Experts and BHG readers answer.
Strange Zinnias and Plant Withering
It sounds as if your zinnias were infected with a virus of some kind. These are often spread by aphids and other sucking insects. If you bought the plants as youngsters, they were probably infected before you bought them. You would be wise to destroy all the affected plant material rather than putting it on your compost pile. Also gather up and destroy any leaves that have droped. Next year fill the space with another kind of plant and put your zinnias elsewhere, just to be on the safe side.