Powdery Mildew

Spray with milk and water solution, 1 part milk to 5 parts water

Milk is good. Skim milk at 1 part of milk to 9 parts water is what I have been told. If the fat content is too high other moulds may start growing on the leaves and it can smell a bit.

The other thing I have heard of is baking powder (1 teaspoon per litre).

Commercial wettable sulphur from your garden centre will halt the spread but will not kill the mildew.

If you find a systemic fungicide that can be used on squashes/courgettes read the label carefully as they all have a time interval between application and eating. Personally I would not use one.

Mildew likes dry conditions so anything that makes the leaf surface so that any water droplets spread out helps. Water with soap or detergent should do some good.   

i agree with salmo about the skimmed milk and the mixture . i would burn the leaves because you will only be putting the spores in the compost