Spireas are tough shrubs but the foliage will turn yellow for a number of different reasons. They do not like alkaline soil so if the you live in an area of little rainfall you may need to add lots of organic matter including peat moss to lower the pH of the soil.  Nematodes or tiny soil worms can also infect the roots of spirea causing knobs or galls on the roots and yellow leaves on a sickly plant. There is a also a scale insect that appears as bumps on leaves and branches. A heavy infestation of scale can yellow leaves. Too much waster and poor drainage or too little water will also show up as yellow foliage. Feel the soil in the root zone by poking a finger into the soil to check for moisture.  Perhaps you can send a photo of the plant and description of the growing conditions and I can make a better diagnosis of the problem. Do the leaves green up after the shrub blooms? If only the lowest or oldest leaves turn yellow this could be just natural leaf drop. Keep growing, Marianne Binetti