A Beginners Guide to Powdery Mildew

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This was going to be a general house plant fungus post (yum), but I thought I’d concentrate on powdery mildew since it’s one of the more likely fungal infections to turn up on your indoor plants, and it’s probably the easiest to diagnose.

A lot of the time, people incorrectly diagnose a plant as having a fungal infection when it’s really a root issue – just because your plant has a black mark with an orange ring around it doesn’t mean that there’s a fungal infection present.

Oh, and one of the main causes of fungus infections is wet leaves, so if you’re insistent on misting, add a tiny bit of castile soap or neem oil to your water to discourage fungus.

What is powdery mildew?

Powdery mildew is the umbrella term for some of the ascomycete fungi that infect plants. There are around 6,400 species of ascomycetes, but they’re not all powdery mildew – truffles and morels are in the same family.

They’re also definitely not all parasites, but powder mildew is.

Powdery mildew is unlikely to kill your house plant*, but it can hinder the development of foliage and make then look a bit weird and grim, which probs isn’t the aesthetic you’re going for.

*It can kill certain plants – the variety that tends to infect tomato plants can do serious damage to the plant.

What are the symptoms of powdery mildew?

As we’ve previously established, there are several species of powdery mildew, so there is some variety in the symptoms. However, they all tend to display:

  • Patches of what looks like mould on the underside of the leaf – the colour can vary from white to dark grey, depending on the species.
  • The leaf edges can curl up
  • Blotches on the leaves
  • Slow growth
  • Stunted growth
  • Leaf drop
  • Lesions
  • Warped new growth

You might also see black spots called cleistothecium, which are the mildew, er, fruits.

What causes powdery mildew?

Like pests, powdery mildew has an incredibly annoying habit of just showing up.

It’s not really a matter of causation – powdery mildew will turn up, and if it likes the conditions you provide for it, the infection will grow.

The cleistothecia can survive without a host, allowing them to survive over winter, and if those structures end up in your home, you’ll get powdery mildew.

How to prevent powdery mildew

Unfortunately, powdery mildew tends to thrive in environments that are great for growing house plants, but there are steps we can take to reduce the likelihood if it taking hold in our collection.

  • Powdery mildew likes warm-ish (around 18oC/65oC), humid environments without a lot of air flow, so adding a fan (or opening a window) can help.
  • It also likes low light conditions, so make sure your plants get ample light
  • If you have dry air, watering the foliage can help because powdery mildew doesn’t like direct contact with water. Don’t do this in high humidity – it can encourage other fungi
  • Keep your plants in good condition – powdery mildew is less able to get established on a healthy plant
  • Try not to crowd your plants – allow good air circulation between them
  • Remove any dead leaves

How do I treat powdery mildew?

A lot of the traditional chemical fertilisers designed for use with powdery mildew are more suited to outdoor plants, and you don’t want to use it indoors (for the sake of your own lungs).

Isolate the infected plant the moment that you see it, and clean the leaves of all the other plants – avoid wetting the leaves, so wiping with a dry cloth should be enough to disrupt the lifecycle of the fungus.

Professionals actually recommend removing any infected leaves, but there are a few home remedies that can work:

  • Soapy water – I like to use castile soap because it’s super gentle on the foliage but should get rid of the fungus. A drop in some water will do
  • Vinegar – a couple of tablespoons in a gallon of water is the recommended dosage
  • Milk – I was skeptical about this one, but apparently, a ratio of 40% milk to 60% water is as effective on powdery mildew as some chemical fungus controls.
  • Baking soda has a few supporters online, but it’s not that effective at killing the fungus when it’s taken hold. It is, however, good at preventing the fungus from spreading, so it’s a good preventative measure. A teaspoon in with your castile soap can supercharge your cleaning solution

Should I destroy plants with powdery mildew?

It’s up to you. If you have a LOT of plants and you don’t have space to isolate the infected one, then it may be easier, in the long run, to sacrifice one plant for the good of the others.

If you’re the kind of person that doesn’t mind taking the time to painstakingly check and clean your plants then by all means keep the plant and work on eradicating the fungus.

On the other hand, if you’re the kind of person that can end up neglecting plants when they’ve got infections/pests because it’s a bit overwhelming, then it might be in the best interests of your other plants to take drastic action.

You don’t actually have to destroy the plant – you could remove all the foliage, flush through the soil, and work on preventing it from taking hold in the future.

What happens if you touch powdery mildew?

Nothing. Powdery mildew in its, er, raw form doesn’t pose a threat to humans, but if you’re allergic to mold and consume a plant that’s been infected then you could potentially have an allergic reaction.

Caroline Cocker

Caroline is the founder and writer (and plant keeper) of Planet Houseplant

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