Tom Volk's Fungus of the Month for January 1997

This month's fungus is Tuber gibbosum, the Oregon white truffle.

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Truffles!!Tuber gibbosum is found in Oregon and Washington and is a mycorrhizal fungus with Douglas fir (Pseudotsuga menziesii.) It is important in the old growth forest ecosystem because it helps the Douglas fir tree to grow tall and strong by providing increased absorption of nutrients from the soil. The Oregon white truffle is also important as a food source: during certain times of the years more than 90% of the diet of the northern flying squirrel and other small rodents is made up of this species, as well as other truffles and false truffles. Of course these small rodents make up most of the diet of large birds, such as the endangered Northern Spotted owl. So Tuber gibbosum is important on several levels to the ecosystem.

The Oregon white truffle can also serve as a delicious food source for people. The flavor of the truffle is very difficult to describe, but once you have tasted it you will always recognize it. This fungus is not eaten as a regular mushroom, but is used for flavoring of soft cheeses, broiled meats, and rice. The flavor is very strong and only a small amount is needed. The flavoring agent is volatile and fat-soluble, so long cooking simply results in your kitchen smelling very good, but most of the flavor is lost. The pictures above are truffles collected by the dogs (primarily the dog named Morel !) in the backyard of my friend Judy Roger, who lives in Estacada, Oregon, at the foot of Mount Hood. Thanks Judy!

Tuber x.s.Tuber x.s.closer

Tuber gibbosum

is a member of the Tuberales in the Discomycetes in the phylum Ascomycota. Most other Discomycetes (like Morchella; see below) have an epigeous (above ground) apothecium, an open ascocarp with the asci in a hymenium. However, the Tuberales have a hypogeous (below ground) ascocarp with scattered asci. The image on the left is a cross section of a truffle ascoma showing the distribution of the asci. The image on the right is a closer view of the asci, showing the spiny ascospores. There are 1-4 ascospores in each ascus.
Morel composite view The image to the left is a Morchella hymenium for comparison. Note that all the 8-spored asci are lined up in a hymenium, which is exposed to the air.

Related species

There are a couple other species of Tuber in the Pacific Northwest, including the milder-tasting Tuber giganteum. The black truffle or Perigold truffle (Tuber melanosporum) grows primarily in Europe, especially Italy and France. In Italy trained pigs hunt for truffles with their owners, while French hunters usually use dogs (or so I've been told). There are various other genera of truffles and false truffles, including Gautieria, Elaphomyces, Geopora, Leucangium, and others. There are truffles and false truffles in all parts of the world, but not all species are edible. None are poisonous that we know of, but most don't have any real flavor. We have found truffles (genus Tuber) and false truffles (genus Elaphomyces) in Wisconsin on several Wisconsin Mycological Society forays. They appear to play a similar although less dramatic role in eastern ecosystems.

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