Oregon State University,

Corvallis, Oregon 97331-5705

and

U.S.D.A., Forest Service,

Pacific Northwest Research Station,

Forestry Sciences Laboratory,

Corvallis, Oregon 97331

Truffles are belowground (hypogeous) relatives of the cup fungi (Castellano et al., 1989; Trappe, 1979). They have evolved a spore dispersal strategy that depends on animals. As a truffle matures its spores, it begins to emit an odor, a chemical signal to animals that a feast awaits (Trappe and Maser, 1977). Nearly all mammals seem attracted to ripe truffles and will eat them whenever they detect them (Maser et al., 1978).

Humans are numbered among the animals that enjoy eating truffles, although only a few of the several hundred known species please the human palate. North America abounds in truffles, and new species are turning up regularly as new places are explored for them. The activities of the North American Truffling Society (Box 296, Corvallis, OR 97339-0296) have been particularly fruitful in this respect.

The growth of interest in the quest for truffles has generated increasing demand for up-to-date keys to identify them. At the request of the editorial committee of McIlvainia, we developed keys to Ascomycete truffle genera on a world-wide basis. Keys to the truffle genera base solely on spore characteristics were published by Castellano et al. (1989) with the special intent of identifying fungi eaten by animals as represented by spores in stomach contents or feces. The collector who has a truffle in hand but no microscope will want a key that permits identification at least to genus by macroscopic characteristics. Even when a microscope is available, use of characters in addition to spores can greatly simplify identification.

Although laymen have generally heard only of the true truffles (order Tuberales), truffle-like fungi are diverse. They occur in many families and genera of the Ascomycetes (true truffles), Basidiomycetes (false truffles), and some Zygomycetes, and the general principles of finding, describing, and preserving them apply to most species. For the purposes of this paper we will use the general term truffles for all truffle-like fungi, but we present keys only to the Ascomycete genera.

Though abundant in much of North America, truffles are seldom encountered by collectors who seek specimens only on or above the ground surface. Having learned the art of finding truffles the collector will find this bounty more often then not. Moreover, the likelihood of turning up new taxa offers an incentive, since little of the world has been well-explored for truffles .

Truffles are typically found near mycorrhizal roots of woody plants in or near forests, groves, and shrub areas. Some fungal species are relatively nonselective in terms of mycorrhizal host species that they associate with; for example, Elaphomyces granulatus associates with trees in several genera and families. Other truffles require hosts from a few or only one genus. An example of such narrow host specificity is Tuber gibbosum with Douglas-fir (Pseudotsuga menziesii).

In habitats that remain moist through the summer, truffles often grow in the litter layer or, occasionally, so close to the surface that they emerge at maturity. Some spring-fruiting species develop on the humus surface under snow banks, to be revealed as the snow melts. More typically, however, truffles reach maximum development near the interface between humus and mineral soil – it is here that mycorrhizae develop most profusely. A few, such as certain species of the genera Tuber and Elaphomyces, characteristically fruit more deeply in the soil; some, indeed, have been found at depths of several feet.

Certain microhabitats are particularly congenial for some truffle species. For example, Hydnotrya variiformis fruits profusely in buried rotten conifer wood.

Every kind of seasonal occurrence is represented among the truffles. In temperate regions there is usually a flush of fruiting in spring, summer, or autumn, depending on rainfall patterns. Maximum truffle production generally coincides with maximum fruiting of wild mushrooms. Many species normally fruit in one particular season, while others appear sporadically or more or less continuously throughout the growing season. The Basidiomycetes tend to be relatively ephemeral; the primordium forms, expands, and the sporocarp matures and decays in a relatively short time. Many Ascomycetes, by contrast, take several months to mature and then decay only slowly. Such fungi can be found throughout the growing season but sometimes cannot be identified for lack of spores until late in the season. Elaphomyces spp. are particularly durable and may even overwinter to mature in the year following initial expansion of the primordium.

The most important tool for unearthing truffles is a garden cultivator or rake. Preferences vary, but we find a four-tined garden cultivator with the handle cut to a comfortable length (about 30 inches) to be most efficient. Hand cultivators are also satisfactory but require more stooping and cover less area than the larger type. Once a likely spot is chosen for examination, the litter is gently raked aside and the exposed humus inspected for specimens. The humus is then raked away down to mineral soil. During and after each stroke of the fork, the material being worked should be carefully watched. Having reached mineral soil the collector will often find it worthwhile to rake several inches deeper especially if there is any suggestion (mycelial network) that something is growing underneath. After completing the search for specimens at a particular spot the duff and soil should be replaced as best as can be done.

Specimens growing in the site examined will either be uncovered or "pop out" from the raking. It pays to look especially for small, dark, inconspicuous specimens because then they will be seen along with the larger or more brightly colored ones. Once a specimen is found, note the depth and kind of material in which it was growing and look for evidence of associated mycelium. Truffles are often gregarious, and many additional specimens may well be nearby in the same substrate or mycelial network.

Having collected all specimens uncovered at the site, the collector should take notes on their fresh color and odor and whether latex is exuded from a cut surface or the specimens change color when either cut or bruised (press the surface firmly). If no immediate color change occurs, the specimens should be rechecked after 30 - 60 minutes and again after several hours. The date, location of a collection, and notes on associated vascular plants, including tree seedlings and shrubs as well as overstory trees, are useful in describing the ecology of these fungi. Specimens can be kept in waxed paper sandwich bags (avoid plastic bags) until final processing.

