Certified GKH Gardening Expert
This appears to be a severe fungal infection. You can try to amend the area with dolomitic lime and wettable sulfur, but you might not see any results until next year. It will be best to plant them in a different place next time, until the soil has time to cure with this treatment.
This article will give you more information on the care of these plants: https://www.gardeningknowhow.com/ornamental/flowers/sunflower/growing-sunflowers-in-your-garden.htm