I would spray them with a mixture of baking soda and organic dish soap--about 2 ml of soda, 4 ml of soap, in 1 l of water. Let stand a few hours, shaking occasionally, before spraying. Spray weekly through mildew season. Potassium bicarbonate is better for the plants, but it may be harder to find.

I get this most years and it does not affect the fruit I have found. If I had the recipe for the spray from Tugbretil I would have used though it as it is very unsightly isn't it!

It's the time of year when your courgettes are getting older and the nights are already cooler. Some of my early ones are just the same now, but I have had a great crop from them. Still time to grow some in your greenhouse.

When mine get the mildew, the effects on the crop aren't drastic, but a bad case can make the plants unthrifty, and slow to bear.