Such a sneaky disease. And boy does it love African violets and begonias, not that it means powdery mildew has stayed away from my other houseplants.
Outdoors it’s a common one for roses, grapes, tomatoes and more.
But let’s look at what causes it in indoor plants, how to deal with it, and most importantly, how to prevent it.
Powdery mildew is MUCH easier to prevent than to cure, but I’ll answer all three below...
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What is powdery mildew and what does it look like?
Powdery mildew is a fungal disease caused by the fungi family Erysiphaceae.
It’s an obligate parasite. Sounds scary. Is scary if you’re a plant. It needs a living plant to feed and reproduce.
It covers plant leaves, flowers, stems and buds with a greyish-white powdery-looking coating called haustoria, which work a bit like root tips, forcing their way into the leaf to feed on the plant.
What conditions does powdery mildew prefer?
It loves warmer temperatures during the day (15° to 27° C / 60° to 80° F preferred), and cooler nights, poor airflow, plants crowded together, damp conditions and swings in humidity, ideally higher humidity at night, with drier conditions during the day.
The combo of closing up our homes during winter, reducing airflow, along with a toasty warm house during the day which dries out the air, then a cold drop overnight when the heating gets turned off, is why powdery mildew and winter often go together.
And just to add to the fun, powdery mildew can spread easily, both by air and water splashes, but unlike other fungi, does NOT need wet leaves to spread.
Why are African violets and begonias more prone to powdery mildew?
The ‘good’ news is the type of powdery mildew that likes African violets often leaves other plants alone.
But it absolutely can spread like crazy to your other African violets (ask me how I know ☹).
Speaking of, it’s one of my favourite things about African violets that makes them so prone to powdery mildew, which is their fuzzy leaves. Perfect for trapping moisture and fungal spores.
Begonias are another favourite with powdery mildew and get their own types too, typically Oidium begonia or Erysiphe begoniicola.
Any type of pllant with leaves that are thinner, fuzzy or furry, or velvety, are easier for the fungi to break into the leaf to feed.
How do you treat powdery mildew?
Another tricky thing about powdery mildew is typical treatment isn’t handled as well by furry, fussy or thinner leaves. Yep, that includes African violets and begonias.
The treatment you might see recommended for roses or grapes for example, might harm both the fungi AND your plant when it comes to an African violet’s and begonia’s more sensitive foliage.
Powdery mildew is much easier to prevent than cure, but if you catch it early, treatment has a much higher success rate.
The good news is there are a lot of fungicide options, some home remedies, some store-bought. Here’s a few options and what’s worked for me…
Step 1: Isolate the plant
Especially away from other similar plants.
Step 2: Trim
If you haven’t caught it early, trim off the worst infected leaves, buds and flowers.
If you’ve caught it early, or can’t bare to chop anything off, treatment might be less effective but cross your fingers and move to step 3.
I err on the side of caution and get chopping either way because removing extra leaves also has the benefit of increasing airflow, which powdery mildew does NOT like.
Step 3: Spray or Wipe
I prefer the non-toxic options but here are a few options to pick from below including wet brush, hydrogen peroxide (my go to for African violets), Enspray (a good all-rounder for most plants and an insecticide also), and Yates Rose Gun (my go-to for begonias).
I've also seen baking powder (or was it baking soda), and milk recommended online but haven't tried them for powdery mildew.
(and wouldn't try milk myself, last time I did that for increasing calcium levels, it turned rancid and I got fungus gnats)
Step 4: Repeat
Powdery mildew produces new spores every 3 to 14 days so that combined with your chosen treatment usually means a higher treatment frequency is needed for the first 2 weeks.
Then one or two more treatments for the last 2 weeks, for 4 weeks total.
Some treatments can be used as a preventative also, usually once a month during higher-risk months.
Water:
Wet a soft brush and wipe the leaves, flowers and buds top and bottom. Clean the brush with a fungicide like hydrogen peroxide afterwards to kill the spores.
Repeat every 3 days for the first 2 weeks, then twice weekly for 2 more weeks.
It's super important to make sure that brush is sterilised in-between with something like hydrogen peroxide so you don’t spread those spores!
If I have nothing on hand for the next step, I’ll do the water brush step while I wait to get supplies.
Hydrogen peroxide for powdery mildew:
This is my non-toxic go-to for powdery mildew on my African violets (my top pick for begonias is the last option, coming up).
Rather than just washing it off, hydrogen peroxide actually destroys the fungus at a cellular level.
Make up hydrogen peroxide as per the directions (below or check the product), and spray lightly but thoroughly to cover everything above-ground, top and bottom of leaves, stems, buds and flowers.
