Your soil looks like it needs a ton of help. No soil should be bare like yours. You need organic mulch which will feed the soil and make it thrive with biological activity. This alone could help your plants better fend off the fungus, your plant's health is directly related to your soil's health, and your soil's health right now is severely lacking.

Bandaid solutions like fungicide should be put off until you have a functional soil system.

And yes, you're right, it might not even be fungus.

The person that suggested the cinnamon probably assumed you had Powdery Mildew, a common problem with Zinnias. If your plants have a light, whitish, hazy substance on the leaves they may well have Powdery Mildew. Other methods of control include a 50/50 mixture of fat free milk and water or mizxing 1 teaspoon of Baking Soda in 1 quart of water and spraying those on the plants, preferrably early in the day before the butterflies start to flit arouns.
While your soil does need attention PM on Zinnias can happen even on plants growing in a good, healthy soil although will less trouble.

You should have a constant layer of organic mulch on your soil, without it your soil is suffering

Powdery mildew seldom develops into brown, crispy foliage. Without seeing to confirm, I'd speculate the issue may be rust - likely one of the most difficult fungal diseases to control. It does not respond well to most fungicides, whether natural/organic or more chemically heavy duty.

If that is what the issue is, your best solution is to try to avoid the problem to begin with. A lot of modern hybrid zinnias tend to be resistant. And make sure you do a thorough clean-up this fall. Rotating your planting or plant in another area. Avoid any overhead watering. FWIW, gribble has th right idea - an organic mulch is an excellent thing to add to any planting area and especially one prone to disease issues. In most cases they will smother fungal spores or otherwise discourage their growth if not generating beneficial microorganisms that will devour any pathogens outright.

My Zinnias got those crispy leaves at the bottom last year, but the plants were so tall and bushy that you couldn't even see the bottom of the plants, and also I had Melampodium planted in front, so it wasn't noticeable.

I do plant the tall Zinnias in a new spot every year. The one year I didn't, they got terrible powdery mildew (could be a coincidence, or maybe not!).

Mulches are wonderful, except that I don't mulch areas where seeds are sowed or seedlings are planted until the plants get about 1 foot tall or so. Slugs are probably the number one hazard to my seedlings - and the mulch encourages the slugs.

I would like to hear the about the butterfly friendly fungicide. My tall Zinnias' do have the fungus problem. Not as much as last year though. My soil is organic (mainly composted cow manure with some blood meal mixed) and organic mulch. I read that they don't like to be wet or they can get the fungus. It's hard for me to prevent it because they are in my front yard where my sprinkler system saturates them while watering the yard. It's better this year because I set the sprinklers to go off at 9 AM so that the sun can immediately dry them. This has helped but still have some fungus. Do you have a home remedy or butterfly friendly fungicide?

What kind of fungus?? It DOES make difference :-)

If just powdery mildew, there are various home remedies that will stop the spread of the disease and not bother butterflies, pollinators or any other beneficials. Milk spray, bicarbonate of soda or Neem oil will all work.

btw, powdery mildew is NOT caused by water on the foliage. In fact, water will often wash off the PM spores. PM is usually caused by excess humidity and a differential between that and soil moisture (deficient). With warm summer temperatures, that offers an ideal breeding ground for the pathogen. It is also encouraged by insufficient air circulation or crowding and not enough sunlight.

Looks like alternaria, which is a fungus. I have about 100 zinneas in a bed with three rows. When the plants start getting tall, I pull off the bottom leaves so air can circulate better. You should also remove infected leaves so it doesn't spread as fast. I spray Serenade as a preventative. It works well for powdery mildew. Maybe it slows down the alternaria but it isn't curing it.