BLACK TRUFFLE CULTIVATION AND COMPETING FUNGI - Inra

Proceedings of the 7 th International Conference on Mushroom Biology and Mushroom Products (ICMBMP7) 2011
BLACK TRUFFLE CULTIVATION AND COMPETING FUNGI
Section:
Mycorrhizal mushrooms
SOURZAT Pierre
Station d’expérimentation sur la truffe,
46090 LE MONTAT,
France
station.truffe@wanadoo.fr
ABSTRACT
Some of conditions required for Tuber melanosporum truffle production are known. The truffle
fungus is introduced with controlled mycorrhized plants, in an appropriate soil (calcareous,
aerated, free draining, with a good biodiversity), under a suitable climate which allows for a
complete life cycle of the truffle. Culture techniques are usually concerned with the care of the
soil, plantation irrigation, protection and pruning of the trees. Nevertheless, we observe the
failure of some plantations because of the appearance of contaminating fungi.
Contaminating fungal species appear in truffle plantation according to the age of the trees
and culture conditions. Many mycorrhizal or saprophytic species have been identified according
to the age of different plantations in the South West of France, under mycorrhizal or sporocarp
forms. A typology of this species was done to determine the level of danger it can cause in a
success of truffle plantations.
These results show that certain species (Hebeloma sp.) can contaminate plants in the
nursery. When Tuber melanosporum is cultivated in oak forest environment, Tuber brumale is
the first contaminating fungus which takes advantage of the fall of biodiversity or some
cultivation methods disturbing young truffle plantations. Tuber aestivum is a late contaminating
fungus or it appears when the climate becomes drier. Many basidiomycetae are observed as
plantations become old. When truffle cultivation is done in open landscape with cereals and
vineyards, contaminations are rare. Tuber melanosporum can dominate the fungal train or
succession for many years.
These observations require preserving the initial mycorhization of the planted trees with
cultivation methods suitable to the environment with or without oaks forest. Bringing additional
truffles spores can strengthen the initial inoculation with Tuber melanosporum. Thinning and
pruning help Tuber melanosporum to be present and stronger in the truffle plantation as it is an
early stage fungus.
Keywords: Tuber melanosporum, truffle, “brûlé” or burnt area, cultivation methods, mycorrhizal
fungi.
INTRODUCTION
Truffle Tuber melanosporum cultivation has evolved since its beginning in the early 19 th century
when acorns were seed in a suitable environment already naturally producing black truffles. The
process was codified on the 19th century and at the beginning of the 20 th by Chatin [1], de
Bosredon [2], de Ferry de la Bellone [3] and Pradel [4]. At this period of abundance, cultivation
consisted mainly in growing oaks in limestone soils, that was to say to scuff the soil and prune
oaks in order to keep the lasting of the production. After a period of declining production,
following the two world wars, Rebiere [5] was the first to reformulate the truffle cultivation
methods with the achievements of the modern agriculture intended to feed Europe. A new era
began with the invention of controlled inoculated plants popularized since 1974 thanks to Gérard
Chevalier. Grente and Delmas [6] specified the inoculation conditions of the environment with
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trees mycorrhized with T. melanosporum and Delmas [7] analysed mostly the characteristic of
suitable soils. First truffles, which were harvested under hazel trees only 4 years after planting,
gave hope for a new rise of truffle production. French public institutions supported new truffle
plantations with grants given until 1986. When it became obvious that truffle production would
not really pick up again, all grants for planting mycorrhized trees, installing truffle irrigation and
renewing old plantations were abolished. In 1994, the 11 th National State Plan proposed that a
step of experimentation in truffle cultivation should take place before giving out new grants for
truffle plantations. In 2003, at the national truffle cultivation day organized by CTIFL in
Cuzance (Lot, France), certain regions decided to encourage again truffle plantation with new
financial aid.
