[Carlh80]

My courgette plants seem to have a mildew or fungal issue.
I’ve researched apparently using baking powder solution seems to be a non chemical solution.

Has anyone tried this? Is it effective?

[Yorkie]

Are you sure it's mildew?  Is the silvery colour powdery when you rub it?  Many courgette leaves look like that

[jambop]

That leaf looks healthy enough! Some courgettes have a silvering in the leaf colour I think Yorkie is correct.

[Carlh80]

Sorry just seen the replies, no it’s not powdery to be honest. Good to know and thanks for both your advice saved me some wasted time.

[Hampshire Hog]

The other possibility could be damage from the seriously sunny and dry weather we have had. Perhaps make sure you have provided enough moisture for the plant as they don’t like a lack of water.