I have several netted melon plants which are in my greenhouse in somerset . They have been growing nicely and we have several grapefruit sized melons on them .
Watering has been consistent and ventilation good & temperatures normal for U.k summer , all was well but over the last week the leaves have started to go as below :-

The leaves start to go brown around the edges first then spreads to the whole leaf , veins and stem & fruits seem unaffected .Some of the fruits have started to go prematurely yellow as well ,as if the plant can't provide for them anymore.
I'm wondering if its downy mildew , but not sure . Either way could you give me your opinion on what you believe it to be and any possible cure .

Hm, I think if it was nutrient deficiency the leaves would have turned pale or yellow first. This looks like something either fungal or viral to me. Possibly a mosaic virus.

Your best treatment would be to remove all affected leaves right away, bag and destroy, don't compost them! If it is something fungal, this might save enough leaves to let your melons mature. If it's viral there's really not much you can do except destroy the plants.

Elaine

"Success is stumbling from failure to failure with no loss of enthusiasm." –Winston Churchill

This is for tomatoes, but I imagine it extrapolates.. Nitrogen cause that yellowing, but there are other things that can be lacking. Magnesium.. I think.. is one that the veins stay green but the rest of the leaf pales.. Potassium and Zinc are two others. Either way, it isn't usually too hard to test the soil and see if that is an issue just to rule it out.