I usually use Forsana Plant Care Plus 3% but another newer option is GT Oxy 3%. (both are available in New Zealand).
For my overseas plant friends, get yourself Essential Oxygen 3% which is plant-safe, as not all hydrogen peroxide is ok for plant use.
(that 3% part tells you it's 3% strength hydrogen peroxide, the strength recommended for plants - save the high-strength stuff and anything with added fragrances for your kitchen and bathroom)
Spray until it just starts to run off leaves. Don’t miss spraying underneath leaves.
One of the Mister 360 or Egmont 360 sprayers work a treat for this.
They produce a super-fine mist spray for better coverage and they spray upside-down to get under leaves without tipping your plant upside-down.
You can adjust the dilution rate (a few options below), to suit how sensitive the plant is.
But if you’re worried, I always recommend a spot test first of a couple of leaves and waiting a couple of days for any reactions.
Spray when leaves are completely dry so the hydrogen peroxide directly contacts the spores.
Same as all these spray treatments, if you have any plants in direct sunlight, shift them out of direct sun or bright light until leaves are dry, so turn grow lights off too during that time.
I use 1 part 3% hydrogen peroxide with 9 parts water for treatment, or 30mls per 1 litre of water for a regular preventative spray.
Enspray 99 for powdery mildew:
Big fan. Enspray is very fine mineral oil and works as both a fungicide and an insecticide.
Some houseplants are sensitive to oils but even then I’ve found they tolerate the finer Enspray 99 no problem, however absolutely do a spot test spray first just in case.
Enspray both suffocates the fungus and helps stop spores from attaching and reinfecting a plant so can be used as both a treatment and a preventative. It’s also certified for organics.
I use it at 20ml per 1 litre water.
Yates Rose Gun for powdery mildew (my pick for begonias)
Remember how roses are on that list for plants that powdery mildew loves?
It loves them so much that roses get their own treatment, but this also works a treat on houseplants and is a favourite with begonia hobbyists (me included).
But Rose Gun is different from shorter-acting contact fungicides like Enspray and hydrogen peroxide. Yates Rose Gun is a systemic fungicide.
That means it gets into the system of the plant and helps fight off powdery mildew from the inside out, so one treatment lasts a lot longer.
I use the premixed Yates Rose Gun so there’s no prep and any leftovers don’t have to be thrown away, compared to products that are concentrates (as once a concentrate is mixed you can’t store leftovers).
For Yates Rose Gun, just shake and spray.
The same guidelines apply for all treatments. Spray on dry foliage. Keep plants out of direct sun while leaves are wet. Turn grow lights off also until leaves fully dry.
You want great coverage also, especially underneath leaves, and to repeat spray every fortnight for at least 1 month.
How often to repeat treatment for powdery mildew:
For hydrogen peroxide: During an active infestation, I spray every 3 days for the first 2 weeks, then I shift to a preventive spray weekly for the next 2 weeks.
For Enspray 99: I repeat spray weekly, once a week, for 4 weeks during treatment, and fortnightly to once a month during risky times as a preventative.
For Yates Rose Gun: Repeat fortnightly for 1 month.
How to prevent powdery mildew
Since powdery mildew has specific conditions it loves, and the spores are likely always around just waiting for those conditions to happen, the easiest treatment is to prevent it in the first place.
Here’s your powdery mildew steps for prevention:
🪴 Avoid crowding your plants together to keep air flowing,
🪴 Quarantine new arrivals for at least 4 weeks to avoid the risk of them introducing new spores to everyone else,
🪴 Keep up good airflow in general but especially in winter,
(a small USB fan does the trick nicely for mine or a room fan on for a couple of a days is plenty too, just opening windows helps too),
🪴 Try to avoid swings in temperature overnight in winter,
🪴 For African violets in particularly, regular separate suckers to keep good airflow between stems and leaves,
🪴 For plants with fussy, furry or velvety leaves, avoid leaves getting or staying wet,
🪴 Top water the soil only or swap to bottom watering.
🪴 Don’t mist.
I do hope you never have the sad job of cutting off buds, flowers or leaves covered in powdery mildew, and that you came here for the prevention not the cure; but either way, I hope these tips and tricks help you get and stay free of powdery mildew.
ABOUT THE AUTHOR: Anna is the owner of plant store Love That Leaf which specialises in products and advice for indoor plants. She is also the resident indoor plants writer for NZ Gardener magazine. Her houseplant care articles appear in print in magazines in-stores NZ-wide, and online on Stuff, The Post, NineHoney and more.