Hi Karen and Welcome to ATP, that sure is a beautiful display of Coleus you've got going. Housekeeping is always the first line of defense for fungus, if you can keep removing every leaf you see infected, as quickly as possible, that will slow the spread, at least. (don't compost them, put in a plastic bag and throw away in the trash)
We've always used a mild baking soda solution to prevent the spread of mildew-type diseases. It works but must be sprayed often since the baking soda is very soluble and if you use sprinklers, it will be rinsed off each time you water, as well as whenever it rains. It works by altering the pH on the leaf surface making it inhospitable to the fungus.
That being said, I also grow orchids outdoors in Florida where we have every fungus known to man floating around in the air. I've "fought the good fight" for 5 years now, to try to use only mild, organic means like Neem Oil, but recently gave up and bought an excellent fungicide that prevents a wide range of fungal attacks. The active ingredient is Thiophanate methyl, the brand I got is Southern Ag's Thiomyl and it is a generic (less than half the price) than the widely used Cleary's 3336 and available on Amazon. It's a systemic so only needs to be used every 6 weeks or so and is rated for use on fruit trees, so the most non-toxic one I could find.
I'm sure you have excellent watering habits, but it's worth saying that watering first thing in the morning is another strong preventative measure for fungus - the leaves dry off during the day and stay dry at night. Never, ever water at night so moisture sits on the leaves through the cool, dark hours. It's an open invitation.
