Powdery Mildew: How to Identify, Control, and Prevent Powdery Mildew, The Old Farmer s Almanac

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Powdery Mildew

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Age-Old Wisdom meets Modern Tools

Wondering about that white fungus on your plants? The fungal disease powdery mildew affects a wide variety of plants and takes away a plant’s nutrients. This causes the plant to bloom less and become weaker.

What is Powdery Mildew?

There are many different species of the fungal disease powdery mildew, and each species attacks a range of different plants. Unlike many other fungal diseases, powdery mildew thrives in warm, dry climates, though it does require fairly high relative humidity (i.e., humidity around the plant) to spread. In cooler, rainy areas, it does not spread as well. That being said, it is capable of infecting your plants under a wide variety of conditions.

When the fungus begins to take over one of your plants, the mildew that forms is made up of many spores. These spores carry the infection to other plants through the wind. Powdery mildew can slow down the growth of your plant. In some cases, if the infection is severe enough, powdery mildew can kill your plants.

Identification

How to Identify Powdery Mildew Damage

  • Plants infected with powdery mildew look as if they have been dusted with flour.
  • Powdery mildew usually starts off as circular, powdery white spots, which can appear on leaves, stems, and sometimes fruit.
  • Powdery mildew usually covers the upper part of the leaves, but may grow on the undersides as well.
  • Young foliage is most susceptible to damage. Leaves turn yellow and dry out.
  • The fungus might cause some leaves to twist, break, or become disfigured.
  • The white spots of powdery mildew will spread to cover most of the leaves or affected areas.
  • The leaves, buds, and growing tips will become disfigured as well. These symptoms usually appear late in the growing season.


Powdery mildew first appears as small white spots on the upper part of the leaves. Photo Credit: The Regents of the University of California, UC Davis.

Control and Prevention

How to Control Powdery Mildew

  • Remove all the infected plant parts and destroy them. Remember, do not compost any infected plant, as the disease can still be spread by the wind and persist in the composted materials.
  • Spray infected plants with fungicides. Effective organic fungicides for treating powdery mildew include sulfur, lime-sulfur, neem oil, and potassium bicarbonate.

Prevent Powdery Mildew

  • Choose plants that are resistant or tolerant to powdery mildew. Many mildew-resistant varieties of cucurbits (melons, cucumbers, squash, etc.) have been developed and can be bought from major seed suppliers.
  • Avoid watering plants from overhead in order to reduce relative humidity.
  • Selectively prune overcrowded areas to increase air circulation; this also helps to reduce humidity around your plants.
  • Spray your plants with the fungicides mentioned above according to the directions included with the products.
  • If you don’t want to use fungicides, try spraying your plants with a bicarbonate solution:
    • Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.

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Reader Comments

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powdery mildew

Submitted by DON W on November 24, 2019 — 3:02pm

I have found that a mixture of 3% hydrogen peroxide with insecticidal soap works very well at controlling both powdery mildew and insect pests. I buy 27% h2o2 at the local farm supply and dilute it 1 quart to 9 quarts water, and add the insecticidal soap in my backpack sprayer. I spray all my fruit trees at the first hint of the fungus, and this keeps the trees happy for several weeks. I too am in the PNW.

Dwarf Japanese maple

Submitted by Brian on August 20, 2019 — 9:47pm

My Japanese Maple looks like it has this powdery mildew. I sprayed a few leaves with some “seven”, hope I didn’t kill it more. A week or so back I noticed something odd, went on vacation and now it has spread quite a lot. I read here that all infected leaves should be removed and destroyed. yikes. Will the other options (Milk & water or baking soda) work without removing the infected parts?

powdery midew remedies

Submitted by sonya Norton on July 19, 2019 — 4:47pm

Bicarb works, but I’m never sure if i overdid it. I’m a firm believer in milk and water as a spray. It doesn’t seem to matter of it’s skimmed milk or whole milk. 50/50 with water, weekly or after rain. Has even beaten the mildew on lupins, my worst sufferer here in the PNW.

BlackBerry plants

Submitted by Marie on July 20, 2019 — 11:57pm

The stink bugs have destroyed majority of my blackberry bushes/plants. Help how do I prevent this for next years crop.

powdery mildew

Submitted by richard h hoyt on July 4, 2019 — 12:46pm

Implement Lactobacillus Serum as a preventive and you won’t get PM. Google search on how to make. Really simple.

Lactobacillus Serum

Submitted by Payton Jett on July 31, 2019 — 10:28pm

Once I make this, how do I implement?

How to make lactobacillus serum

Submitted by Annie Herbert on October 5, 2019 — 4:44am

Will I have to dig up new plants .honey suckle .

Crystals on my rose-scented geranium?

