Powdery mildew is a common garden problem appearing as a gray-white powder on flowering plants including fruits and vegetables. While there is no cure, applied early, a milk solution may discourage spreading.
If you start plants from seeds indoors, you may also get another common fungal problem called damping-off disease.
Powdery Mildew in The Garden
It starts like this. One day you’re walking around your garden—usually midsummer or a little later—admiring how nicely things are growing and then you see it.
Some sort of gray-white powder is suddenly covering the leaves or stems of—let’s say—your squash vine—but it could be some other flowering plant as well.
It’s chalky and gross-looking and attacking an otherwise healthy plant.
Prior to this you may have also noticed a few little splotches on the leaves but didn’t really think much about it.
But now the leaves are coated in the powder and—yuck!
You may also see curling or yellowing leaves or some misshaped flowers.
Upon further examination, one thing stands out: while the infected plant looks terrible, other plant species growing nearby look fine.
So why is this one plant infected but others around it are not?
That’s how powdery mildew works.
There are hundreds of fungi species we call powdery mildew. Each one affects either a single plant species or a few species within the same plant family.
For the squash example, those powdery mildew fungi can only infect other plants within the cucurbit family (pumpkins, squash, cucumbers, melons), not other unrelated plant species.
We wish it stopped there but there’s more.
Because there are so many types of powdery mildew fungi, different infections can occur at once.
Your infected squash or roses or lilacs or peas aren’t going to cross-infect each other but each can be experiencing powdery mildew at the same time.
Keep reading to learn more about this fungal disease along with tips for discouraging future infections.
If you’ve got powdery mildew and want a simple solution, unfortunately there isn’t one.
Here’s what we know.
- Your infected plant is unlikely to die but it may struggle.
- There is no getting rid of powdery mildew entirely (it will return).
- Any effective commercial or unconventional treatments (including spraying with a milk solution) are preventative only and will not work once you have an outbreak.
- Any treatments you use may also have undesirable side effects.
- Moving forward, use these prevention tips to discourage it.
- What is Powdery Mildew?
- How to Prevent Powdery Mildew
- How to Treat Powdery Mildew
- Frequently Asked Questions
- Powdery Mildew Fungi & Their Host Plants
What is Powdery Mildew?
Powdery mildew is a broad term for an entire group of fungi species that infect plants. This group includes over 900 fungi that infect over 10,000 flowering plants species—many of them familiar to our home gardens including ornamental flowers, fruits, vegetables, and grasses.
These plants serve as host species for the fungi. You’re probably familiar with hosts and specialist species for butterflies. One example is how monarchs are a specialist species that cannot survive without specific milkweed plants (hosts) to feed their larvae.
Same for these powdery mildew fungi: they are what we call obligate parasites and specialist species that cannot survive without their specific host plants (or family of host plants).
With so many fungi species in this group, there are going to be some variations in the details of how they work, but they are similar enough that we group them all together under this one disease name.
While the numbers seem overwhelming, your garden is not doomed. Because the fungi are host-specific, each species only attacks certain plants. If you’re just noticing the mildew on one plant, that might be the extent of it. The infection on your peas will not spread to nearby roses or lilacs or any other unrelated species. But other peas growing nearby are susceptible and other, separate infections could occur as well.
From the perspective of someone trying to get rid of powdery mildew, the reality is, fungal spores are everywhere and fungi as a whole are vital to life on earth. This is why we must be careful with the use of any fungicides in general.
In agriculture, powdery mildew is a multimillion-dollar problem with plenty of ongoing studies searching for solutions.
In home gardens, it’s more of a nuisance. Yes, it looks unsightly and can reduce harvests, but the garden as a whole carries on. We may not like it but it’s never truly going away.
How It Spreads & Thrives
With so many different powdery mildew fungi species, their preferred growing conditions vary, but the one thing they all have in common is the quest to survive (through reproduction).
While we associate fungal diseases like damping-off with moist conditions, some powdery mildew fungi can spread in dry conditions.
Same for humidity—some need it high, others not so much.
Overall, most don’t do well with sun or heat.
When temperatures get beyond 90°f (32°C), the fungi become inactive, and, beyond that, may not be able to recover. But the limits depend on the species. If there was one set of rules for this group, we’d probably have a better handle on them.
How The Mildew Forms
Once a plant has been infected, the spore’s hyphae (branching filaments that make up the mycelium of a fungus) expand on the host plant leaves or elsewhere on the plant. Viewed under a microscope, these hyphae look like a network of little branches or threads.
