Irpex lacteus is in the phylum Basidiomycota, class Hymenomycetes, order Aphyllorphorales, family Polyporaceae and genus Irpex. Common name is “Milk-white Toothed Polypore.” Fruiting body is effused-reflexed growing on the underside of fallen hardwoods. Fruiting body appears to be similar in color to milk, but may range from white, to off-white or cream colored. Pores that break up into short teeth (less than 3 mm long) when mature. Fruiting body is dry and stiff (Bessette et al.).

Pictures:

 Picture of Irpex lacteus from messiah.edu
 
 
 
 

Photo by William Tannenberger

Distributed across North America but very rare in the Southwest. Also known from Europe and elsewhere.

Irpex lacteus is the only species in the genus Irpex_. To identify the genus; if the teeth are less than 3 mm in length (_Irpex), if teeth are longer than 3 mm in length when mature you may have Spongipellis pachyodon. Irpex is a saprophyte on hardwoods and infrequently conifers causing white rot, but maybe parasitic on Cherry trees. Spore print white with spores ranging from 5-7 × 2-3 um that are smooth, cylindrical and inamyloid (Bessette et al.).

Essentially no look alikes because it is the only species in the genus Irpex and the two characteristics, effused-reflexed and teeth less than 3 mm, make this species very easy to identify. Spongipellis pachyodon is a toothed polypore that looks somewhat similar but the teeth are usually longer than 3 mm in length.

<img src=“http://www.mushroomobserver.org/...; />
 Picture by James V. Gallagher IV
 
 Schizopora paradoxa may sometimes look similar, but it is more poroid, and its hyphae have clamp connections.

It is found on the underside of fallen hardwoods, rarely on conifers, where it is a white rot saprophyte. It may be parasitic on Cherry trees.

Irpex lacteus is inedible and isn’t commonly used by people.

However, it is an important saprophye in nature, returning nutrients back to the ecosystem.