Karyogamy is the merging of two nuclei during sexual reproduction.
Plasmogamy is the sexual reproductive phase of fungus.
During this phase, the protoplasm of two-parent cells fuses, and the process continues without the fusion of nuclei, eventually resulting in two haploid nuclei in the same cell.
Karyogamy does not immediately follow plasmogamy in ascomycetes and basidiomycetes, resulting in the creation of the dikaryotic stage.
In other fungi, plasmogamy is followed by karyogamy.