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Milk-white Toothed Polypore - Irpex lacteus

Good evening, friends,

This week's mushroom is one of the more common winter mushrooms you'll find in Central Park and the surrounding tri-state area, it's the Milk-white Toothed Polypore (Irpex lacteus). This white toothed mushroom that often appears to be painted on the underside of branches will actually defy it's common name and discolor brown with age. The photo below was taken in Manitou on 12/9/2022, one of the only days with any snow this winter :(, but I find this mushroom routinely throughout the year. While rather common in appearance and occurrence, it definitely has some interesting qualities that you can use to impress your friends when you find it.


Irpex lacteus

Not So Fun Fact

I. lacteus is a pretty aggressive decomposer, as we'll dive into deeper in the ecology section, but it's one of those vigorous fungi like Schizophyllum commune that can actually infect humans with compromised immune systems. They isolated a strain of I. lacteus from a "pulmonary abscess" (a lesion in a lung) in a poor 9 year old girl that was already in the hospital battling leukemia (Reference 3).


Irpex lacteus

Fun Facts

A neat feature of the mushroom is that it grows entirely resupinate, meaning it will grow on the underside of a dead branch, orienting itself perpendicularly with the pull of gravity, and create those small "effused-reflexed" caps that curl off the branch to extend the mushroom's fruiting body and keep it perpendicular. Though it grows like a crust fungus, it is still considered a polypore.


The etymology is kind of neat. Irpex comes from Ancient Greece & Rome where an irpex (or urpex, or hirpex) was an iron-toothed rake, drawn by an ox, that was used to weed and till fields. Lacteus is from Latin and means "milky" (where "lactose" comes from too) which relates to the color of the mushroom.


Irpex lacteus

When I find a resupinate polypore on the bottom of a branch I always check to see if there is a neighboring lichen, like the shield lichen (Parmelia) shown below growing on top of the branch. Lichen are a symbiosis of multiple organisms from up to three different kindoms - fungi, plants, and monera (single-celled organisms). They will always have a fungal component and then will recruit either an algae (plant) or a cyanobacteria (monera) - and sometimes both - to conduct photosynthesis. That's like if we joined together with a palm tree and a milk-white toothed polypore to create an entirely new organism. I always get a kick out seeing the mushroom on the bottom of the branch oriented with gravity and the lichen on top soaking up the sun.


Irpex lacteus

Ecology

I. lacteus is saprobic, digesting dead wood. We sometimes delve into whether a fungus causes a "white rot" or a "brown rot" - terms that refer to which parts of the wood the fungus prefers to digest - but what's interesting about I. lacteus is that it is an equal opportunity rotter. It will digest the lignin (usually prefered by white rot fungi) and the cellulose (usually preferred by brown rot fungi), however is considered a "simultaneous white rot" fungus. These synchronous usurpers are almost always found on hardwood, as is the case with I. lacteus, and rarely on conifers/softwoods. I have found this mushroom most frequently on the invasive Ailanthus tree (Tree of Heaven).


The fungus grows commonly in temperate North America, and temperate regions across both the northern and southern hemisphere. It grows year-round but is most evident in the winter when leaves have fallen off trees and that milk-white really pops. Although it appears to have teeth, those are in fact pores that quickly break apart and leave a toothy appearance. It has white spores and when fresh I find it smells kind of waxy/fungal. It's nearest look-a-like is probably Xylodon paradoxus (featured below), but X. paradoxus has maze-like ("daedeloid" if you really want to impress) pores and often forms in circular patches.


Xylodon paradoxus

I think I saw the comet, or at least I told myself I did - and that's 90% of it. Have a great week,

Aubrey


References:

1) Kuo, M. (2007, April). Irpex lacteus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/irpex_lacteus.html

3) Buzina W, Lass-Flörl C, Kropshofer G, Freund MC, Marth E. The polypore mushroom Irpex lacteus, a new causative agent of fungal infections. J Clin Microbiol. 2005 Apr;43(4):2009-11. doi: 10.1128/JCM.43.4.2009-2011.2005. PMID: 15815046; PMCID: PMC1081321.

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