Houseplants Plant diseases

When Mildew Attacks

I have fungus.

Well, yes, that’s how intimate we get, I’ll tell you when I’ve got bugs, but also when I’ve got fungus!

Unfortunately, fungus can’t really be cured; you have to remove the infected part and wait for healthy parts to grow back, treating them to prevent the fungus from contaminating those parts too.

… What?

Stop looking at me like that! I’m talking about fungi that attack plants! What were you thinking?

Houseplant Fungi

This is the first time it’s happened to me in several years, and unfortunately I didn’t react quickly enough. The truth is that the appearance of powdery mildew on houseplants is quite subtle at first. That is, until the leaves fall, just like in autumn!

For me, it happened when I brought my plants into the house at the end of the summer. They went from a sunny, airy balcony to a dark corner of the house. I procrastinated about installing my lights, and the warmer weather meant I didn’t turn on the ventilation or heating, but opened the windows instead. And bam! The humidity in the house rose, the plant lacked light and ventilation, and fungus settled on the leaves.

Where does this mushroom come from? There’s no real answer to that. It’s a very small fungus that can be found just about anywhere, waiting for the right opportunity to develop. An plant could therefore be the only one in your collection to be affected and never contaminate the others, or, on the contrary, if your plants were all in a “weak” situation they could be contaminated by the tiny spores floating in the air.


There’s no point in isolating diseased plants in the same way as unwanted insects, for example. Spores are too volatile to remain confined to a room. The best way is to ensure good air circulation and light to give the plants energy.

Clean up any dead leaves that fall on the soil, ensure good drainage and water only when necessary (when the soil is dry in the first inch for most plants). That’s all there is to it!


Have you ever had nail fungus? There’s nothing you can do to heal it: the affected nail has to grow back. To prevent the fungus from taking hold, treat the healthy nail for regrowth, and wait patiently until you can cut the affected part. It takes a long time: it can take a year before your nail has grown back enough to remove all the fungus!

For plants, you can cut off ALL the affected parts at once. It hurts, but that’s what it takes! Because, in fact, it’s like fingernails: the parts that have been affected won’t heal, and can only contaminate the other leaves…

So arm yourself with scissors (and courage) and… cut!

My beautiful baby-gonia when I took it in and inspected it…
… and a month later, the result of the cut.

Did I cry when I fleeced her? Pfff! No, it’s just a plant!

OK, yes, a little, I admit it…

After the massacre, it’s time to treat the plant to prevent other spores from taking hold: air, light and fungicide. A spoonful of bicarbonate of soda from your kitchen in a liter of water, sprayed on the leaves, will do the trick.

WARNING: Moisture stagnating in the soil or on the leaves encourages the proliferation of powdery mildew. After spraying, be sure to place your fan directly on your plant. Repeat the treatment as new leaves grow, removing any that show signs of infection.

That’s it… I’m going back to inspect the baby leaves again, just in case!

Audrey Martel is a biologist who graduated from the University of Montreal. After more than ten years in the field of scientific animation, notably for Parks Canada and the Granby Zoo, she joined Nature Conservancy of Canada to take up new challenges in scientific writing. She then moved into marketing and joined Leo Studio. Full of life and always up for a giggle, or the discovery of a new edible plant, she never abandoned her love for nature and writes articles for both Nature sauvage and the Laidback Gardener.

3 comments on “When Mildew Attacks

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  2. I don’t understand. While a serious case of powdery white mildew can kill a plant, and a moderate case may quite likely result in the loss of some leaves, typically it’s not so serious. Spraying with rubbing alcohol (70% isopropyl), or even dusting the plant with cinnamon or another desiccant will manage it, combined with, as you say, better air circulation, better light, and less humidity.

    There’s no need to resort to a fungicide for powdery white mildew, although of course those could be needed for more serious situations, like rust. But I’ve never seen powdery white mildew require surgery. Of course a hard pruning isn’t likely to do any harm to a winter dormant plant, but I’m confused. Are there more virulent types of powdery white mildew? We can’t kill off all the fungi spores in an area, but this one isn’t typically a grave threat or particularly difficult to remove an active case from a plant. A plant that’s in very poor health may still not recover, but most of the time, minimal treatment and better conditions will resolve powdery white mildew, although of course, we cannot “cure” whatever damage was done. I am so confused.

    • Just so you know, All three things, potassium bicarbonate, cinnamon, and rubbing alcohol ARE fungicides. Just because you can get/make it at home, does not mean it’s not a pesticide. But please do not put rubbing alcohol on your plants. It is phytotoxic, and the worst of the three. In fact, even potassium (and sodium) bicarbonate, if sprayed in large quantities in a late stage infection, can also be phytotoxic. You are right though that resorting to pesticides is absolutely not ideal. They should be used cautiously, and as a last resort, and ideally as species selective as possible, and the least toxic possible, while still being actually effective.

      The things that matter most when it comes to mitigating biotic disease are genetic resistance and proper cultural practices. Typically when a plant becomes infected/infested with something, that means it was stressed via inappropriate cultural situations, and an investigation as to what that factor is, is of importance.

      Coming in at a close second in disease mitigation is sanitation. That means removing infected plant material, including dead plant material from the site, and avoiding cross-contamination by practicing things like disinfecting pruning shears between cuts, and especially between plants. And despite what the article states, quarantining plants is absolutely still beneficial, as many diseases are spread via infected water splashes in addition to wind, including the mildews. Mulching is also a good idea to prevent infected soil splashing up on the leaves.

      Powdery and downy mildew can be an exception, but generally, the majority of foliar diseases are cosmetic at worst, and sometimes the best thing to do is in fact nothing

      For the record, powdery mildew are part of Ascomycota, and therefore do no make mushrooms. It is Basidiomycota that possess the trait of making mushrooms.

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