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Feature Article of the Month


Introduction by Jerry Henkin, NNGA Librarian

A valuable additional crop that can be harvested by nut growers is truffles. At this year's NAFEX/NNGA conference in Tifton Georgia, Dr. Tim Brenneman, discussed the history and current status of truffles on the roots of pecan trees.

This article discusses the background of truffles, the climatic and soil requirements for growing truffles, and the necessary cultivation practices. An up-to-date bibliography on truffles can be found on the Wikipedia entry https://en.wikipedia.org/wiki/Truffle.

Black and Burgundy Truffles as Agroforestry Crops on the Roots of Hazel and Oak Trees*
by Gretchen Pruett and Johnn Bruhn, Division of Plant Sciences, University of Missouri, Columbia

Introduction

European truffles including Tuber melanosporum (Périgord Black Truffle) and Tuber aestivum syn. T uncinatum (Burgundy Truffie) are valuable food commodities selling for hundreds of dollars per pound and may be profitable commercial crops in the south central USA. These underground fungi are native to Europe, growing mutualistically on the roots of compatible host trees, including white oak (Quercus spp.) and hazel (Corylus spp.) (Chevalier 1978). Tuber melanosporum and T aestivum have environmental requirements (Lulli et al. 1999), (Wedén and Danell 2001) compatible with the south central U.S. and are good candidates for production on sites with proper soil pH and precipitation. Generally, T. melanosporum has more stringent growth requirements for precipitation and edaphic features than does T. aestivum (Chevalier et al. 2001), but also commands a higher Price.

In Europe, truffles are cultivated artificially by germinating host seeds, inoculating the host seedlings with truffie spores, growing the seedlings in greenhouses until the mutualistic mycorrhizal relationship is well established and then out planting the seedlings (Chevalier et al. 2001). Ongoing and future research in the USA involves evaluating various growth media, lime types, inoculation techniques, and truffle sources based on their ability to generate well-colonized seedlings. Identification of fungal competitors in the greenhouse and in the orchard will provide insight into the effectiveness of management techniques.

Background

Fungal ascomycetes in the genus Tuber produce underground fruiting bodies known in the common vernacular as truffles. These fungi form ectomycorrhizal relationships with several genera of host plants, including Quercus, Corylus, Carpinus, Betula, Fagus, and Pinus (Chevalier 1978). The fungus forms a mycelial sheath, called a mantle, around the tree's fine root tips. The mantle constrains fine root development, giving ectomycorrhizal rootlips distinctive morphologies (Agerer 1996). From the mantle, hyphae penetrate the root cortex, surrounding cortical cells with a jigsaw-puzzle-like hyphal network called the "Hartig-net" (Agerer 1996). Other hyphae grow into the soil from the mantle, effectively foraging for water and minerals, some of which are provided to the tree. In exchange, the tree provides carbohydrates to the fungus.

Truffles are found naturally on the west coast of the U.S., throughout most of Europe and in Russia, Africa and China . Tuber melanosporum, the Périgord Black Truffle and Tuber aestivum, the Burgundy Truffle are highly prized species based on their gastronomic properties (Hall and Yun 2001).

Truffles vary in color and surface texture. Both T melanosporum and T. aestivum are subglobose or lobed with angulated warts covering their outer surface. Inside the leathery rind is a marbled firm interior that contains the reproductive spores. Because of the truffles' good flavor, forest animals dig them up, eat them and disperse the spores in their excrement.

Climatic and Soil Requirements

Tuber melanosporum and Tuber melanosporum were chosen as research organisms for our lab because they grow natively in areas with soils and climates similar to what is found in the central U.S. Worldwide, attempts to cultivate truffles outside of their native range have been limited to the U.S., New Zealand and Israel and because of this their true climatic and edaphic ranges are unknown. They may be able to survive and even thrive in areas substantially different from their native ranges in Europe. Consequently, native climate and soils data should be used as a guide, not a rulebook, when selecting acceptable planting sites here in the U.S.

In general, Tuber melanosporum has more stringent environmental requirements than T. aestivum and tends to produce best in rocky, warm, open orchards in areas with moderate winters and regular rainfall during the summer (Sourzat2002). In its native range, Tuber melanosporum occurs in iron-rich, free draining limestone soils with pH between 7.5 and 7.9 (Lulli et al. 1999). Tuber melanosporum appears most frequently in soils with minimal topsoil, low sodium content, and low to moderate organic matter (Lulli et al. 1999), Climatically, average rainfall of 23-60 inches, average summer temperatures of 64-72 degrees Fahrenheit , and. average winter temperatures of 34-46 degrees F are optimum for Tuber melanosporum (Hall et al. 1994). On the other hand, T. aestivum tends to grow well in a wider range of soil types and climates than Tuber melanosporum which may be due in part to ecotypes specialized for divergent climates and soil types (Wedén et al. 2004). In its native range, T. aestivum can be found in oceanic settings with little seasonal change and evenly distributed rainfall, mountainous settings with warm dry summers and long cold winters, and continental settings with hot dry summers and cold rainy winters (Chevalier et al. 2001). T. aestivum produces fruiting bodies in thinner soils with less organic matter and tolerates lower soil moisture and increased soil porosity (Wedén and Danell 2001). For optimum fruiting body production, T. aestivum requires soil pH > 7.0, C/N ratio in the 9-12 range and calcareous soils (Lulli 1999). Additionally, Tuber melanosporum and. T. aestivum produce truffles at different times of the year. Because T. aestivum truffles mature in the autumn as far north as Gotland, Sweden, we think this species is better suited to cultivation farther north in the USA than Tuber melanosporum. Because Tuber melanosporum produces fruiting bodies during the winter in Europe, we think that this species will be more productive farther south in the USA where winters are milder.

