HI Rosemary, perhaps this article will help you identify what the problem is with your sunflowers - https://homeguides.sfgate.com/brown-spots-newly-planted-sunflower-leaves-29776.html
Sunflowers are a moisture-loving plant. Without sufficient water, new plants are susceptible to fungal wilting. Fungi living in the soil attack sunflowers, moving upward and causing dark brown spots on the leaves and stem of the plant. New plants also appear wilted and dry.
IT is now the end of the season (fall), they naturally die off. They are annuals. You have to replant next spring.