If you’ve been gardening long, you know the white powdery mildew that can affect zinnias. It’s a fungus that likes warmth and humidity. Unfortunately, growing mass quantities of cut flowers require them to be planted close together for various reasons. These include saving space and promoting longer stems. This close spacing creates a perfect breeding ground for this mildew. Because they are annuals that I’ll be throwing out soon, I don’t worry about it. It doesn’t affect other plants; I just remove all of the leaves from those plants and use the flower only. This is another reason to succession plant because usually, the ones I plant right after the last frost are the first to get hit with mildew.
If you have the space and hate the look of powdery mildew, you should plant your zinnias 18” apart.