Spirea Die Back
I have a mass of spirea. Last year leaves on one or two plants turned yellow and brown; this spread to the entire plant and then to adjacent plants. Eventually entire plants succumb. This year it has continued spreading. So far, surrounding adjacent spcies, e.g., hosta, turtlehead and a grass, have not been affected.
Without actually seeing the plants, we can only guess at the problem. If it were too wet soil, it would affect all the plants equally. This is what I found on the University of Illinois Extension page and it seems to match the appearance of your shrubs: Bridal Wreath & Vanhoutte Spirea - Spiraea prunifolia & Spiraea x vanhouttei Dieback/cankers - Nectria cinnabarina is a saprophytic fungus that will invade and kill stressed plant tissue. Under the right conditions, the canker will kill the entire plant. In dead areas on the woody tissue during the growing season, spore structures develop. The spore structures vary in color from coral pink to pinkish orange to purplish red. As they age, the color changes to tan, brown and almost to a black like appearance. During summer months and into fall, additional spore structures that are round and orange red in color develop among the other spore structures. These can persist into the winter. Wet weather helps disperse the spores in these structures. The spores infect dead buds and other winter injured plant parts. The spores also infect through pruning wounds. Keep plants healthy by growing the plants in the right location and use good cultural practices in maintaining them.
I would suggest you remove all infected plants. Clean up the area thoroughly including all woody material and leaves. If you have pruned your shrubs, disinfect your pruners in a solution of 1 part bleach to 10 parts water. Do not plant any more spireas in that area for a very long time until you are sure the disease fungus is no longer viable.