Nosema disease of bee is caused by

Nosema disease of bees is caused by Protozoa.
Nosema is a serious disease of adult European honey bees including queen bees. In some years, nosema may cause serious losses of adult bees and colonies in autumn and spring.
The disease is caused by the spore-forming microsporidian, Nosema apis. Spores of this organism can only be seen using a light microscope.