Yikes! Mildew on the new growth of a mature shrub is most likely from cool damp weather or the plant dried out  or  poor air circulation.   I also had this happen to one of my older spiraea Magic Carpet  and I was guilty of having this shrub in a pot for too long and the roots dried out.   The good news is you can prune back the infected part, water the roots and new growth should appear that is clean and mildew free. You need to investigate to see why this normally hardy shrub fell prey to mildew. If you did not allow the plant to dry out it could be weak due to root damage from burrowing rodents. Your shrub should be able to grow new roots around the area that was damaged so give it some time and extra attention this summer. Keep growing, Marianne Binetti