SIngular of fungi is fungus.

The taxonomic kingdom of lower plants; Also called: kingdom Fungi, fungus kingdom.
(Pun) The one who buys the drinks.
Aerobic, multicellular, nonphotosynthetic, heterotrophic microorganisms. The fungi include mushrooms, yeast, molds, and smuts. Most fungi are saprophytes, obtaining their nourishment from dead organic matter. Along with bacteria, fungi are the principal organisms responsible for the decomposition of carbon in the biosphere.

Množina: fungicides

ETYM Fungi + -cide, from Latin caedere to kill.
A chemical agent that destroys fungi.
Any chemical pesticide used to prevent fungus diseases in plants and animals. Inorganic and organic compounds containing sulfur are widely used.
Substance killing fungus.