Hi Steve, The white substance is powdery mildew, a common ailment of Euonymus shrubs. The cool, wet spring weather has been favorable for this disease this year -- we have see a lot of it on these plants in particular. Given the degree of this infestation, and since this is a recurring problem, it may be easier to remove the plants and replace them with an alternative shrub. Or, you can prune them and treat the new growth to protect them from powdery mildew. Repeated treatments with horticultural oil or other fungicides (check the label for mention of powdery mildew) would be needed.

Spraying with a fungicide will not do anything for the leaves that are already infected. A fungicide can prevent powdery mildew but will not cure it. Here is further information about powdery mildew and how to manage it.
https://extension.umd.edu/hgic/topics/powdery-mildew-trees-and-shrubs

Christa