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Figure 282. A sporeless strain of P. ostreatus. Under
the microscope, the gill planes are entirely free of
basidia.
GROWTH PARAMETERS 315
Description: Cap convex at first, expanding to
broadly convex, eventually flat and even upturned
in age. 5-20 cm. (+) in diameter. White
to yellow to grayish yellow to tan, rarely with
pinkish tones, to lilac gray to gray-brown. Cap
margin smooth to undulating like an Oyster
shell. Color varies according to the strain,
lighting, and temperature conditions. Stems
are typically eccentrically attached to the cap.
Flesh generally thin. Some strains form clusters;
others form individuals.
Distribution: Distributed throughout the temperature
and tropical forests of the world.
Natural Habitat: Common on broad-leaf
hardwoods in the spring and fall, especially
cottonwoods, oaks, alders, maples, aspens,
ash, beech, birch, elm, willows and poplars.
(From an evolutionary point of view, this
mushroom has been very successful, given its
ability to saprophytize a broad range of tree
species.) Although seen on dying trees,
P ostreatus is thought to be primarily
a saprophyte, but behaves as a facultative para-
site at the earliest opportunity. Occasionally occurring on composting bales of straw, and in Mexico,
on the waste pulp from coffee production. (The occurrence of P ostreatus on this last habitat might be
a result of this species escaping from the woodland environment and taking advantage of a niche provided
by the coffee industry.) P ostreatus, and particularly P ostreatus var. columbinus, are
occasionally found on conifers, especially A bies. The most abundant fruitings of this species is in low
valley riparian habitats.
Microscopic Features: Spores white* to slightly lilac to lilac grey, 7.5-9.5 x 3-4 p. Clamp connections
present. Context monomitic.
Available Strains: The genome of strains for this species is vast and increasingly explored by home
and commercial cultivators. Cold and warm weather strains are available from numerous culture libraries.
Amycel's #3001 and Penn State's # MW44, cold weather strains, are popular. A warm weather
strain I cloned from mushrooms growing on a fallen oak in a ravine near San Diego, produces an attractive,
white mushroom in as short as 10-12 days from inoculation onto wheat straw. (See Figure 285).
* From my experiences. Oyster mushrooms from river-bottom habitats in western Washington and Oregon produce a
white to grey buff spore print, and not distinctly lilac as reported for the eastern forms. Furthermore, I have recently
collected a pale rose variety of P ostreatus on alder (Alnus rubra) from western Washington which I have never
encountered before. The pale rose color has been described for P pulmonarius, but not for P ostreatus.
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