The image doesn't reveal much. It doesn't look like powdery mildew, but if it's a new planting it could well be the result of what we commonly call sunburn. Light levels well above what the plant is currently conditioned to tolerate can cause a chemical reaction in the leaves which results in creation of an oxidase you're likely familiar with - hydrogen peroxide. Like bleach, hydrogen peroxide is a very strong oxidizer and will attack (and "bleach) the first organic molecule it comes in contact with. Since the reaction that causes sunburn occurs in the layer of cells where the plant's photosynthesizing machinery is located, the green pigment, chlorophyll, becomes the direct target of the free oxygen molecules in hydrogen peroxide. The more scientific name for sunburn is photo-oxidation.
Snipping off the spent blooms will free up food/energy that can be utilized by the plant to increase the number of roots, then leaves and branches.
Al