Within hours, asexual spores are produced and it’s the hyphae and spores that together form that powdery-like stuff we see on our plants.
Spore Spread & Germination
Spores spread by wind or can be carried by anything that comes into contact with them including insects or other animals (including humans), and by things like dirty garden tools or water.
From the time a spore lands on its host plant, germination can happen in under a minute. You can see why fungi are so prolific: they do not waste time.
By spreading their spores so rapidly and widely, the fungi are safeguarding their genes for survival. Our podcast episode on powdery mildew (below in Resources) discusses more about the differences between their asexual and sexual spores and how they survive the winter if you are interested.
How It Affects Plants
As spores germinate and mildew coats the leaves, the host plant’s ability to perform photosynthesis can be reduced. If extensive, this could affect nutrient delivery, growth, and productivity.
A weakened plant is also less able to fend off pests and other diseases.
As the fungal infection spreads within a plant, a series of specialized structures and cells end up penetrating the plant’s cuticle and cell wall.
If all goes well from the spore’s perspective, it can start syphoning off nutrients from the plant—but not so much that the plant should die. After all, these spores are obligate parasites meaning their survival depends on the host’s survival.
The loss of nutrients in the plant may result in things like curling or yellow leaves, loss of leaves, misshapen flowers or fewer of them, which in turn can mean lower yields.
How to Prevent Powdery Mildew
If there was a sure-fire way to prevent powdery mildew, we’d all be doing it.
That said, there are some things you can do to at least discourage the spores and perhaps lower the numbers or decrease their spread.
Some of these options are basic good garden practices, others are drastic measures you may or may not like depending on how severe the problem is or how much it irks you.
- Whether you’ve ever seen it or not, assume powdery mildew spores (and other transmissible plant diseases) are present in your garden.
- Clean your hands, gloves, and tools frequently, and remember that spores can travel on things like clothing too.
- Don’t crowd plants—allow room for good air flow.
- If the infection is substantial, consider removing infected plants and any related species in your garden.
- Only handle infected plants if you are removing them and then take precautions to avoid dispersing the spores further.
- Seal the infected plants in disposal bags.
We know the spores become inactive in temperatures over 90°f (32°C) but it takes a lot more to kill them off. It varies by species but sustained temperatures in the range of 140-160°F (60-71°C) work for some—similar to the temperature of a hot compost pile. Your municipality may also offer a disposal service.
Some recommendations also suggest smothering the soil (plant site) in thick plastic to heat-kill the spores in the soil. Keep in mind this will affect everything in the soil.
- Avoid growing known host plant species for at least a year.
- For future plantings, look for powdery-mildew resistant varieties. I’ve listed some suggestions in the Resources section. These are often marked on plant tags and in seed catalogs as PM or noted within the plant description.
There are additional suggestions in the Treatment section.
How To Treat Powdery Mildew
Most powdery mildew treatments are preventative and not effective once you have an outbreak. If your plant is covered in mildew, save your money on products or remedies—it’s too late.
While there are commercial fungicides that can work as protectants to stop powdery mildew from forming or in other cases reduce the severity, there are several things to consider first.
Note that they say “stop powdery mildew from forming.” This means some products must be applied preventatively, at a particular time in certain conditions, prior to infection. Once you see your plants coated in that mildew, it’s too late.
Others may reduce spore numbers. Again, they will never be entirely eliminated.
For large-scale agriculture, even a small improvement in percentages can provide a big financial return, but for home gardeners, any benefits gained should be weighed against any collateral damage or side effects of any treatment (plant or soil damage or harming wildlife) as well as the cost of the product.
If you opt to try a commercial product, first, know what type of fungi you have to determine the right product for the job. The host plant species is your clue. I’ve listed some common ones here.
Check the warnings on the label to understand possible adverse effects or side effects and whether it will harm certain plants (phytotoxicity), animals, or other living things in your garden.
I avoid commercial fungicides entirely.
This is one of those rare instances where the “home remedy” has some science to back it up.
There is research indicating that cow’s milk can be diluted with water and applied as a spray to slow the spread of powdery mildew. Studies show it can be as effective as some popular fungicides.
With over 900 fungi species and 10,000 plant species involved, we cannot know how it works for all infections. There are reports it can be effective (preventatively) with the fungi that infect squashes including zucchini, pumpkins, grapes, and roses.
20% milk / 80% water
Also, the milk to water ratios vary with each study ranging from 10 to 50-percent (half milk, half water). Several studies tested 10-20 percent solutions with some success.
