Powdery mildew on courgettes: identifying and treating white mould


With a passion for growing installed at an early age, I have always been happiest outdoors in nature. After training as a professional gardener and horticultural therapist, I currently run horticultural therapy and community kitchen gardens in the UK, helping others access the many physical and mental health benefits of growing vegetables, fruit and plants.

Favourite fruit: apples and pears
Favourite vegetable: asparagus

One of the most common problems home-growers face when growing courgettes is powdery mildew. Read on to learn more and how to identify and treat powdery mildew on courgettes.

Courgette on plant with mildew
Powdery mildew can affect a number of plants, including courgettes. [Photo: kanphotoSS /Shutterstock.com]

Courgettes (Cucurbita pepo), also known as zucchini, along with many other types of squash (Cucurbita), are sadly susceptible to the fungal diseases of powdery and downy mildew. With limited treatment options available to the home gardener, prevention is all the more important.

Recognising powdery & downy mildew on courgettes

Powdery and downy mildews on courgette plants are thankfully easy to identify and distinguish between. Powdery mildew, also known as white mould, is most often caused in warm and dry weather by the airborne spores of Erysiphe cichoracearum and Sphaerotheca fuliginea fungi. These fungi cause a white powdery coating to grow and spread all over the plant, especially on top of the leaves.

Powdery mildew on courgette leaf
Powdery mildew is identifiable by its white dusting on affected plants [Photo: Paul Maguire/ Shutterstock.com]

Downy mildew is less common and differs in that it is most prevalent in cool and damp conditions when the foliage is unable to dry out. Caused by the Pseudoperonospora cubensis pathogen, symptoms of downy mildew begin with small yellow and brown mosaic-like patches developing on the leaves. In turn, these lead to the grey “downy” substance forming on the underside of the foliage.

Courgette plant with downy mildew
Downy mildew can be first identified by its clearly defined marks on the foliage [Photo: Grandpa/ Shutterstock.com]

Controlling powdery mildew on courgettes

Most common during hot and dry spells over the summer months, powdery mildew starts as small white areas before quickly spreading all over the foliage. To avoid this, check your courgette plants regularly and if spotted, remove any leaves infected with powdery mildew and dispose of. Do not add infected leaves to your home composting systems, as the spores can potentially overwinter and contaminate the compost. At the end of the growing season, the same goes for any infected plants; remove all plants and dispose of in your household waste.

Thankfully, powdery mildew on courgette leaves does not make the fruits toxic, although care must be taken as it can cause allergic reactions in some people. When harvesting courgettes, make sure to wash them thoroughly and if any fruits appear rotten or smell, do not eat them.

As well as being infected with diseases, courgettes can also succumb to mutations and crossbreeding, which can lead to the fruit tasting bitter, signifying that they are poisonous and should not be eaten under any circumstance.

Spraying courgette with powdery mildew
Along with chemical controls, home remedies can be helpful in the fight against powdery mildew [Photo: FrankHH/ Shutterstock.com]

Home remedies for powdery mildew on courgettes

Since powdery mildew spreads on courgettes so quickly, staying vigilant and acting swiftly is essential to try and prevent it from affecting the whole plant, as well as any others nearby. At the first sign of any white spots on courgette leaves, home remedies, including bicarbonate of soda and milk, can be tried before resorting to fungicides.

Diluted milk sprays have been proven effective against powdery mildew in several small-scale studies, but strangely enough, the jury is still out on why it seems to work. Similarly, bicarbonate of soda, commonly used in baking, acts as a fungicide against powdery mildew. It is said to be more effective when combined with liquid soap and water.

Combat powdery mildew with chemical controls

If powdery mildew has advanced on your courgette plants so that home remedies prove ineffective, then it may be necessary to move on to a fungicide approved for use on edible plants. Here in the UK, tebuconazole with trifloxystrobin is approved and available for the home gardener to use on courgettes with powdery mildew. Currently, there are no fungicides approved for home-growers for the treatment of downy mildew, making prevention even more essential.

Courgette leaf with powdery mildew
Once powdery mildew has advanced, home remedies may prove ineffective [Photo: Tomasz Klejdysz/ Shutterstock.com]

Preventing powdery mildew on courgettes

Since treatment for powdery mildew on courgettes is limited and for downy mildew even more so, prevention is key.

Some recently bred courgette cultivars have shown to have better resistance to mildew and may prove effective if your courgettes have been affected by mildew in previous years. For more information on these disease resistant cultivars, read our article on courgette varieties.

Methods to help prevent courgette mildew include increasing air circulation around plants, keeping the soil moist through regular watering while avoiding wetting the foliage, and using thick-layer mulches.

Courgette plant with fruits
Correct plant spacing can help prevent mildews from spreading between courgette plants [Photo: Natalia van D/ Shutterstock.com]

Promoting healthy and robust plants can help prevent mildew, as stressed plants are more susceptible to being affected. Fertilising courgette plants will help ensure they receive enough nutrients to encourage strong and healthy growth and subsequent bumper harvest. Our Plantura Liquid Tomato Food is ideal for feeding courgettes and can easily be applied at the time of watering. More information on this and other tips can be read in our article on courgette plant care.

Liquid Tomato Food, 800ml
Liquid Tomato Food, 800ml
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  • Perfect for tomatoes & other vegetables
  • Liquid fertiliser for healthy plant growth & an abundant harvest
  • Quick & easy application - child & pet friendly

It can be tempting to squeeze in as many courgette plants at home as you can. However, this may lead to overcrowding, making it easier for mildew spores to spread. Read our article on planting courgettes for more advice on the recommended courgette spacing and more.