Pleurotus ostreatus, the Oyster mushroom, Tom Volk's Fungus of the Month for October 1998

Tom Volk's Fungus of the Month for October 1998

This month's fungus is Pleurotus ostreatus, the Oyster mushroom

For the rest of my pages on fungi, please click TomVolkFungi.net

One of my favorite edible mushrooms is the oyster mushroom, which usually goes under the species name Pleurotus ostreatus. (I'll have more to say about the name later). It is a delicious edible mushroom and is found throughout the north temperate zone, almost always on dead hardwood (angiosperm) trees. It can also be (relatively) easily cultivated on a variety of substrates, so it is making its way onto many supermarket shelves. In the wild it can often be found in abundance during this time of year, but I've found it every month from March to November in Wisconsin! It can be found every month of the year in more southerly locations.

Pleurotus ostreatus Pleurotus species are characterized by a white spore print, attached to decurrent gills, often with an eccentric (off center) stipe, or no stipe at all. They always grow on wood on nature, usually on dead standing trees or on fallen logs. The common name "oyster mushroom" comes from the white shell-like appearance of the fruiting body, not from the taste. The taste of the oyster mushroom varies from very mild to very strong, sometimes sweet with the smell of anise (licorice). It varies in texture from very soft to very chewy, depending on the strain and what time of the year you pick it-- they tend to be chewier (and thus more interesting) during the colder months of the year. You can make a delicious "Oyster Mushrooms Rockefeller" and a variety of stir-fry dishes.

There is some controversy about what the species should be called. Apparently what we have been calling Pleurotus ostreatus in North America, should be classified in at least three species, including Pleurotus ostreatus, Pleurotus pulmonarius, and an additional similar species that grows on Populusspp. (aspen and cottonwood) species called Pleurotus populinus. P.ostreatus and P. pulmonarius are difficult to distinguish from one another. Young fruiting bodies of P. pulmonarius are, in fact, lung-shaped (as you might guess from its name), while those of P. ostreatus are more dimidiate (split into two equal parts). Perhaps the most valuable distinguishing character is seasonality: P. ostreatus is a late fall and winter fungus in Europe and North America, while P. pulmonarius fruits from mid-summer through early fall. See the excellent online treatise on the biological species in Pleurotus by Ron Petersen, Karen Hughes, and Nadezhda Psurtseva.

Despite what you may have heard or seen or thought about this genus, there are some species of Pleurotus with substantial stipes. One of the most interesting is Pleurotus elongatipes, shown to the left. The bottom part of the image is a few of the fruiting bodies removed from the large cluster (shown above) which is usually attached right at the base of the host tree. It's very different from the true Oyster mushroom with which you may be more familiar. I've only seen it a few times. There is another species, Pleurotus dryinus, which actually has a ring around the stalk.

One common species that used to be in the genus Pleurotusis the elm oyster, Hypsizygus ulmarius. In our area it is common on the knot holes in living or dead box elders, but it does grow on elm and a number of other hardwoods. It was moved out of the genus Pleurotus because Pleurotus species cause a white rot and Hypsizygus causes a brown rot. For a discussion of white vs. brown rot see this page on Phanerochaete chrysosporium.

Crepidotus There are some similar looking fungi that you wouldn't want to eat. One of them is pictured to the left, Lentinellus ursinus, the bear Lentinellus. They say it is "the only mushroom that raccoons will spit out." It is extremely bitter and hot to the taste. It has white spores and belongs in the Tricholomataceae like the oyster mushroom, but it has serrate (saw-toothed) gill edges. If you look at the basidiospores under the microscope they are small and amyloid (blue) in Melzer's reagent (the active ingredient of which is iodine). In addition, the trama (the flesh of the gills) has many amyloid hyphae in it. It is not known to be poisonous, but I certainly don't recommend eating it.

Another common mushroom that can be confused with Pleurotus are Crepidotus species, shown here to the right. They have a similar lack of a stipe and are attached directly to the wood on which they grow. They can easily be distinguished if you take a spore print-- the spore print of Crepidotus species is brown, rather than the white of Pleurotus. So far as I know they are not poisonous, but I do not recommend eating them-- you certainly will not have a pleasant dining experience. Their flesh is very thin, and they do not have a pleasant taste.

I mentioned earlier that Pleurotus species are among the easiest to cultivate. The picture to the left is Pleurotus sapidus being grown in sawdust substrate. You can also grow it on natural logs-- it's really very easy. Almost any substrate that contains cellulose can be used-- straw, waste hulls from agriculture and even toilet apper rolls! For more information on growing Oyster mushrooms (and also shiitake, morels, hen of the woods, and an number of other specialty mushrooms) I suggest you contact Joe Krawczyk and Mary Ellen Kozak at Field and Forest Products in Peshtigo Wisconsin. Their email address is ffp@mari.net or their 800 number in the USA is 1-800-792-6220. They sell spawn and also kits to grow your own mushrooms on a wide variety of substrates.

For a more seasonal Halloween fungus check out the Holiday fungi page, including the jack-o-lantern mushroom, ergot, the witch's hat, and the ghost plant.

Tom Volk's Fungus of the Month for October 1998

This month's fungus is Pleurotus ostreatus, the Oyster mushroom

If you have anything to add, or if you have corrections or comments, please write to me at volk.thom@uwlax.edu

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