There is no specific recommendation on which fat level of cow’s milk to use. There are various theories about how the fats or proteins may interact with the fungi.
To apply it, the milk-water solution is sprayed on all parts of the host plant (both sides of leaves, stems, fruits) before powdery mildew is apparent.
Presumably, you’ve had powdery mildew previously, know which plants are vulnerable, and apply it there before any infection appears or when small splotches appear.
Drawbacks include the cost, inconvenience, and the smell of sour milk in the garden.
Also, while most recommend weekly applications, you may need to apply it more often if rain washes it away.
On a related note, there is also research showing that soaking garden tools in a 20-percent milk solution disinfects to some degree but not as effectively as using a 10-percent bleach solution.
We did a podcast episode on milk and gardening which you can listen to in the Resources section below.
Baking soda (sodium bicarbonate) is recommended for so many garden uses and most of them are nonsense. You can read more about the problem with using baking soda in the garden here.
There are countless DIY recipes online claiming to be fungicides including “The Cornell Formula” which is apparently a made-up thing that takes advantage of the the Cornell name.
These recipes use various combinations of baking soda and oils (vegetable oil, mineral oil).
Studies suggest limited or inconsistent results.
Plus, if you’re considering the bigger picture, we do not want or need more sodium in our soil or waterways.
Alternatively, potassium bicarbonate (KHCO3) is considered an effective fungicide allowed for use in organic farming.
We looked at neem oil as an insecticide here. Knowing that butterflies, moths, and some bees are highly sensitive to it, there is no sense considering it for any use in the garden when plants are in flower and these creatures are active.
I mentioned plant removal as a preventative measure but you can also regard it as a treatment—albeit a drastic one.
The idea is, since powdery mildew fungi are host-specific, by removing their host plants, you remove what they need to survive.
This is a common disease-prevention tactic for lots of vegetable garden crops—either through crop rotation (better suited to mass agriculture, not small home gardens) or by skipping growing seasons to stave off any offending diseases. An example is ceasing tomato growing for a few years to avoid blights.
There are no guarantees for the future since new spores can arrive at any time, but it can give a fresh start and perhaps a reprieve if your garden has been plagued by certain harmful fungi.
In my own garden, powdery mildew was relentlessly affecting a purple bee balm (Monarda) but not any of the red ones, so I removed just the purple ones and have not seen the problem since. Sometimes even within the same species, one cultivar or variety is resilient and the other is not.
Fruit from plants infected with powdery mildew is considered safe if it is in good condition with no signs of disease or damage. As with all harvested crops, wash thoroughly before use. Anyone sensitive to or allergic to fungi should avoid contact.
Powdery mildew and downy mildew belong to two different orders. Powdery mildew fungi belong to Erysiphales and downy mildew belongs to Oomycetes, also known as water molds. There are many differences between the two groups including how they spread and reproduce. Visually, powdery mildew looks like gray-white powder on leaves while downy mildew appears as yellow patches with brown spots, usually originating on the underside of leaves. The tips for dealing with both are the same.
Powdery Mildew Fungi & Their Host Plants
There are over 10,000 flowering plant species that act as hosts for powdery mildew fungi. Here are some examples.
Plant | Fungi Species
- Apples, pears | Podosphaera leucotricha
- Artichoke, onions | Leveillula taurica, Oidiopsis taurica
- Bee balm (Monarda), Phlox | Erysiphe cichoracearum
- Cucurbits: pumpkins, cucumbers, summer squash, winter squash, watermelons, muskmelons | Podosphaera xanthii also called Curcubit powdery mildew (CPM); Erysiphe cichoracearum; Podosphaera fusca
- Eggplant | Leveillula taurica
- Grape | Erysiphe necator (or Uncinula necator)
- Legumes, soybeans | Microsphaera syringae
- Lilacs | Microsphaera syringae
- Pea | Erysiphe pisi
- Strawberry | Podosphaera aphanis
- Norway maple, Japanese maple | Sawadaea tulasnei
Powdery Mildew-Resistant Plants & Seeds
Here are some examples:
- Oregon Sugar Pod Pea | SeedsNow (US Shipping)
- Strawberry Roots & Bundles – Albion | SeedsNow (US Shipping)
- Honeynut Winter Squash Seeds | Botanical Interests (US Shipping)
- Emerald Delight Summer Squash Seeds | Botanical Interests (US Shipping)
- Contender Bush Bean Seeds | Botanical Interests (US Shipping)
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~Melissa the Empress of Dirt ♛