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Our Mushroom Monday posts have been scant for the last year, and with good reason. In the midst of a drought, with no rain forecast through at least the end of the first week of January, 2013 has now been officially ruled as the driest year on record in the San Francisco, and Monterey Bay, areas.

However, considering how bone dry the farm is at the moment, you can imagine my surprise on Christmas Eve when I bumped into a rather robust cluster of oyster mushrooms (Pleurotus ostreatus), growing on a fallen decaying tanoak (Lithocarpus densiflorus) trunk.

These are growing a mere 20 feet behind the chicken coop! Aren’t they gorgeous?

I’ve been looking for oyster mushrooms on the property since we first moved here. Instead I’ve found a variety of other culinary favorites, including chanterelles (Cantharellus californicus), black trumpets (Craterellus cornucopioides), and even a black morel that popped up in the goat yard after we built the barn. However, up until now, the relatively common oyster mushroom had evaded me.

Knowing that here on this property I would be most likely to find oyster mushrooms growing in association with oak, I’ve frequently scoured our oak trees, and downed logs, for any sign of them.

As if to taunt me, last January I did find some beautiful Mock Oyster Mushrooms (Phyllotopsis nidulans).

They are similar in overall structure, albeit their color, and texture, is distinctly different. Close, but not close enough. So considering our paucity of recent rain, I was especially thrilled to finally find this cluster. In fact, I walked that entire hillside this morning, and these were the only specimens I found.

Although both the mock, and true oyster mushrooms are agarics (gilled mushrooms), and both were once in the genus Pleurotus, they are now classified not only in different genera, but also in entirely different families. These true oyster mushrooms are in the Pleurotaceae family, whereas the Mock Oyster is now in the Tricholomataceae family.

Phylogenetics aside, perhaps the most important distinction between the two is that where the true oyster mushroom is edible, and sought after by both professional, and home chefs alike, the mock oyster mushroom is severely lacking in any culinary appeal.

There is little likelihood of confusion in identifying them though. The caps of Pleurotus ostreatus are smooth, whereas Mock oysters are comparatively fuzzy. The caps of Pleurotus ostreatus are distinctly convex, typically ranging between 4-15 cm at maturity, and may become more plane, or centrally depressed with age.

The cap margin is curved under when young, and becomes more wavy with maturity.

This species is often considered by many to lack a stem, but the caps are often anchored by rudimentary eccentric, or lateral, stems to their substrate, and the stipe may be more apparent in some specimens than others. I noted some with more distinct stems that were growing from underneath an elevated section of the log, and growing upward at an angle.

The gills are decurrent, and extend down the stem, and the spore print of this species is typically white to grey. Note that Mock oyster mushrooms have a pink spore print.

There are apparently three known species of Pleurotus, distributed throughout much of North America, that were once all considered to be of the same species, and are quite similar in appearance to each other. Pleurotus ostreatus, is more commonly found in the colder late fall and winter months. These must be particularly hardy considering the recent cold snap we had here. The less cold-tolerant Pleurotus pulmonarius species is usually found late summer to early fall. These two species can be difficult to tell apart morphologically, however, as both are similar in general appearance.

A third species, Pleurotus populinus, is also similar in appearance, but is only found growing on Populus species wood including Cottonwood and Aspen.

Although nature gifted me with my first oyster mushroom sighting on the farm, just in time for Christmas, and I can finally cross oyster mushrooms off my list of fungi to find here, I still have a few other species I’d like to find. However, unless we finally get some significant rainfall in the next several weeks, this might be the first, and last, Mushroom Monday post of the season! Cross your fingers for rain!