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Your zinnias are planted too close together. This blocks sunlight from reaching the entire plant and it reduces air circulation. Both make disease more common. From the picture, the lower leaves are mostly gone because light could not reach them. That reduces photosynthesis, the plants source of energy. The result is they grow rapidly but weakly trying to reach more sun. I would take pruners and snip at ground level at least half of your plants. If you have the seed pack or know the cultivar, you can look up how far apart (the spacing) is recommended.

Finding ants on your plants can mean you have an infestation of another type of pest. The pest residue, called honeydew, will attract ants.
Treat the plants with Neem Oil.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

The plant looks healthy overall. Bottom leaves that get shaded as the plant grows often turn yellow, then brown. They are "senescing", a nice term for getting old. Once a green leaf has turned yellow, it is no longer capable of photosynthesis. I remove them for aesthetic reasons. Fighting off diseases and bugs is an on-going battle for commercial growers. Removing off-colored foliage reduces the chance of infecting more leaves. Keep annuals healthy by watering regularly and giving them fertilizer. Don't crowd zinnias; they need good air circulation since they are prone to powdery mildew in hot, dry weather.

Yes, this looks like powdery mildew. Zinnias and melons/pumpkin are notorious for powdery mildew susceptibility. With zinnias, it is important to not crowd the plants; they need space in between plants for air to circulate. This lessens the risk of disease and makes it less likely to jump from one plant to another. Watering from above puts plants at risk so drip irrigation or soaker hoses are ideal. Keep moisture levels consistent. At this point, remove severely affected leaves. At the end of the season, remove all plant debris as PM can overwinter in the soil. Rotate crops so those not as susceptible to PM are put in where infection was a problem the previous year. The same is true for zinnias. Here is a good article: http://ipm.ucanr.edu/PMG/PESTNOTES/pn7406.html

The drying leaves are likely from uneven watering or uneven sunlight conditions.
Zinnia will need 6 plus hours on direct sunlight and you may need to water twice a day in temperatures above 80 degrees F.
https://www.gardeningknowhow.com/special/containers/container-plant-watering.htm