What is causing parts of my spirea to twist and turn silvery?

This is powdery mildew. Powdery mildew likes to grow in warm moist conditions. It has a silvery appearance and also causes the affected branches to twist. University of Memphis Director of Landscape Joellen Dimond suggests waiting until after the plant flowers in the spring, then pruning out about a third of the branches at ground level. This will allow for more air circulation which will help prevent the powdery mildew.
​
Related Videos:
Spirea
Powdery Mildew on Coneflower

Related Resources:
Powdery Mildew [Shrubs and Trees]
Powdery Mildew