First, check under the leaves for sucking insects. There are two diseases which can damage zinnias. Both occur mostly when the plants are very stressed. Too bad you had already removed most of the affected leaves, as that makes it hard to say which leaf spot this might be.

Thank you very much! I have been checking for insects and have not been seeing many. I do not see sucking insects under the leaves, although some other nearby plants do appear to have small amounts of leaf miner and thrip damage. I have seen aphids in the garden on dahlias and roses but not on the zinnias. Would more photos help? Do you need photos of underneath the leaves? I can certainly get some more pictures as more leaves become affected. 

Feel free to send some photos of serious leaf damage. Glad you are not dealing with sucking insects.



Now some plants my garden about 70 yards away from the other one are having a different leaf issue. One potted dahlia and one potted zinnia had some damage show up rapidly (within the last couple days). They are right next to each other. Both have tiny black spots on the leaves and also some light spots. The zinnia has tiny dark spots and then the edges of the lower leaves are dying. The dahlia has tiny dark spots and also little light spots. I do not see any aphids or mites but in the past have seen aphids on these plants. Is this something neem oil would help with? Other nearby plants seem unaffected but is there a way to prevent issues on them?

Thanks for the plethora of photos. The white spots are definitely from a sucking insect, aphids, mites, etc.  The spots in the blocks are anthracnose. The blocks tend to dry out and heat up more quickly, so it may be stress related. Use of any oil in the coming heat wave will burn the plants. The blocks have limited root space, so they need to be monitored for moisture levels. All our gardens are struggling some with the weather fluctuations.