This sounds like a condition with the root of too much water. With all the rain we have had it's not surprising. Also, zinnias love lots of sun, so if they aren't getting enough that might well contribute to your problems.
I have had success with this situation by mixing cornmeal into the soil (if it is already moist, don't bother watering more) and then a good compost tea foliar spray with garlic. That cured the problem completely within 5 days.
Perhaps this will work for you. It's quick and simple.
Kathe