Which microbe causes hay fever?

None. It is due to an allergic person inhaling pollen and sometimes mold spores. Causing nasal congestion sneezing etc.
Pollen. Pollen are microscopic "seeds" essentially, that help plants expand and grow in new areas. They are designed to float in the air and travel to these new areas. When inhaled, the pollen can can cause typical allergy symptoms.
None. Hay fever is not an infection, its an allergy to pollens. Microbes don't cause hay fever. Http://patient. Info/health/hay-fever-leaflet.