Thanks for your question about your zinnia leaves.  This falls into the "if you always do what you've always done, you'll always get what you've always gotten" category!  Soils and what's on them carry--from year to year--pathogens ('things' that do damage) and, if you continue to plant susceptible plants in the same soil year after year, the same pathogen(s) will infect them consistently.   Were the problem an insect, most likely the damage would be seen on the outer edges of the leaf, where they (beetles, slugs, etc.) chewed.  You may have that as well, but I can't see their edges well enough in your photos to diagnose.

Zinnias are susceptible to a disease called bacterial leaf spot.  Here's a link to an OSU article on the subject, with photos of the leaves before the centers drop out of the infected tissue.   At the end of this season, remove ALL plant tissue from these plants, and dispose of them in a method other than composting.  Next year, plant something you like from the list contained in this Purdue Extension article.  And, in the future, as a general rule, rotate your plants so you never plant the same ones in the same soil.

Hope this is helpful.  Good luck!

Thanks Kristena! Lesson learned -- I definitely won't plant zinnias in this spot next year. I'd still like understand the problem. After checking the link on leaf spot you included and my zinnias, I didn't see any brown/yellow spots, just holes. Attached are 2 closeups of affected zinnia leaves. Some holes are on leaf edges, while others are in the centers of the leaves. Does this point toward any cause more conclusively? And question #2: Is there any organic way to eradicate pathogens from the soil between seasons? Appreciate your help!

The reason that you don't see the yellow and brown spots is that after the leaf tissue is necrotic (dead), it drops out, leaving the holes. Your plants' leaves show several stages of disease progression, meaning that it has been ongoing for weeks or months. I think the outside notching is probably caused by slugs, who adore zinnias. You can hand-pick them off (if you're awake when they're out), apply diatomaceous earth (which tears up their bellies as they crawl over it), or make shallow beer traps. There is no organic 'cure' for these pathogens--at any time of the year. The soil is teeming with microbes--a billion bacteria in one gram! The only solution is to keep the susceptible plants away from the ones destructive to that species. Good luck!