Looks to me like the beginnings of powdery mildew.

What can I do to fix it?

you can buy some horrible poison to spray on it or you can just mix 2 tablespoons of baking soda in quart of water and spray on it ...get one of those small sprayers you use for irioning clothes.....changing the PH of the leaf surface will stop the powdery mildew.....you have to catch it early or it will kill the plant.....re spray every 2 to 3 days or after a rain....

It will generally start on one or two leaves, then spread to other leaves. Hopefully you have many leaves, so cut the infected leaves off and sterelize your cutting instrument when your finished so you won't spread it to other plants. You may also see it on cucumbers, melons, etc. DON'T let the infected leaves come in contact with remaining leaves on the plant.

I just found this forum today and the URL to your pic isn't working for me. Anyway, about powdery mildew, if the baking soda mixture didn't work, try neem oil. Neem is great stuff and you can now find in almost any store that has a gardening section. It's approved for organic gardens. You can soak a plant down with it and not burn it. It kills fungus, mites and insects but isn't a poison.