Submitted by Sian on June 22, 2019 — 2:42pm

Hey, there! I just noticed a peculiar thing I’ve never seen before; three leaves of my rose-scented geranium have what appears to be salt in their gullies. I thought at first I was looking at powdery mildew (a familiar foe), until I noticed the loose, sparkling quality of the substance. Are the leaves leaching salt? I do not fertilize, never need to, and the effected leaves still look as healthy as ever (I have still removed them in case the substance is otherwise nefarious). What could this be? For information’s sake, this geranium is planted in a half-gallon SmartPot.

Powdery Mildew

Submitted by Ron Lane on June 7, 2019 — 1:45pm

This is one of the better PM articles I have seen on the net. One correction is that baking soda does not stop powdery mildew, it only helps to prevent it. You may spray it on and think your PM is gone, but if you come back in a couple of hours your PM has returned.
Pristine is a BASF product that actually eliminates PM. Some form of it might be available to homeowners. Of the softer chemistries, Circadian Sunrise is the only one I’ve seen that consistently stops PM cold. The problem with that product is that the smallest unit presently sold is a 2.5 lb concentrate, which is far more product than most homeowner will use in their gardens over the course of a year or more.

White power on leaves

Submitted by Anna on November 1, 2018 — 8:51pm

I have white powdery substance on my zinnias. Will this make the seeds in the flowers be bad for next year?

Yes, the mother plant has

Submitted by dave on November 16, 2018 — 3:40pm

Yes, the mother plant has been infected and will pass along those genetics to the seedlings which will also carry the disease.

That’s not how genetics works

Submitted by Marc on June 11, 2019 — 9:27am

That’s not how genetics works, Dave.

powdery mildew on bee balm

Submitted by fredric lacarbonara on October 8, 2018 — 1:52pm

the info I have is to spray with milk/water mix or baking soda/water mix. I am doing that , but is it a one time treatment or multiple treatments. please advise how many times and how often to spray. ty

Powdery Mildew Treatment

Submitted by The Editors on October 9, 2018 — 4:13pm

It will likely require multiple treatments to keep the fungus at bay. Test the mixture on one plant first, waiting at least 24 hours to be sure that there are no negative effects. Then spray all affected plants at least once a week. After three or four applications, wait to see if symptoms return before spraying again.

White powder fungus

Submitted by Lisa in Texas on October 5, 2018 — 10:39pm

I have been fighting this in my house for almost 4 years. I have tossed out so many plants I can’t tell you how much money its cost. I am wondering if two giant chinese evergreens are causing it — the have never had it at least not visibly. I am ready to torch the house or at least throw away every plant I have and fumigate / steam clean every nook and cranny including AC ducts. I am at my wits end.

White powdery mildew hay spread to much cherry tree

Submitted by Jack G. on November 13, 2017 — 8:59pm

I’m just sick, I just got 2 new cherry trees 1 Bing, 1 Black Tartarian cherry tree and I spread hay down first before I mulched around the bottom of the trees. The hay I used has white powdery looking stuff in the middle of it, not knowing I put it down and put mulch on top of it. It is Nov 13th here in Wisconsin and cold out. Does the hay have white powdery mildew and will this now infest the trees next spring? Should I dig up all of the mulch and get rid of it all? What should I do?

Late reply ;but worth mentioning I thing.

Submitted by Rocky on February 14, 2019 — 8:24pm

Jack everything you’re describing to me sounds like the hay possibly has mycelium growing on it and while PWM is a fungal culture Mycelium in and of itself can be very very beneficial to plants/trees.
The Mycelium will grow throughout the same substrate the roots also colonize and create a symbiotic relationship with the roots themselves, while mycelium itself is started with other mediums hay is a much loved substrate to colonize after the mycelium has been birthed.
By using a layer of hay beneath your trees you’ve given the mycelium that was either spores lying dormant in wait or living mycelium beneath the hay previously a nice secondary substrate to grow through before expanding into the substrate your roots have begun colonizing.
I’m not going to pretend to know all the names of the different ways the relationship is symbiotic ;but I have recently been researching this topic a lot and can say for sure it’s not just beneficial for one or 2 little things. I can’t say that every strain of mycelium is going to be equally as beneficial obviously or even beneficial at all, some fungus (mycelium fruits fungus/mushrooms) like PWM use the plants themselves for hosts as mycelium itself does actually consume food rather than absorb nutrients through roots like plants ;but only certain types are threats to plants/trees.
Chances are the Mycelium you seen on the hay would take a very very long time (many seasons) to reach the top layer ;but if it does it could very well start fruiting some mushrooms annually if that’s the type of mycelium you seen (not all types produce mushrooms of course). Many strains of mycelium have a natural fruiting time in Oct./ Nov. and also really thrives at that time of year so it makes sense for the time of year you seen it, which is another part of what you said that makes me feel it wasn’t PWM at all.