Cultivation process

Establishment of a producing truffle orchard involves germinating host seeds, inoculating the host plant with the truffle fungus, growing the seedling in a greenhouse until the truffle/host mycorrhizal relationship is well established, and then outplanting the infected host tree into an orchard (Chevalier et al. 2001). In the first step, effective host selection is critical to ensure the long-term success of orchards (Chevalier et al. 2001). Ideally, hosts should be adapted to local environments, able to tolerate the high pH soils required by Tuber and capable of forming mycorrhizal relationships with Tuber.

After the host is established it must be inoculated with the truffle spawn. There are two methods of inoculation, by spores and by liquid culture (Hall and Yun 1998). Each has its advantages and disadvantages. Inoculation by spores provides greater genetic variability and is best suited for situations like the U.S., where it is unknown which truffle individuals will thrive in our soils and climate. The disadvantage with spore-based inoculation is the expense involved in buying truffles at market value, grinding them up and using them for inoculum. Inoculation by mycelium in liquid culture, on the other hand, provides greater control over traits because you can select from the field the traits you desire. It is also potentially cheaper because inoculum can be continually regenerated from a single sample. Unfortunately, Tuber spp. are often difficult to grow and maintain in culture.

Once the seedling is inoculated, the host must be grown in the greenhouse for one to two years for the mycorrhizal relationship to develop. The goal of seedling production is to produce a healthy seedling with a well-developed lateral root system bearing numerous root tips colonized by the desired Tuber species. In Europe, most truffle tree seedlings are grown in long, skinny cylindrical tubes called book planters. Personal observation by Johann Bruhn indicates that in this type of container, mycorrhizae appear most densely at the top and bottom of the container where aeration is greatest. Long term, this may reduce the rate of seedling establishment and fruit body production. We are modifying the RPM™ (Root Production Method) tree production technique developed by Forrest Keeling Nursery, Elsberry, Missouri, in an effort to produce Tuber-infected seedlings which will occupy the orchard site more rapidly and completely than seedlings produced by more traditional methods. The patent pending RPM™ process involves planting seeds in February, culling, transplanting and inoculating the plants in March, transplanting again in May, and out planting the following spring (Lovelace 1998). we are also investigating the effects of different types of lime on levels of Tuber-infection and on seedling root system development.

After the seedlings have been in the greenhouse for at least one year, they are outplanted to a field site with proper soil and climatic requirements (Hall et al. 1994). Trees can be colonized in the greenhouse and in the field by competitive fungi. In order to develop effective management techniques to limit colonization by competitors, we need to know which competitors are present.

Summary

European truffles may be profitable commercial crops in the south central USA when grown on the roots of hazelnuts and oaks. Truffle research in the U.S. is currently focused on identifying appropriate native hosts, site characteristics, and cultivation techniques to make this crop a reality for U.S. orchard owners.

Agerer R. 1996. Characterization of ectomycorrhizae: a historical overview. Descriptions of Ectomycoruhizae. Agerer R., Danielson R.M., and Egli S. et al. Munchen, Germany: Universitat Munchen. 1: 1-22.

Chevalier G. 1978. L'espece Tuber aestivum Vitt.: II. Ecologie. The Tenth International Congress on the Science and Cultivation of Edible Fungi, France.

Chevalier G. and G. Gregori et al. 2001. The cultivation of the burgundy truffle. Second International Conference on Edible Mycorrhizal Mushrooms. New Zealand.

Hall I. and G. Brown et al. 1994. The black truffle: its history uses and cultivation. Christchurch, NZ. New Zealand Institute for Crop & Food Research Limited.

Hall I. and W. Yun. 1998. Methods for cultivating edible ectomycorrhizal mushrooms. Mycorrhiza Manual. Varma A. Berlin, Springer-Verlag: pp. 99-l14.

Hall I. and W. Yun. 2001. Truffles and other edible mycorrhizal mushrooms - some new crops for the Southern Hemisphere. Second International Conference on Edible Mycorrhizal Mushrooms, New Zealand.

Lovelace W. 1998. The root production method (RPM) system for producing container trees. Combined Proceedings of the International Plant Propagators Society 48: 556-557.

Lulli L. and G. Bragato et al. 1999. Occurrence of Tuber melanosporum in relation to soil surface layer properties and soil differentiation. Plant and Soil 214: 85-92.

Sourzat P. 2002. Guide Pratique de Trufficulture. Le Montat, France Quercy- Cahors.

Wedén C. and G. Chevalier et al. 2004. T. aestivum (syn. T. uncinatum) biotypes and their history on Gotland Sweden. Mycological Research 108: 304-310.

Wedén C. and E. Danell. 2001. Scandinavian black truffles - distribution and habitats. International Conference on Edible Mycorrhizal Mushrooms. New Zealand.


* 96th Annual Report of the Northern Nut Growers Association, pp. 51 - 54 (2005)

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