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Powdery mildew (PM) is a fungal disease which flourishes on courgettes, cucumber, summer squash & pumpkins when the weather is warm and dry. It can reduce the crop and eventually kill the plants.
Infected courgette plants are covered in a dusty, furry-looking greyish-white mould, mainly on the upper surface of the leaves and young shoots.
Treatment
Fungicides have little effect: save your money.
As powdery mildew does not like wet conditions, wetting the leaves reduces the severity of attack. A "wetter", eg detergent, has to be added (just a squirt) ~ plain water will just run off the leaves.
Scientists ... in Russia observed that powdery mildew spores that landed on wheat seedlings sprayed with comfrey tea did not germinate, and the wheat ...did not become infected.
source
Weeds can over-winter the fungus, so good garden hygiene is essential.
Remove and bin or burn infected leaves, don't compost them.
Prevention
Ensure plants have adequate water, especially those in containers, and apply mulches over moist soil if grown in open soil.
Avoid overcrowding plants to give good air circulation and minimise humidity.
Don't feed plants too much nitrogen-rich fertiliser because this encourages lush sappy growth which is more susceptible to infection.
Courgettes Dundoo, Soleil & Tosca have good resistance to powdery mildew, apparently. I might try these next year, as I get PM every s*dding year.
Infected courgette plants are covered in a dusty, furry-looking greyish-white mould, mainly on the upper surface of the leaves and young shoots.
Treatment
Fungicides have little effect: save your money.
As powdery mildew does not like wet conditions, wetting the leaves reduces the severity of attack. A "wetter", eg detergent, has to be added (just a squirt) ~ plain water will just run off the leaves.
Scientists ... in Russia observed that powdery mildew spores that landed on wheat seedlings sprayed with comfrey tea did not germinate, and the wheat ...did not become infected.
source
Weeds can over-winter the fungus, so good garden hygiene is essential.
Remove and bin or burn infected leaves, don't compost them.
Prevention
Ensure plants have adequate water, especially those in containers, and apply mulches over moist soil if grown in open soil.
Avoid overcrowding plants to give good air circulation and minimise humidity.
Don't feed plants too much nitrogen-rich fertiliser because this encourages lush sappy growth which is more susceptible to infection.
Courgettes Dundoo, Soleil & Tosca have good resistance to powdery mildew, apparently. I might try these next year, as I get PM every s*dding year.
I'm new to veggies, but trying ! 
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