What causes purple brood (honey bee larva that looks blue or purple, instead of white)?

The plant Cyrilla racemiflora, most often called southern leatherwood or summer titi, can cause larvae to turn purple when nectar or honey made from titi is feed to larvae. In 1932, Burnside discovered titi to be the cause of poisonings of half the bee colonies in Florida that year. He named the malady, “purple brood”, because affected larvae appeared blue or purple. Burnside reported that purple brood comes on suddenly and all the colonies in the affected area develop symptoms at the same time. Purple brood has been found throughout the southeast, linked to the presence of summer titi and is still considered a serious problem periodically in Florida, Georgia, Mississippi and South Carolina. At least a few purple larvae have been observed in honey bee colonies outside of the known range of the titi plant.
-John Skinner, University of Tennessee