This disease isn't coming from infected seeds, it is airborne, and probably on the surface of your soil, or on other plants in your garden. You need to spray something on the leaves to prevent it.
I'm using a biologic fungicide called Actinovate. I've used it for years on my orchids, but it is an organic-approved solution for this type of disease on edibles, too. I get it off amazon. I'm in Sarasota, too, my Malabar spinach so far is completely clean and I haven't had to spray it yet. But yes, the leaves are fine to eat if you just remove the spotted parts.