Newsletter Article
September 1995

Pleurotus tuber-regium

Introduction

Pleurotus tuber-regium is a tropical sclerotial mushroom which has been gaining some interest in the U.S. Being sclerotial, the mushroom produces a sclerotium, or underground tuber, as well as a mushroom. Both the sclerotium and the mushroom are edible. The mushroom looks somewhat like an oyster mushroom (Pleurotus ostreatus) except that, when mature, the cap curves upward to form a cup-like shape. The sclerotium is spherical to ovoid and can be quite large - up to 30 cm (11.8 inches) or larger in diameter [8]. It is dark brown on the outside and white on the inside.

As far as we have been able to determine, no one is commercially cultivating this species although research being conducted in Nigeria is aimed at encouraging cultivation in that country. According to Ralph Arnold, members of the Oregon Mycological Society are getting good results with trials they have been running. Ralph provided us with the references we list and will be happy to supply a complete set. Since we have not experimented with this species, this article is based upon our review of the eight references.

Uses

In Nigeria P. tuber-regium is used as both a food and a medicinal. The sclerotium, which is hard, is peeled and ground for use in a vegetable soup [8]. The sclerotium is expensive and considered a delicacy [5]. The mushroom itself is simply chopped and used in similar soups [8]. It may also be dried for future use [5]. Nigerian native doctors use various combinations of herbs and other ingredients in their medicine. P. tuber-regium is used in some of these combinations that are intended to cure headache, stomach ailments, colds and fever [8] as well as asthma, smallpox and high blood pressure [2,8]. We came across no research indicating the effectiveness of P. tuber-regium as a medicinal.

Food Value

One study [4] indicates that the mushroom is about 16.5% dry matter and of the dry matter 7.4% is crude fiber, 14.6% is crude protein and 4.48% is fat and oil. Protein levels compare to shiitake at 18%, P. ostreatus at up to 30%, wheat at 13% and milk at 25% (all based upon dry weight). Fat levels are comparable to other mushroom species. Total sugar content is about 18.6% with high concentrations of galactose and low concentrations of glucose and maltose. Levels of oxalic acid, which can reduce the food value, were low as were levels of hydrocyanic acid which can be toxic. The mushrooms also contained low levels of vitamin C.

Cultivation - Mushrooms from Sclerotia

The easiest way to produce mushrooms of Pleurotus tuber-regium is to plant the sclerotia of the fungus and keep it damp. Apparently this technique is commonly used by Nigerian farmers who find the sclerotia growing wild in their fields [1]. If you want to try this technique, contact our advertisers to see if they can supply you with sclerotia of this species. The simplicity of this approach makes this a good way to introduce amateur mushroom growers to Pleurotus tuber-regium. The sclerotia seem to be pretty hardy (their role in nature is to bring the fungus through tough environmental conditions) and production of mushrooms seems to be almost a sure thing. One study [7] showed that a sclerotium can be peeled and both the inner, edible white portion and outer peeling can independently produce mushrooms. Thus, you can eat the inner part of the sclerotium and plant the peelings and still expect to get some mushrooms. The same study found that loam is the best type of soil for planting sclerotia, apparently because of its water-holding capacity. In addition, the bigger the sclerotia that is planted, the more mushrooms that are produced. All of this suggests that this technique will not be a particularly good approach for commercial growers because it simply crops the sclerotia rather than converting a substrate into mushrooms. Production will be limited by the size and number of sclerotia that are planted.

Cultivation - Sclerotia

The cultivation of sclerotia in a nutritive medium has the advantage of creating fungal mass rather than simply converting it from sclerotia to fruit-body. The sclerotia itself has value as food, as a mushroom kit for the type of cultivation discussed above, and as an inoculant in the type of cultivation discussed below. One study [5] that looked at the possibility of growing sclerotia found that when grown on drill dusts of the wood of Elaeis guineensis and Daniella oliveri trees (these are trees of Nigeria, we are not familiar with them) the fungus would produce sclerotia, but on other tested substrates it would produce fruit bodies without sclerotia. It took at least 65 days to produce sclerotia in these tests. In nature, sclerotia typically form in response to adverse growing conditions as a method of carrying the life of the fungus through difficult conditions. Thus, as the growth medium dries out or available nutrients are used up the fungus responds by forming a sclerotium. One study [5] suggests that the type of wood or perhaps the physical and textural properties of the substrate play a role in sclerotia formation.

