Enfoque Social

In the early 19th century, acorns planted near older truffle-producing trees were removed as seedlings with the expectation of infection of these plants by the truffle fungus. The success of this technique remains speculative given the mystique and secrecy surrounding the harvesting and selling of truffles during these times. Its success may be indicated by its continued use on a commercial scale late into the 20th century (Singer and Harris 1987). The method of cultivation of truffles with such seedlings differed and was often an adaptation of pedoclimatic conditions that were consistent with proven truffle grounds. Land of poor productivity for other crops was utilised with minimal capital outlay. Subsequent management practices often followed family traditions and involved only tilling by hand and weed removal. Generally, such truffières were not irrigated and the host plant canopy was managed to reduce exposure of the soil thereby reducing water loss. Typically yields were poor and sporadic with an extended period from establishment to production ranging in the order of 15 to 25 years. Fungal competitors readily established under such management conditions and had the potential to degrade productive truffle grounds.

In the latter half of the 20th century there was a resurgence in the cultivation of truffles with the intention to increase supply. Only relatively recently have inoculation procedures been developed that allow the artificial mycorrhization of Quercus and

Corylus spp. with truffle producing fungi (Palenzona 1969; Chevalier and Dupré 1988;

Pinkas et al. 2000). French and Italian scientists developed the techniques in the late 1960's and early 1970's and since this time there has been increased planting of inoculated (many certified) plants in much more intensively managed truffières in France, Italy, Spain and later in non-traditional regions of the world (Chevalier and Poitou 1988; Sourzat 1994). Determination of conditions for spore germination in Tuber species has seen moderate success with the development of a range of ascospore treatments. Pacioni and Comandini (1999) report spore germination is enhanced following consumption by a hydnophagous animal. There is an inherent difficulty in preparing spore inoculum via this method hence spore slurry suspensions are generally utilised for inoculation of seedlings. Thickening agents (e.g. crosslinked polymers) are added to spore suspensions to aid adhesion of spores to actively growing roots.

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Seedlings that are well mycorrhized with T. melanosporum have proven essential to successful cultivation of the black truffle.

With the production of artificially inoculated seedlings in the mid 1970’s, changes to cultivation practices occurred. Sourzat (1994) describes two discrete methods of truffle cultivation that are utilised to varying degrees in Europe. The first of these is the “Pallier method” on which some of today’s more famous truffières are based. It espouses the search for calcareous land more suitable for truffle growing with intensive tilling of the soil, irrigation and the application of lime to increase soil pH to a range within ~7.5 to 8.2. Pruning is more vigorous in response to increased branch growth in response to irrigation. The use of chemicals is encouraged to control certain pests and to remove suckers of host plants. Pruning is carried out with the intention of increasing penetration of sunlight to the ground beneath trees to warm the soil and encourage fungal growth at the onset of spring. Most often, trees are pruned to an inverted cone shape. Planting density is between 400 and 800 trees ha-1 with regular spacing and often with alternate plantings of host species. The configurations often vary and many were reviewed by Hall et al. (2001).

The application of organic fertilisers is also recommended using the Pallier method although little detail is given by Sourzat (1994) of the chemical composition or relative effectiveness of application. Early production is observed with this method as hazel trees have produced truffles in the fourth year. Truffle yield ranges between 15 and 30 kg ha-1, 15-20 years after establishment. Byé (2000) argues that the Pallier method focuses efforts toward improving productivity of the host tree rather than on truffle production. The orchard style management of the Pallier method could be the result of a general lack of understanding of the physiological mechanisms governing growth, development and fructification in truffle fungi

The second model of truffle cultivation is commonly termed the Tanguy method or “method of managing truffle space” (Sourzat 1994). The early to mid 1990’s saw modification of the Pallier method whereby the ground is sown with grass, and the working of the ground and the chemical control of grasses and weeds around trees ceases after the third year of establishment. Planted grass is maintained by mowing and growth of the plant host is reduced to favour growth of the fungus although how this is

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done is not clear. Reduction in the use of fertiliser is one means of reducing host vigour and indeed there is minimal fertiliser input other than to maintain soils within a range considered appropriate based on existing truffières (discussed further in Chapter 2). Cultivation of the ground is not repeated unless truffle yields decline and may be done to reinvigorate the root system to encourage new mycorrhizal root development. It is suggested that regular tillage of truffières aids in the dispersal of fungal competitors and the practice is minimised as a result. The level of production of this method has not yet been fully evaluated as many truffières using this method were established during the mid-1990’s. Nevertheless, production begins rather tardily and full production is not likely to begin for 10 to 12 years following establishment. Truffières of New Zealand and Australia generally subscribe to the Pallier method although there is no strict conformity to either method, hence a mixture of techniques and practices exist.

