How to Identify and Treat Powdery Mildew

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

Today we will talk about how to know your plants are suffering from powdery mildew and what you can do about it when it you find 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 that often begin 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. This is the powdery mildew.

Photo by: David B. Langston, University of Georgia, Bugwood.org

It can also appear a bit less obvious, lightly covering the surface of the leaf. You can see an example of this in the photo below.

Photo by: Julie Beale, University of Kentucky, Bugwood.org

There are a few conditions that can allow powdery mildew to thrive:

It loves humid, cool, stale conditions.

In other words, plants that are close together or overgrown, creating pockets of little air circulation with ample humidity and cool conditions.

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

How to Treat Powdery Mildew

Move affected plant 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 affect plants away from others to try to control spread.

Remove severely affected leaves, stems, buds, or whole plants

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

Spray 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.

#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 plant before applying to 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.

#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 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.

Do you need to repeatedly spray? Yes!

How often should you spray? Weekly.

Which ever 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.

AND 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 but cutting back plants or spacing them out further.

Preventatively treat 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.

Big Takeaway

Powdery mildew loves humid, stale conditions. If plants are super close together, they can create pockets of stale air and humidity that invites powdery mildew in.

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

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