Zinnias can be vulnerable to several types of leaf spot infections, both bacterial and fungal (like Alternaria and Cercospora); these symptoms look fungal, and there is also a comparatively minor amount of powdery mildew. No fungicide can cure existing disease, and while it helps to protect healthy growth from infection, it's not practical to use this late in the growing season, especially if there is little to no uninfected foliage left at this point. Additionally, fungicide residues could risk harming pollinators or other organisms, so for plants as popular with pollinators as zinnia, we don't generally recommend their use.

If you are a cut flower business, you can work with your county Extension agent to determine if a fungicide is warranted, though if the leaves are removed upon harvesting the stems, then it probably would not be.

If you are a home gardener, are you irrigating the flower bed when the weather stays dry? If so, does the hose or sprinkler water get the zinnia leaves wet? Leaf surfaces that are wet can be easier for disease spores to infect, so one method to reduce the risk of infection is to only water the soil, or water early enough in the day so that the leaves can dry before nightfall.

If you replant zinnias next year, look for cultivars that say they have above-average disease resistance, as some are more vulnerable to infection than others. When these plants are done for the season, remove all debris and dispose of it (don't compost) so infectious spores are not harbored into next year by old fallen leaves or plant stems.

Miri