Answer: What you describe sounds like the fungal disease called powdery mildew. The disease can develop under relatively dry conditions in spring and summer so long as the air is humid. Plants in shade tend to have more problems because that environment favors disease development and multiple disease cycles occur during the growing season. to control:
Avoid overcrowding plants.
Use resistant cultivars such as Blue Bird, Cameliard, Galahad, and King Arthur.
Destroy plant debris at the end of the season.
Fungicides will protect your delphinums before symptoms appear, but won't bring your plants back to health. Since this is an ongoing problem, plan on using protective sprays early next season. Safer Garden Fungicide (Ready To Use 0.4% sulfur) thoroughly sprayed over the entire plant should help it avoid powdery mildew.
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