Welcome to the National Gardening Association forums.
I think the zinnias in your photos are too far gone to save. They most probably are infected with two different fungus diseases
, Alternaria Blight and Cercosperia Leaf Spot
. The common Powdery Mildew of zinnias is not strongly indicated in your photos.
There could be some Bacterial Leaf Spot present, but nothing positive for that appears in your photos. Since there is no "cure" for Bacterial Leaf Spot (other than prevention by sanitary practices) let's assume you have "just" the two fungal diseases on your zinnias.
You can prevent zinnia fungal diseases by spraying them with a good fungicide. Prevention is a better strategy than cure. I prefer to use a systemic version of fungicide, which is absorbed into the plant sap, travels throughout the plant, and protects for several weeks from one application. But frequent applications of a non-systemic (like Green Cure, which is based on Potassium bicarbonate and a good wetting agent) can also do the trick. You just need to apply it more frequently.
But, like I said, these zinnias in the photos are too far gone to save. It is interesting that there is a black "mulch" or "compost" material present in your photos. I frequently see that black stuff in photos of zinnia plant disease problems. It is a frequent source of plant disease infections. In your first photo the black stuff appears to be heaped up around the plant, partially burying the plant. Why was that done?