Callot [8] underlined the importance of biological activity in truffle soil to explain the lack of
good results. Sourzat [9] stressed the fact that soils had not changed in one century and biologic
surrounding conditions had shifted as the farming methods had negative impacts. Fungal
successions (fungi train) of wild truffle trees with those of plantation truffle trees where the soil
was tilled were compared. In plantation, on shallow soil, no fungal succession but a disorder was
observed. This disorder means generally that competing fungi species are favoured, particularly
Tuber brumale. The principle of precaution in truffle cultivation, which was defined in “Truffe et
Trufficulture” [9], recommends a technical itinerary with 3 stages:
- Stage One: in the first two years of planting, one must be sure to achieve the best possible
success rate for the T. melanosporum infected seedling in the first two years of planting.
- Stage Two: it is important to focus on “a not too much” on the rapid growth of the mycorrhizal
trees to avoid contamination by other types of mushrooms; the modification of the environment
induced by the truffle itself (i.e. soil structure and biological activity) during the period of brûlé
formation has to be considered.
- Stage Three: once fruiting has started, the aim is to improve the quality and quantity of the
black truffle whilst ensuring a good perennial annual yield.
In 2008, the syntheses of truffle cultivation experimentations in France were published [10].
They enlighten the results provided since 1994 during the 11 th and 12 th National State Plans
State-County with the subsidies from FranceAgriMer (Oniflhor, Viniflhor) and the Regions.
These syntheses handle particularly of the truffle soils, host trees and seedling quality, technical
methods (soil managing, truffle irrigation, pruning), T. brumale contaminations. Considering
truffle soils have not really been altered for one century (in truffle traditional areas), cultural
methods today are quite well known and seedlings are correctly mycorrhized, why is it so
difficult to increase the level of the actual truffle production? The common answer is generally
that climate has become warmer and dryer. Truffle growers who are irrigating their plantations
credit regular production to this effort. Nevertheless, results are still heterogeneous. One
observes that certain non-irrigated plantations can give quite regular results whereas irrigated or
non irrigated other ones have bad results. Why?
The research programme SYSTRUF, funded by National Agency of Research (ANR),
attempts to answer this question by studying particularly the biology of the truffle. The way of
fungal competition by different species and observation of what happens in the fields are
explored by the Station of experimentation on the truffle at Cahors-Le Montat. Why is the
presence of such fungi more negative today than in the past? In other ways, why is the black
truffle less dominant than in the past, why does it defend less against its potential aggressors?
MATERIAL AND METHODS
The three types of investigations involved surveys of the fungi species in the truffle environment,
formation on the natural “truffières” on the edge of the plantation, contamination of truffle trees
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in wooded environment or not wooded environment. The coherence of these three studies is
motivated by the last question above and will be discussed in the conclusion.
Fungi surveys in truffle environment in Lot district. These surveys were done on four types
of environments:
Limestone grassland surrounded by Quercus pubescens woodland.
Young truffle plantations (from introducing plants to the first harvested truffles).
Actually producing truffle plantations.
Old truffle plantations.
Study of the natural “truffières” on the edge of woodlands on natural pastures or
Limestone grasslands. The conditions of formation of natural “truffières” are analyzed in
traditional truffle areas. This type of truffières appears mostly in natural pastures, grassland or
cleaned fallow land (moorland), linked to Q. pubescens woodland on the edge of the field in
limestone areas. These situations are common in Lot district or other parts of Midi-Pyrénées
region (Tarn, Tarn et Garonne, etc.). One example is selected on the commune of Le Montat to
illustrate the phenomenom. This site is called grassland of Haute-Serre (pelouse de Haute-Serre)
(Fig. 1).
This state is briefly discribed :
Oaks on the edge, aged about fifty years, constitute a woodland relatively homogeneous, in
average 6 to 8 meters high , with 2 to 8 meters between every trees.
Tuber melanosporum « brûlés » appear on a strip located at 8 to 12 meters from the oak
basis. This strip is 3 meters wide and is called the « melano strip » (plage à melano ».
Some brûlés where Russula delica fruitbodies are observed at few meters from the basis of
the oaks (2 to 4 meters).
