If your spirea has cankers on the branches, they most likely are infected with Fire Blight. That's the disease that creates cankers.

Fire blight (Erwinia amylovora):  Bacteria oozing from cankers on previously infected spirea branches migrate to healthy tissue via insects or splashing spring rain. They penetrate through open flowers, wind or insect damage or natural entries such as the pores on the foliage's undersides. Fire blight is most active during humid or wet weather when temperatures range between 75 and 85 degrees Fahrenheit during the day and remain above 55 degrees F at night. It blackens and shrivels tender new foliage and open blossoms. Twigs bent to resemble shepherd’s crooks are one telltale sign of infection. On young spirea plants, fire blight may be fatal. Fire blight is a bacterial disease that is spread by insects and rain. It is one of the most destructive diseases of some spireas, blighting young twigs and limbs. Leaves die, hang downward and cling to the blighted twigs. Dying branches, which appear scorched, are conspicuous during the summer and ruin the shrubs' ornamental value.

If the bush is not pruned to remove the unhealthy branches, the entire bush can die. Removal of dead or dying branches will stop fire blight from spreading to the entire bush. Prune the branch to include at least 12 inches of good branch.

Then spray shrubs with a streptomycin antibiotic wettable powder product to protect blossoms. Do not over fertilize, because excess nitrogen makes the shrub more blight-prone. This disease is rarely serious unless the spirea is planted near pears, apples or hawthorns which are also susceptible to
it.