It’s actually any dust and/or spores and/or pollen sufficiently high in the atmosphere. In addition, if a cosmic ray strikes the dust/spores/pollen, they pick up a charge that attracts water vapor and help form clouds/rain. Hence, when the sun’s magnetosphere is quiet (see Maunder Minimum and current solar activity), we get more rain, cooling, and NASTY winters.
PS That is a very short summary of the interactions involved. It’s a bit more complicated, but the underlying principles are known. “Scientists” pushing AGW most likely (a) are ignorant of these interactions or (b) are likely ignoring them to keep the grant money flowing.
Dust has been forever, but if mushroom spoors are an effective enhancer, then they should try some of the same stuff they did with Silver Iodide. Try seeding some potential rain clouds with spoors and see what happens.