Spirea is susceptible to verticillium wilt, but my first guess on this plant is a powdery mildew or other fungal problem. Would you post photos of one of the other plants, to show the overall pattern of leaf damage? Also, pick a couple of leaves and do a closeup photo of each side.

Do you have a magnifying glass to examine a leaf, or even a camera/phone that will take a photo really close?

Here’s the UC IPM page for Spirea, to share a resource I’m using.

I'm sorry to keep asking more. Perhaps you feel we are playing a game of "20-questions"? There are fungal issues that cause leaf spots, especially with the warm wet weather. The dry April has also been a problem for some plants. There are normal stress responses to transplanting too, and I'm not sure which issue(s) you are dealing with.

Would you share a photo from a distance showing the whole planting?

Describe how you planted these, and what you found when you pulled those you replaced. How did the roots look?

Rather than tilling up pathogens, I'm wondering about the soil structure being an issue, as water doesn't move well between soils of different textures. This means an area with tilled-in amendments might have layers of exceptionally dry or water-logged soil.

How did you treat the root balls of your new plants prior to planting? Did you examine them for circling roots and correct any you found? Were they planted at the natural root flare-depth, or possibly too deeply?

Woody plants can take a year to establish in a landscape, so possibly you'll need to wait and see what happens. You have so many plants, though, that I'd like to offer more of a solution for you. After a additional information from you, I may refer this to another expert.

Consulting with another expert, there’s a chance these are stressed from our dry spring, and watering well on a slope can be a challenge. Check the root-balls with your fingers. Is moisture moving into the base of these plants? Too wet is also a problem, as roots can essentially suffocate in poor drainage.

Oh, and consider shade cloth for these super hot weeks while plants are establishing. It shouldn’t touch the plants, so a support is best. Folks set an old deck chair, or drape a tomato cage over plants. Amount of wind in your location is something to consider when being creative with shade cloth.