Powdery mildew is a disease that usually inflicts curcubits - like pumpkin, zukes etc, and sometimes tomatoes... I know that both EM bugs and milk/ water mix work and yellow ladybirds eat it so are also good. I have read that a bicarb soda recipe also works but haven't tried it yet.
Often it is made worse by watering of the leaves which increases the humidity, and it is recommended to water the roots only and early in the morning or late in the afternoon - unfortunately the rain isn't quite so accommodating! Another thing is to ensure that plants are well ventilated and have enough room so that the micro climate isn't so humid (I am so bad at this personally, whenever I look at my gardens it is either feast or famine... one day I will get the spacing right - but it hasn't happened yet!)
This is one of the main reasons I am trying different veggies to see what grows better as an alternative to ones that are constantly affected by our hot humid summers and suffer from a number of problems. An alternative to zukes is the angled luffa which doesn't seem to be affected but can be used in the same way when cooking.
As far as I know it is spread by aphids and other sap sucking pests and could even be wind borne...
One question I have is can I compost the affected leaves or should I throw them in the bin and why?