Figure 1 : grassland of de Haute-Serre on east side with the 2 dotted lines delimiting the strip where
Tuber melanosporum brûlés appear caused by the oaks on the edge. The 2 white and red stakes (1 m) at
the right frame a producing brûlé. At the left, the tall red and white stake gives an idea of the size of the
trees (one colour division = 0,5 m)
Beyond this report, we have investigated the fungal species repartition in the grassland of
Haute-Serre by the way of the mycorrhizae indentification (microscope) plus classical surveys of
fungi.
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Mycorrhizal mushrooms
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Conservation characteristics of initial mycorrhizal status by Tuber melanosporum or
contamination by other fungi species in truffle plantations surrounded by woodlands or
not. Examples chosen were explored in spring 2011.
Truffle plantations in wooded environments. These two explored plantations are located in
Aujols (Lot district) and Daglan (Dordogne district). They are established on shallow and stony
soils (rendosol), with mycorrhizal trees (Q. pubescens, Quercus ilex, Corylus avellana). They are
well maintained with care and producing despite relatively dry summer. Investigations on these
plantations concern the mycorrhizal status of the analyzed trees by the way of binocular loup and
microscope. At Aujols, roots samples were taken mostly on producing trees whereas in Daglan,
no producing trees were targeted.
Truffle plantation in cereals and vineyard plains landscapes. Explored plantations are located
near Angoulême (Charente district) in vast plain of cereals cultivation and wineyard region for
the Cognac production. There is neither woodland nor hedge of woods at least 300 meters from
explored plantations. They had been done on “terres de Groie” (rendosols deeper than those at
Aujols and Daglan) with mycorrhizal trees (Q. pubescens, Q. ilex, C.avellana), cultivated with
care. Irrigation was by bringing water with big tanks (3000 liters) and watering only producing
“brûlés”. Growers brought spores from crushed truffles the 3rd year of plantation on the “brûlés”
of young mycorrhizal trees which were not yet producing.
These plantations are producing better than those of the Lot and Dordogne districts. Harvest
started earlier (4 years old) and percentage of producing trees was also higher (90 to 95 %
instead of 30 to 60 %). Same type of investigations than above was done on these plantations.
RESULTS AND DISCUSSION
Results and their acquirement conditions are summarized without going into details in order to
keep this article a reasonable size.
Mycological survey in truffle environment in Lot district. It was impossible to present the
exhaustive mycological survey in the limits of this article. A synthesis was elaborated to
understand the essential.
Limestone grassland, suitable for T. melanosporum, presented common mycological
surveys. We observed that, both in grassland with Bromus erectus and Festuca ovina on edge of
woodlands and in moorland with sparse Qu. pubescens and Festuca ovina, T. melanosporum
could live with certain species (Inocybe jurana, Hebeloma edurum) but it excluded regularly the
other fungi out of its production area (located in the “brûlé”). Scleroderma verrucosum (Fig. 2)
was frequent at the limit of the T. melanosporum “brûlé” just starting or already producing. Its
presence did not seem to penalize the truffle production. Its frequent observation in this
condition means that S. verrucosum is preparing the field and helping to adapt and spread of the
truffière, transforming organic matter and structuring the soil in front of the brûlé.
Tuber brumale presence in young plantations is generally a consequence of a disturbing
technical itinerary which does not respect the fungi species dynamic and their repartition in
space and time. The ecosystem is disturbed by some cultural practices such as frequent tilling
with tractor-drawn tools. In these conditions, T. brumale, a well flexible species, occupies the
ecological niche initially planned for T. melanosporum. A bad choice of establishment
(insufficient drainage, soil with bad structure) or cultural methods unsuitable (soils compacted
with mechanic tools, fresh organic matter ploughing in soil, and excessive irrigation before
production) favours T. brumale at the expense of T. melanosporum.
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Section:
Mycorrhizal mushrooms
Figure 2: Scleroderma verrucosum Pers. on a truffle « brûlé ».
