In agriculture, a "bumper crop" refers to a particularly productive harvest. if there is a bumper crop for wheat at the same time that more people become allergic to wheat and all else is held constant, what will happen to the equilibrium price and quantity for wheat?

The equilibrium price will go down and equilibrium quantity will be indeterminate.
Bumper crop refers to a situation when a certain type of crop exeprience sudden bump in productivity. If at that exact time more people become allergic to this crop, the crop would be overly stocked in the warehouse and the owner would most likely sell it at lower price.