I've never known a cosmo to have any real blight problems. But, when you water, do you wet the foliage? Best to water cosmos (and most plants) at ground level to avoid the powdery mildews, etc. that can arise with wet foliage and possibly poor air circulation in crowded gardens. If it's a wet situation, let them dry out and see if the cosmos, which are pretty vigorous growers, grow out of it.

Actually, in the right kind of environment, cosmos can be troubled by powdery mildew, which is a common garden fungal disease.

I water only onto the ground but i sprayed with a deer spray. Im wondering if that could be the culprit.

Should I remove them from the garden?

Possible, but not likely if it was Liquid Fence or something similar. Do you think it might be the spray residue especially if you sprayed heavily? I notice white residue on my lily leaves many days after using Liquid Fence.

Google images for powdery mildew - lots of pictures of it on different plants and leaves. That might help you determine if that's what you have on your cosmos. If so, I'd remove (or treat with a fungicide if you think they're worth going that route) since it can travel to other things. Powdery mildew is seldom fatal, but it'll speed up defoliation and will definitely weaken plants.

I've never heard of it on cosmos.

I used the deer off which does have an oil in it I think. Im just wondering if it will spread to other species?
Ill google for pics thanks. any other info is welcome.