Zinnias are susceptible to  several leaf spot diseases and it looks like yours have Alternaria  leaf spot disease. This is an infection encouraged by overhead watering and poor air circulation. Try not to wet the foliage when you water and let the plants dry out a bit next summer. This fall you can just uproot the plants and turn the soil to bury any disease spores. You may want to plant a different type of flower in the area next year until the fungal spores die out. The problem could also be a bacterial leaf spot but as this is the end of the summer season the cure if the same. Uproot the plants, loosen and turn the soil and keep the foiliage of these sun lovers as dry as possible. Keep growing, Marianne Binetti