A good indicator of truffle activity is the presence of fresh small animal digs. Squirrels and mice usually unearth truffles one at a time, leaving a small pit; sometimes they will nibble the upper part of a specimen and leave the lower part. Usually they will dig only a few of the specimens present, leaving others to mature later. Once a pit is found, careful raking of several square feet of surrounding ground to about the same depth as the bottom of the pit is likely to yield specimens. Needles, leaf fragments, other debris, or spider webs in a small animal dig usually indicate it is so old that further exploration will be unprofitable. Tunnels usually are not closely related to occurrence of truffles; if the bottom of a hole cannot be felt with a finger, it should not be regarded as an indictor of truffles.

Other animals, too, are attracted to truffles. Wild pigs, deer and bear sometimes paw up sizeable tracts that harbor specimens. Such areas can be profitably searched, and sometimes specimens can be found lying on the surface.

Truffles are also found where the soil is somewhat compacted or where the humus layer is thin. Under such circumstances even relatively small specimens will hump up the humus enough to be detected by a perceptive searcher, while large species may emerge through the surface. Thus campgrounds, old tractor trails, abandoned roads, and road banks are particularly productive places to look.

With experience, a collector will recognize microhabitats likely to produce truffles. Generally, they are most numerous and diverse in places with relatively little ground vegetation. Often the soil adjacent to rotten logs is good, particularly the "microbenches" on the upper sides of long-fallen trees lying along the contour of moderate slopes.

Finally, random searching can provide fine collections. Seemingly unlikely sites can harbor some species that do not often occur elsewhere. In one instance, a splendid group of Tuber specimens were discovered during cultivation of a rose garden in a city residential area.

Each specimen should be cut in half. Many species resemble each other on the surface but differ in the interior; examining the interior minimizes the chance of including more then one species in a single collection.

For several reasons, well-dried specimens are more useful and easier to maintain than those preserved in liquid. Specimens are preferably described and dried as soon after collection as possible; once deterioration begins, much of their value for later study is lost. This applies particularly to Basidiomycetes, although specimens in very good condition can often be stored in a refrigerator for several days if absolutely necessary. Each specimen should be halved before drying; large ones can be cut in several vertical slabs.

Best preservation results from rapid drying at relatively low temperatures. A circulating oven or food dryer set at 30˚C works well. Without good air circulation, specimens will warm without drying rapidly and deteriorate rapidly. Silica gel provides a good way for drying specimens in the field (Hoseney, 1963). Small containers (half-pint and one-pint sizes) with lids and a tightly sealed container of oven-dried silica gel can be conveniently taken on collecting trips. When a collection is ready to be dried, the specimens are cut in two or more pieces, depending on size, and covered with silica gel granules. Pieces should not touch each other. No more than one collection should be put in a given container because, when dried, species can often be difficult to tell apart. One to two days will dry specimens thoroughly by this method if enough silica gel has been added in proportion to the volume or material to be dried.

In special circumstances where it is not practical to take silica gel, e.g., in wilderness areas, light-weight frames covered with fine mesh screen are useful. The screens can be set up for drying in the breeze and sun or near a campfire.

Truffles have a limited number of macroscopic characters for use in keys. Having evolved the belowground fruiting habit, they often lack distinctive features such as stems or brightly colored caps. Many resemble small potatoes or dirt clods more than a mushroom or cup fungus. Consequently, a key based solely on macroscopic characters has limitations. We present a first attempt at a macroscopic key and have tried to use common words in lieu of technical terms when possible. The first challenge is to determine whether a specimen is an Ascomycete or Basidiomycete. No single macroscopic character differentiates the two groups. Accordingly we present Key No. 1 for this first determination. If the specimen is an Ascomycete, one can turn to Key No. 2 if no microscope is available. Key No. 3 is an alternative for use with a microscope. In Key 2, that utilizes macroscopic characters, some choices depend on the specimen being mature, for example the application of a drop of iodine to see if the specimen gives a blue or black reaction. It often won't unless it is at least partially matured, but how does one know without a microscope to check whether asci are filled with spores? We have not determined a way around such a dilemma. Fortunately, most collections will include some mature specimens.

We also use geography in the macroscopic key, for example by specifying that a genus occurs only in the Northern Hemisphere or only in association with Southern Hemisphere trees such as eucalypts or only in deserts. Normally we prefer to avoid this approach, because a genus known today only from certain kinds of places may turn up in other places tomorrow as the search for truffles expands into new territory. Still, we needed all the options to make the macroscopic key workable.

In some cases a particular genus keys out in two or more places. The genus Tuber, for example, does so because it is diverse in form and color and yet difficult to describe in a readily understood way without recourse to microscopic characters.

In a second key we include microscopic as well as macroscopic characters, again with minimal use of technical terms. Those that have access to a microscope will generally find the second key to be more satisfying. Moreover, it enables one to check the characters of the specimen in hand with those given in descriptions in sources such as Trappe (1979) or Castellano et al. (1989).

After the keys we give references to the best available keys to species of each genus. In many cases, no reliable, updated keys are available. We are working towards correcting that deficiency, but it is a large task.

9. Chamber single, in youth containing cottony hyphae, at maturity spore powder 10

23. Truffle surface smooth to scabrous or lobed and furrowed but not warty 28

30. Truffle lacking well-defined cavity, but depressions or furrows may be present 32

36. Pockets of truffle interior yellow to brown; truffle surface brown; spores spiny or reticulate Terfezia

9. Spores ellipsoid 11

23. Truffle surface smooth to roughened; paraphyses at least as long as asci 24

29. Spores spiny, warty, or covered with amorphous material; hymenial palisade even 30

34. Asci multispored 35