Another study [1] reports that banana leaves, corn cobs, cotton waste and rice straw all were capable of sclerotia production. This study produced the sclerotia using sterilized substrates incubated in perforated plastic bags in the dark at 28 to 32 degrees C (82 to 90 degrees F) for 3.5 months. This is very similar to the techniques commonly used for production of other Pleurotus species in the U.S., although these temperatures are higher. Biological efficiencies (BE, the weight of product divided by the dry weight of the substrate) for this technique ranged from 13.58% on banana leaves to 30.11% on cotton waste. We suspect that BE's can be increased considerably with substrate additives or different substrates. Fasidi and Olorunmaiye [2] found that glucose stimulates mycelial growth most from among several carbon compounds and yeast extract is most stimulative of the nitrogen compounds tested. A carbon to nitrogen ration of 1:4 or 1:5 as well as 4:1 or 5:1 seemed to produce the best mycelial growth. This suggests that P. tuber-regium can make good use of either carbon or nitrogen.

Cultivation - Mushrooms on Cellulosic Substrate

Okhuoya and Okogbo [6] used scleroia to inoculate oil palm fruit fiber as a spawning material. They generally found that if they used the sclerotia as spawn, they would get more mushrooms than if they used oil palm based spawn. This seems logical in as much as the sclerotia has already stored all the nutritive value needed for fruiting while normal mycelia would have to gather these nutrients from throughout the substrate. They grew the mushrooms in trays and generally found that casing the substrate, after the spawn run, with garden topsoil increased yields from 150% to 800%, depending upon substrate and inoculation method. Several substrates would not fruit without casing. River sand, used as a control, would not produce mushrooms when inoculated with the palm-based spawn yet the sclerotial spawn produced more mushrooms in river sand than in uncased yam peelings, rice straw or corn straw. This suggests that these materials donÍt quite make it as a substrate.

Conclusion

Overall, these studies lead us to suspect that P. tuber-regium can be grown in essentially the same ways as other Pleurotus species with due care given to temperature needs. This fungus seems to be adaptable to a wide range of materials. These studies naturally focused on materials available in Nigeria, but we suspect that various hardwoods as well as wheat and barley straws, perhaps supplemented with alfalfa would do as well or better. Tray culture using a casing technique will generate higher yields, but it may be more economical to forgo some of that yield in favor of a technique that avoids the expense of casing. This information should give you a good start on your experiments with this fungus. We have located an experienced P. tuber-regium grower and hope to have more details for you in an upcoming issue.

REFERENCES

[1] Fasidi, Isola O.; Ekuere, Usukama U. 1993. Studies on Pleurotus tuber-regium (Fries) Singer: cultivation, proximate composition and mineral contents of sclerotia. Food Chemistry 48: 255-258

[2] Fasidi, Isola A.; Olorunmaiye, Kehinde S. 1994. Studies on the requirements for vegetative growth of Pleurotus tuber-regium (Fr.) Singer, a Nigerian mushroom. Food Chemistry 50:397-401

[3] Hibbett, D.S.; Thorn, R.G. 1994. Nematode-trapping in Pleurotus tuberregium. Mycologia 86(5): 696-699

[4] Ogundana, S.k; Fagade, Oba. 1981. The nutritive value of some Nigerian edible mushrooms. In: Mushroom Science XI, Proceedings of the Eleventh International Scientific Congress on the Cultivation of Edible Fungi, Australia; 123-131

[5] Okhuoya, J.A.; Okogbo, F.O. 1990. Induction of edible sclerotia of Pleurotus tuber-regium (FR) Sing. in the laboratory. Annals of Applied Biology 117: 295-298

[6] Okhuoya, J.A.; Okogbo, F.O. 1991. Cultivation of Pleurotus tuber-regium (Fr) Sing on various farm wastes. Proceedings of the Oklahoma Academy of Science 71:1-3

[7] Okhuoya, J.A.; Etugo, J.E. 1993. Studies of the cultivation of Pleurotus tuber-regium (FR) Sing. an edible mushroom. Bioresource Technology 44: 1-3

[8] Oso, B.A. 1977. Pleurotus tuber-regium from Nigeria. Mycologia 69: 271-279