Figure 3: Tuber melanosporum mycorrhizae Figure 4: Hebeloma mesophaeum seen
(red ellipse) among infestation with Hebeloma sp. in a truffle plantation
on root system of inoculated plant.
Figure 5: Scleroderma verrucosum (first ellipse with metallic tool)
closer to the trunk than truffles (2 nd ellipse)
In producing plantation, “brûlés” which are producing T. melanosporum presented a very
few species except some Incoybes like I. jurana and I. splendens; sometime, there are
Hymenogaster (H. luteus) or Hebeloma sp. on the edge of the brûlé. On the other hand, trees
without truffle production were associated with many other epigeous fungi (Inocybe sp. and
Tricholoma sp. are frequent) and hypogeous (Genea sp., Hymenogaster sp., Tuber sp.), then
Russula sp. and Boletus sp. when trees were becoming older. These fungi were not located
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anywhere in the truffle plantation, particularly according to the “brûlé”. These species are those
of “old roots”, located generally closer to the trunk of the tree, under the organic litter.
In old truffle plantations, according to the density of closure of the canopy, we observed
thermophilic and limestone fungi usually found in Q. pubescens forest on calcareous soil. They
are species which are very difficult to eliminate from renovated old plantations in order to
produce again Tuber melanosporum. To succeed in renovating old plantations, it’s necessary to
open or clear and thin them, creating a variety of ecological niches. A good example exists with
an old plantation which is producing again T. melanosporum at Laburgade (Lot district) [11].
These mycological surveys confirmed that each species or group of species have their
specific ecological requirements corresponding with necessary conditions for the black truffle in
the dynamic of its environment [12]. These groups or species have to be located at a particular
place in the fungal succession (fungal train) observed with the environment change. Tuber
melanosporum has its good place or stage as well as many other species.
Table 1 of main fungi which are not Tuber and living in the truffle environment.
Fungi quite negative
Boletus luridus Mycorrhizal fungus in open environment with grass. There is no truffle under trees
producing this mushroom.
Russula lepida Mycorrhizal fungus in closed environment, mainly in old truffle plantation. Russula
lepida never lives with Tuber melanosporum under the same tree.
Russula delica Mycorrhizal fungus in open environment. It creates « brûlés » similar with those of
Tuber melanosporum
.
Russula maculata Mycorrhizal fungus in old truffle plantation and on the edge of woodland
Amanita strobililiformis
Amanitopsis
lividopallescens
Hebeloma sinapizans
Hebeloma edurum
Section:
Mycorrhizal mushrooms
.
Mycorrhizal fungi living in open limestone environment. When they are colonizing
« brûlés » producing T. melanosporum, truffle production disappears.
Mycorrhizal fungi living mainly in old truffle plantation or limestone oak plantations.
They persist in the soil when the forest is cleared. It was observed outside the “brûlé”,
doing a circle 0.50 to 0.80 m from the limit of the “brûlé”.
Armillaria mellea Dangerous parasite of the trees. It destroys truffle trees, mainly hazel.
It’s highly risky to plant truffle trees where this mushrooms is seen.
Fungi quite neutral
Incocybe jurana Mycorrhizal fungus quite frequent in truffle plantation. It is named « le truffier » in
Provence where people eat it. Present on the edge of the producing « brûlé », it
appears harmless for the truffle.
Scleroderma verrucosum Myccorhizal fungus frequent on the outer fringe of the brûlé. It does not seem
to interfere with Tuber melanosporum. It can coexist and fruit inside the brûlé.
Tricholoma scalpturatum
Tricholoma terreum
Mycorrhizal fungi living in old truffle plantation. It follows the decline of the truffle
production. It happens when there is an ecological disorder.
Helvella crispa Non Mycorrhizal fungus frequent in truffle plantation when autumn is wet and hot.
Clavaria aurea
Morchella esculenta
(and other morchella.)
Non Mycorrhizal fungus living in limestone oak groves and rare in truffle plantation.
