milk is really great at controlling powdery mildew!

You can make a spray by mixing 2 parts milk to 3 parts water.

This spray, while effective, can sometimes be stinky (especially when used indoors).

Researchers aren’t exactly sure why milk works, but have confirmed that it really does work!

#2 Baking soda spray

Baking soda has a very high PH, which powdery mildew cannot tolerate.

Mix 1 teaspoon of baking soda and a few drops of mild dish soap in 1 quart of water.

Apply as a spray.

#3 Neem oil spray

Neem oil is able to disrupt powdery mildew’s spore production, which controls and eventually eliminates the fungus.

To use neem, mix 1 teaspoon of neem oil and a few drops of mild dish soap in 1 quart of water.

Some plants are sensitive to neem oil, so you may want to test the spray on a small section of the plant before coating the entire plant.

Ferns, for example, are not happy with me when I spray their thin, delicate leaves with neem oil. So, I typically try to use other sprays for them when issues arise.

To learn more about neem oil, check out my post here: Is Neem Oil Actually Effective at Houseplant Pest Control?

#4 Copper fungicide

If you prefer to buy something rather than make your own spray, one great option is Bonide’s Copper Fungicide.

Copper fungicide can control several different fungal diseases.

The instructions will provide you with the amount to add to a gallon of water.

These amounts are normally provided in ranges, like .5 ounces to 2 ounces per gallon.

.5 ounces might be preferred for first-time users to ensure plants are safe or to control a very small fungal issue.

2 ounces is the maximum dosage and might be preferred for severe infestations or for people who want to ensure the problem is controlled immediately.

Repeatedly spray

How often should you spray? Weekly.

Whichever spray you choose should be applied liberally all over the plant. It is recommended to spray weekly for a few weeks to ensure the fungus is gone.

There is some concern that using the same spray weekly might allow powdery mildew to develop resistant strains. So, if you want to be really safe, you can rotate between a couple of different sprays.

Increase circulation

If your plants are very close together, it can create the perfect microclimate of humid, stale air that powdery mildew can thrive in.

To help eliminate the fungus, space plants farther apart or even add a little fan to provide air circulation among the plants.

I had this problem last year in my vegetable garden where my squash plants became huge and started crowding out some of the other plants. (It was my fault for planting things too close together)

The squash plants quickly developed powdery mildew and I spent some serious effort getting it under control.

Squash is known for its susceptibility to powdery mildew, just like some of our indoor plants are.

Begonias and African violets, for example, are known to be susceptible to powdery mildew.

How to Prevent Powdery Mildew

Increase circulation by cutting back plants or spacing them out further.

Preventatively treat powdery mildew with baking soda, using the mix above for a spray. You can thoroughly coat susceptible plants.

Quarantine new & suspect plants so they cannot spread powdery mildew to others and can be treated right away.

Check plants regularly, particularly those that are placed close together or are particularly susceptible to powdery mildew (like Begonias).

Big Takeaway

Powdery mildew loves humid, stale conditions.

If plants are super close together, they can create pockets of stale air and humidity that invite powdery mildew in.

Powdery mildew, when caught early, can be managed through repeated sprays.

In later stages. powdery mildew may cost the plant its life or give you no choice but to compost in an effort to protect other plants.

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