How to control powdery mildew on courgettes

If you have ever grown courgettes before then the chances are that you have already experienced a powdery mildew infection. So prone to infection are some cultivars that the plant's entire foliage can turn silver due to a complete covering of the fungus. While to some this may provide an attractive, ornamental effect, it will seriously reduce the courgette plants ability to produce a crop, and left unchecked can eventually cause the death of the plant.

Of course for future crops you can avoid the heartache of watching your plants succumb by planting mildew resistant cultivars. One of the best performing is courgette ‘Soleil’, but also consider courgette 'Tuscany' and 'Defender'.


As mentioned previously the most striking symptom is the typical powdery-white, or off-white coating which spreads from discrete patches to cover the leaves which eventually take on a characteristic tattered and frayed appearance. Both the blooms and the fruits can also be affected. The incidence of powdery mildew is most prevalent from the end of the summer when the cooler night temperatures following warm dry days and dry soil conditions all favour development of this fungus.

Organic control of powdery mildew

Plant courgette plants in a position of full sun and allow plenty of space between plants to help ensure good ventilation. When you consider that each plant will cover at least 1 square metre then leave at least a metre between plants. Powdery mildew spores will overwinter in the soil, ready to re-infect plants the following year, so avoid planting courgettes in the same position year after year.

Keep the soil moist as water stress with make the plant less able to resist or even cope with the fungus. Remove and burn the worst affected leaves and and any infected leaves that have fallen to the ground. Provide a dry mulch such as bark chips or gravel to help keep moisture in the soil and reduce humidity around the base of the plants.

You can consider applying an organic fungicide such as a proprietary potassium bicarbonate spray, or create your own formula mixing 1 teaspoon of baking soda in two pints of water, with a few drops of olive oil or one made from 40% milk and 60% water.

Chemical control of powdery mildew

If you want to do it 'old-school' then apply copper-based or sulphur-based fungicides. Alternatively treat with myclobutanil based products. Alway read manufacturer's recommendation before application.

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