Non Mycorrhizal fungus (?) rare in truffle plantation
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Table 1 summarizes perception of species roles for the most frequent species living in
truffle ecosystem in Lot district.
About the genus Hebeloma, Hebeloma mesophaeum, ubiquitous species all over the
planet, is a big concern in greenhouses where nurserymen make mycorrhizal plants. They fight it
with drastic sanitary precautions (Fig. 3-4).
Natural “truffières” on the edge of woodlands in natural pastures or Limestone grasslands.
Fungi observed in the grassland of Haute-Serre belong to the species characteristic of the
ecosystem of “natural pastures or Limestone grasslands on the edge of old Q. pubescens
woodlands”. A brief view of the main species observed with T. melanosporum is presented in
relation to the fungal succession (Fig. 6). We defined where Tuber melanosporum was located in
the limestone ecosystem previously described (Fig 7).
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Mycorrhizal mushrooms
Figure 6: Fungal succession observed at Haute-Serre.
Figure 7 illustrating where Tuber melanosporum is expanding in the conquest space.
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Proceedings of the 7 th International Conference on Mushroom Biology and Mushroom Products (ICMBMP7) 2011
Tuber aestivum is a pioneer fungi in Limestone grassland and moorland. Nevertheless, if
Tuber melanosporum fruits in this fungi succession, Tuber aestivum has a tendency to be
quiescent (“sleeping”), that is to say that Tuber aestivum waits for the closure of the environment
or woodland before it begins to fruit. In the meantime, Tuber aestivum is observed only on the
form of mycorrhizae. Its mycorrhizae do not tolerate other kind of mycorrhizae like those of
Tuber melanosporum.
The fungal succession observed at Haute-Serre is common in many areas in Lot district
and French south-west.
Pioneer species like T. melanosporum are established in the young root system whereas
basidiomycetae are on old roots. Tuber aestivum is located in front of or around the “brûlé” of T.
melanosporum.
Evolution of the mycorrhizal status in relation to the type of environment. At Aujols, roots
of producing trees were contaminated by basidiomycetae (Hebeloma sp.) on a radius of several
tens centimetres from the trunk. Beyond, T. melanosporum was mostly present under the form of
clusters of mycorrhizae (Fig. 8-11).
Figure 8: plantation at Aujols. Figue 9: sampling method under the tree and
every 60 cm at Aujols
Figure 10: view (with a measurement scale) of Figure 11: backlighting of a mycorrhizae
typical cluster of mycorrhizae of cluster of Tuber melanosporum which underlines
T. melanosporum. young mycorrhizae
At Daglan, roots from non producing trees were contaminated with many undetermined
fungal species. Inoculated trees with T. melanosporum on the edge of the woodland grew slower
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than the other ones inside the plantation (feeding competition?). They were strongly
contaminated from the oaks of the close woodlands.
We observed very dense root hairs on contaminated root system. On the other hand, roots
inoculated with T. melanosporum revealed a sparse root hair. Phenomenon of the “brûlé” from T.
melanosporum affected plants which can grow in the soil near the trees and the root system. Both
were attacked directly or indirectly by Tuber melanosporum.
Figure 12: comparison of 2 root samples Figure 13: close-up of the sample 8
respectively from a contaminated area and a
producing part of a Tuber melanosporum “brûlé”.
Taken with an equal soil volume, the two samples above (Fig. 12) illustrate what we could
observe in truffle plantation at the good age to produce. Sample 6, from a non productive area,
taken on a Q. pubescens shows a dense root hair. Sample 8, taken at 50 cm from the trunk of a
good producing Quercus pubescens, in the part of the “brûlé” where truffles were found out,
shows how the root hairs become sparse when the “brûlé” start to produce T. melanosporum.
Close-up above of the sample 8 (Fig. 13) shows how, in the productive part of the
« brûlé », mycorrhizae were mostly grouped together in a cluster which the maximum size is
about 8 mm as against 28 mm at Aujols. Note the tortuous shape of the root at the right side
which indicates many scares of abscission. These scares look like puffiness. Depletion of the
root system or lost of the root hair was typical inside the producing “brûlé”. It seems T.
melanosporum is attacking the root hair with a lost of some “branches” or ramifications.
