Hello Alchemilla, thanks for your post and I am sorry to hear about your melons.
If some older leaves survive they may give good fruit.
Downy mildew on cucumbers, squash etc is normal in late summer, but may be reduced by watering plants more. It should not kill your plants because they still have healthy new leaves. Its a ‘one leaf at a time’ infection, not in the plant sap and not at all like, say, potato blight
