The symptoms you are seeing on the leaves might be an indicator of too much water. Plants in the sunflower family are susceptible to several types of leaf spot diseases, but we don't think this looks like a disease symptom. These are annual plants that start to decline naturally after mid-summer. The only thing we would recommend for next year would be to make sure they do not get too much water. Keep them in a well-draining pot (or plant them in the ground).

ckc