«I spread hay down first before I mulched around the bottom of the trees. The hay I used has white powdery looking stuff in the middle of it, not knowing I put it down and put mulch on top of it. It is Nov 13th here in Wisconsin and cold out»

PWM wouldn’t be living on hay at that time of year outside ;but mycelium definitely could! Laying down mulch on top of the hay is an interesting touch as many beneficial mycelium species really love wood as well!
Anyways I know my reply is very late ;but hope you still find some useful information here and if you enjoy gardening at any level I highly recommend further investigating yourself as mycology is a very interesting hobby to get into.

Powdery mildew

Submitted by Linda Schwin on July 28, 2017 — 8:14am

I have powdery mildew on my peonies. I sprayed with neem oil but now the plants look dry and damaged. Should I cut the plants all the way down? Will they come back next year if I do? Is there anything I can add to the soil? This is the second year this has happened to these plants. Last year I did not treat them with anything.

How to keep mildew of peonies plant

Submitted by Simon viola on June 18, 2018 — 8:06pm

Every year after the season I start getting fungus on my peonies what can I use to prevent this.

PM on peonies

Submitted by The Editors on August 27, 2018 — 11:29am

Are your peonies crowded? You might try thinning them out, per above to improve circulation: Selectively prune overcrowded areas to increase air circulation; this also helps to reduce humidity around your plants.

You could also try the spray solution listed above:

  • Mix 1 teaspoon baking soda in 1 quart of water. Spray plants thoroughly, as the solution will only kill fungus that it comes into contact with.

Powdery mildew

Submitted by Ken Diehn on July 20, 2017 — 10:10pm

You mention to destroy plants and not compost. What method is best? Will burning cause the mildew spread to other plant? How about other diseases on plants, will burning spread disease?

Burning powdery mildew

Submitted by Cindy on October 22, 2017 — 2:33pm

Did you ever find if powdery mildew is spread by burning infected plants?

Burning Powdery Mildew

Submitted by Oliver on October 18, 2018 — 7:59pm

I have been doing a lot of research on powdery mildew. Temperatures above 90 degrees kill it.

Grape Vine

Submitted by Peter on July 20, 2017 — 11:11am

Our concord grape vine seems to have «expired», after 5 years of fairly reliable fruity growth. How can I tell if I need to start over, or find some «life» in what remains? Sad. The neighboring vines are doing well.

Rosebush fungus ?

Submitted by Dev Prashad on June 12, 2017 — 9:17pm

Would like to know what some white fuzzy looking stuff on the rosebuds might be and
what can be done to eliminate it.
Looks like the fuzz n peaches but is a bright white in colour

powdery mildew on roses

Submitted by The Editors on June 13, 2017 — 1:48pm

It sounds like powdery mildew. See guidance above and

• plant roses in full sunlight.

• space plants to allow ample air circulation

• fertilize to maintain vigor but do not overfertilize

• avoid wetting leaves when watering; use drip irrigation, if possible

• prune affected canes and collect and destroy cuttings (do not compost)

• use fungicide with care

• in future, select rose varieties that are resistant

Sounds more like aphids to me

Submitted by Cass on June 5, 2019 — 9:25am

Sounds more like aphids to me. Roses are certainly prone to white mealy bugs. The “bright white” is what convinced me. Mildew isn’t typically bright white but the infuriating little critters definitely are.
If it’s them, alcohol will kill them only if it actually touches them. Neem will repel them. Most commercial pesticides are useless thanks to the typical resistance. I knock them off using water pressure and then use Neem to keep them off. Repeat as needed, at least 3 times since they’ll also dig into the soil plus you’re dealing with the juveniles and eggs.
If it’s a small houseplant I’ll pick them off or kill them individually with isopropyl alcohol. Again, retreat as necessary and keep in quarantine. The awful things can lie in wait without a plant to feed on for a long time so be careful about the site and disinfect.

Powdery Mildew and Environmental parameter

Submitted by Pawar Bhalchandra on October 27, 2016 — 12:04pm

Would like know about relationship between disease powdery mildew and environmental factors specifically Temperature, Relative Humidity, Rain Fall and Wind speed. Is there any quantitative MODEL which defines relationship amongs them in control of disease.

Red Spider Mite on hydrangeas

Submitted by Alison on October 16, 2016 — 2:42am

Can you tell me what to spray to rid the plant from this pest. It has spread to other plants in the area.

Red Spider Mite on Hydrangea

Submitted by The Editors on October 18, 2016 — 5:11pm

Mite infestations often occur when plants are stressed from insufficient water, so be sure you are watering enough. General purpose insecticides can exacerbate the problem. Instead use an insecticidal soap according to the directions on the label to knock the population back.

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