Good news. From the photo you sent it looks like only the oldest or most mature zinnias are turning brown and that is natural as each bloom finishes up its life cycle and fades to make seeds. This is the time of year when you need to dead head or clip off all fading or brown blossoms to keep annuals like your zinnias from going to seed. Then the plant will keep producing new flowers until a hard frost kills the plants. The brown flowers only appear later in the summer as it takes time for the first blooms to mature and turn brown. Keep growing, Marianne Binetti