Production levels in commercial truffières appear variable and may be the result of a range of factors. Estimating production levels of commercial truffières in regions of Europe and elsewhere is difficult as truffière owners are well known for their heightened discretion with such details. Chevalier and Frochot (1997) provide the most detailed account of production levels in individual truffières in France. Average harvests of 30 kg ha-1 at 10-13 years after planting and 50 kg ha-1 after 13-16 years after planting have been observed in a well managed truffière in Dordogne, SW France. This truffière was established with mycorrhized plants from the French Institut National de la Recherche Agronomique (INRA) and the success of the truffière has been attributed to well mycorrhized plants. Sourzat (1994) suggested the Pallier method will yield between 15 and 30 kg ha-1 of T. melanosporum at 15-20 years of age. Based on the results of experiments conducted in a truffière in the Cote-d‘Or department (NE France), yield from hazel trees at 10 years of age of between 20 and 50 kg ha-1 should be possible provided the truffière is correctly managed and irrigated during the warmer months (Callot 1999). Again exceptions to this are known; for example a truffière also of the Côte-d’Or, produced 20 kg ha-1 at 4 years of age and 110 kg ha-1 after 14 years (Le Tacon et al. 1988).

More recently, Chevalier and Frochot (1997) cite a study where three truffières of the Périgord region (SW France) were maintained according to the Tanguy method and had quite high levels of production during the 1993/94 truffle season. One of these truffières

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(age 11 years) produced 100 kg ha-1 and the remaining two yielded 150 kg ha-1 (age 13 years). This illustrates the potential but often varied yield of some truffières. The point at which a truffière is likely to yield a consistent crop is unknown and is no doubt dependant on close management of truffières to encourage fungal growth. It is extremely difficult to estimate the harvest quantity prior to or even during a season. In general terms it is considered that the first appearance of a brûle indicates truffles are only a year or two away but equally it is not guaranteed (Chevalier and Frochot 1997; Hall and Brown 1989). Generally, production is progressive from the time of the first truffle harvest but is greatly dependant on correct maintenance and appropriate seasonal conditions. A greater understanding of processes that influence production and yield will greatly benefit the truffle industry.

The long period associated with the development of the truffle may see a number of factors affect the quality and quantity of the harvest. The reported independence of the ascocarp and its ability to take up water and nutrients from the soil medium suggests the benefit of irrigation in managed truffières. It has been shown that dryness in October reduces formation of ascocarps and dryness in March and April slows development and encourages imperfect maturation (Le Tacon et al., 1982; Mamoun and Olivier 1993a; Mamoun and Olivier 1993b). Therefore, yield is likely to be improved with irrigation during the warmer months and early autumn prior to seasonal rains. A recent report by Roux (1999) indicated a reduced harvest of T. melanosporum in the absence of rain over the warmer periods of January and February particularly if dry periods are extended beyond 35 days. Recent research at an experimental truffière near Cahor, SW France suggests that a good truffle year will be had if there is sufficient rainfall during February as defined by the formula: 3T < P < 4T; where T is mean monthly temperature (°C) and P is precipitation (mm) (Sourzat, unpublished). It is important to acknowledge this formula may be site specific and is likely to vary depending on many factors, most notably soil physical characteristics.

As alluded to earlier, the effect of fertiliser application in truffières is largely unknown. Literature reporting the use of fertilisers in natural or artificial truffières is rare and what data is available indicates no distinct advantage of particular fertiliser additions regardless of existing soil fertility. Research has been conducted on nutrient supply and

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mycorrhizal interactions in other host-fungi examples. This moderate body of material will be explored and deductions will be drawn from this material in a later section of this thesis.