Answer: I'm afraid I can only speculate about the cause. Gaillardia are said to prefer an evenly moist, well-drained loam soil in full sun, and coneflower, even better drainage. If your soil contains clay, it may retain too much moisture for these plants. My guess is that the vigorous growth in spring can make use of the moisture, but by the end of summer, when growth slows, and humidity is high, the stage is set for a fungal disease to form and rot the leaves near the crown. If this is the case, move the plants to soil that drains better, and make sure there's plenty of air circulation around the plants. If this doesn't fit your situation, then you may need to contact a plant pathology specialist at your county extension service (ph# 612/385-3100). Best of luck!
Q&A Library Searching Tips