Answer: There is a species of insect that bores in the stalk of the zinnia flower. The insect may still be inside the stalk -- cut one open and see if you can find it. You can probably salvage your zinnias by cutting back the flower stalks, and letting them grow up again. I believe there is only one generation of these insects produced per summer, so future blossom stalks may not be affected.
The adults lay eggs in fall, and the eggs overwinter on weeds and flower stalks, so prevention requires fall cleanup around the flowerbed. Remove the debris from the zinnias and surrounding growth, and compost it far from the garden. Sorry I can't give you more info than this -- best of luck!
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