There Is A Fungus Among Us.

Sweden
Mushrooms
August 20, 2015
The stamp set depicts Cantharellus cibarius and Coprinus comatus.

Poland
Mushrooms in Polish Forests
August 31, 2012
The stamp set with labels feature Russula virescens, Amanita phalloides, Morchella esculenta, Gyromitra esculenta, Macrolepiota procera, Amanita pantherina, Armillaria ostoyae, Hypholoma fasciculare.

Gibraltar
Mushrooms of Gibraltar
September 15, 2003
The stamp set features Lepista Nuda, Clitocybe odara, Hypholoma fasciculare, Agaricus campestris.

Georgia
Mushrooms
December 15, 2000
The stamp set features Cantharellus cibarius, Agaricus campestris, Armillariella mella, Russula adusta, Cortianarus violaceus.

Cuba
Mushrooms
January 10, 1989
The stamp set features Pleurotus levis. Pleurotus floridanus, Amanita caesarea, Lentinus cubensis, and Pleurotus ostreatus.

Moldova
Edible Mushrooms
April 23, 2007
The stamp set features Steppe Morel (Morchella steppicola), Gold leaf (Phylloporus rhodoxanthus), Hedgehog Amanita (Amanita solitaria), Black-hued Boletus (Boletus aereus).

Bosnia and Herzegovina
Mushrooms
July 30, 1998
The stamp set depicts Morchella esculenta, Cantharellus cibarius, Boletus edulis, Amanita caesarea.

Dominican Republic
Mushrooms
September 13, 2001
The stamp set features Pycnoporus sanguineus, Morchella elata, Mycena epipterygia, Coriolopsis polyzona.

Saint Pierre and Miquelon
Fauna and Flora
November 8, 2018
The stamp depicts Fairy ring mushroom (Marasmius oreades).

This 43 p British "Millennium Series" stamp (SG #2082) of March 2, 1999 depicts a different kind of fungus, mould. It is a culture of penicillin mould, which became mankind's first antibiotic, thanks to an accidental contamination by a laboratory assistant which led to the discovery by Alexander Fleming.

Great Britain's first commemorative stamp depicting penicillin, issued in 1967, can be seen on page 5 of this thread.

New Caledonia
Mushrooms
September 16, 2020
The stamp set depicts Panus lecomtei and Ryvardenia campyla.

Belgium
Mushrooms
October 26, 2020
This stamp set depicts Clathrus ruber, Clathrus archeri, Crucibulum crucibuliforme, Geastrum quadrifidum, Calocera viscosa.

Republic of Korea
Mushrooms
May 30, 1994
The stamp set features Cortinarius purpurascens, Oudemansiella platyphylla, Gomphus floccosus, Morchella esculenta.

Cuba
Mushrooms
February 15, 1988
The stamp set depicts Boletus satanas, Amanita citrina, Tylopilus felleus, Paxillus involutus, Inocybe patouillardii, Amanita Muscaria, Hypholoma fasciculare.

Ivory Coast
Mushrooms
August 4, 1995
The stamp set depicts Lentinus tuberregium, Volvariella volvacea, Dictyophora indusiate, Termitomyces schimperi.