How to prevent and treat powdery mildew in your garden

Powdery mildew is a fungal disease that is prevalent on plants in summer and autumn, and is made a lot worse by the waves of humid, wet and extreme dry weather we are having in New Zealand.

How to identify it

It manifests itself with white spots (spores) that appear on the top of the leaves and stems of the plant. Powdery mildew affects a variety of vegetable plants and grape vines. The main plants affected by this fungal disease in the garden are zucchini, cucumber, melons, pumpkin; it can also affect dahlias, sweet peas, roses, hydrangeas and grapes.

You may not know…

Mildew usually appears later in the season and can affect the production and life span of the plant. The fungi are usually type specific so if you have mildew on your roses it won’t spread to your cubits (zucchini, cucumber etc).

Many of you would have seen the black and yellow ladybirds on these plants and think they are doing a good job, well I would say think again! These ladybirds are the only species that don’t feed on aphids – what they do is feed on fungi and they carry it from plant to plant under their wings, infecting the new plant.

How to treat it – a recipe for success

Powdery mildew does not thrive on wet leaves; the water actually suppresses the spores, stopping them from multiplying. You can try mixing one part milk and nine parts water and spraying on the leaves. The bacteria in the milk helps to suppress the fungi and also boosts the immunity of the plant. Regularly re-apply to help keep the leaves wet and stop this disease from taking hold.

For roses you can mix half a teaspoon of baking soda to a litre of water and spray on the leaves. Be careful to follow this recipe though, as too much baking soda will burn your leaves.

Top tip

Remember NOT to compost infected plants or leaves as this will keep the fungi spores alive and keep the cycle going. I usually cut off and burn my infected leaves of the zucchini. However, I must say that this year, I have no sign of powdery mildew at this stage. Fingers crossed!

Next week I shall talk about blight – as ever, stay tuned and do keep in touch with your questions!


Happy gardening!


Photo by Pollinator: Wikimedia Commons

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