What to do about mould or mildew on cucumber and squash plants

powdery mildew growing on cucumber leaves in vegetable garden

It’s that time of year: the leaves of the cucumber and zucchini and other squash plants are starting to go mouldy. What can you do? Well, first of all, like any sort of problem-solving, you need to understand what the problem is. And it is not actually mould! It is a kind of mildew known as powdery mildew.

How can I get rid of powdery mildew on my cucumber and squash plants?

Well, the bad news is that once it has started you can’t really get rid of it. But don’t stress out about it: powdery mildew is pretty much expected if you grow cucurbits in your garden (cucurbit is the plant family that includes both squashes and cucumbers, as well as pumpkins and melons too).

You probably will never totally get rid of powdery mildew. You just need to learn how to control it so it doesn’t affect your plants to the point of reducing your produce yield.

squash plant growing in a flower pot

If you are growing in pots, move the pots to a sunny and breezy area, and turn them occasionally so all leaves get some exposure to the sun.

pollinated butternut squash and unpollinated fruit starting to rot

Remove any leaves or dead flowers or unpollinated fruit that may start to rot or mould or mildew.

For example, here I have one butternut squash that pollinated and is growing , but the little guy below didn’t make it. He will rot – so I will pick him and remove him before that happens.

Be careful when trimming mouldy leaves, though, because you don’t want to go overboard!

squash plant with many leaves trimmed off die to mildew

If the leaves only have a little bit of mildew on them, it is best to leave them, at least for a while.

Plant needs the leaves for photosynthesis – in order to have enough energy to produce fruit.

For example, I would not remove as many of the leaves as this gardener did.

powdery mildew growing on squash leaves in vegetable garden

Here, I removed the one leaf that had a lot of mildew on it early, so it won’t spread to the others.

But I will wait and see – leaving the remaining leaves on as long as possible before, and will only remove them once they have a lot of mildew on them.

Some people recommend fungicides or other sprays, but I have never used them and I do not recommend them. (You don’t want to spray that stuff around your food!) Rather, your goal should be never to let the powdery mildew get that bad, so you don’t even consider using a fungicide spray.

How can I prevent getting powdery mildew in the first place?

  1. Make sure that you promptly remove mouldy or mildewy leaves, stems, blossoms and fruit. Then – do not put them in your compost, or you will be seeding their spores back into your garden next year!
  2. Plant your cucurbits – zucchinis and other squashes, cucumbers, melons, etc. – in areas where they get good airflow and lots of sunlight. Don’t crowd the plants or you will create an environment where powdery mildew will grow. Space potted plants well apart (like my cucumbers on my balcony, below left). Try to keep leaves and fruit of garden plants from touching the ground as much as possible (for example, by using a squash fence, below right).

For more tips about how to grow vegetables in your home garden or in containers on your patio or balcony, join the Food Garden Club!

Published by Jacqueline Windh

I'm a writer, photographer, and radio broadcaster who is concerned about our planet and how we live our lives - hoping my work helps people to find new ways of thinking about issues such as personal health, wilderness, the environment, food security, thinking about the future. These things are all connected, you know...

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