If this aggression on a part of its root system does not kill the host tree, it is not the same
for many plants which disappear when the « brûlé » spread. Cistus albidus sometimes produces
T. melanosporum. In Corbières (Pyrénées-Orientales district), when « brûlé » is formed around
the shrub the year « n », truffle is harvested on year « n+1 », and Cistus dies the year “n+2”.
Tuber melanosporum « brûlé » accompanying production becomes fatal for the host shrub.
We have done analysis with molecular tools on the roots of plants usually living in truffle
“brûlés” or on the edge (Fig. 14). These plants were mainly Sedum sediforme, Festuca ovina,
Bromus erectus, Carex halleriana, Juniperus communis, Vitis vinifera. We had found out the
presence of T. melanosporum in the root tissues. This discovery in 2009 [13] validates the
hypothesis that the truffle is not only a mycorrhizal fungus. It could be a parasite of plants that
do not accept ectomycorrhizal fungi. When these plants are attacked by T. melanosporum, they
can die.
When black truffle is observed in a very clear or net « brûlé » (said virulent), which is
regularly gaining ground on the conquest space, we can observe that mycorrhization with T.
melanosporum is very exclusive in the production area. It is not the case of many other species
of fungi which accept cohabitation, particularly T. brumale.
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« Brûlé » expansion is an indication of « virulence » or aggressiveness of the black truffle
on its environment. The growth of the « brûlé » (10 to 30 cm per year in average) is the same on
the good truffle trees, regardless of the region or the kind of environment. Truffle virulence or
aggressiveness, pointed out in other words in books from the 19th century (de Bosredon, 1887),
is a permanent characteristic. We could observe these effects both on the root system and the
surrounding plants. We think that, in the past time, agricultural practices and pastoralism (or
shepherding) were stimulating this property of T. melanosporum to be aggressive.
Figure 14: plantation at Daglan with a good level of production. Contaminated trees on the right side
have not grown.
Figure 15: truffle plantation in Charente Figure 16: sampling of mycorrhizae near the trunk
district in a landscape with cereal and of very good truffle tree in Charente district
wine cultivation.
In Charente (Fig. 15, 16), T. melanosporum mycorrhized trees were poorly affected by
fungal contaminations. Contaminations (Hebeloma sp.) were located in a radius of 10 cm around
the trunk of the host trees (Fig. 17). We observed that the clusters of mycorrhizae formed by T.
melanosporum had a small size. They had few mycorrhizae in comparison with those from
Aujols and also from Daglan. It was difficult to find mycorrhizae on the roots inside « brûlé » of
the good truffle trees (Fig. 16). Fungi succession is limited or smaller than those we have seen in
traditional truffle areas.
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Figure 17: mycorrhizae of Hebeloma sp. With Figure 18: mycorrhizae said AD common
a characteristic mycelium reminding « cotton » in « brûlés » on edge of woodland
which catches soil in its net.
.
We are trying to understand these differences in the fungal succession and the size of
mycorrhizae clusters. When there is no woodland in the environment, it is logical that
contaminating mycorrhizal fungi like AD (Fig. 18) are rarer. Forming big cluster of mycorrhizae
could be a strategy to protect T. melanosporum against threat from other fungi. Maybe truffle can
be a threat against itself, after an intense vegetative phase, when « brulé » appears. “Brûlé”
appearance is concomitant with a depletion of the root system as if a truffle parasitic phase is
beginning. Big clusters of mycorrhizae could also be a mean to have a big quantity of mycelium
required for fruiting and feeding the fruit body when it is growing. These questions resulting
from the evolution of the mycorrhizal status, regarding the environment, make so many
hypotheses to study
CONCLUSION
Many species of fungi were observed in truffle environment. This fact emphasizes the difficulty
to stimulate only one species cultivation, such as T. melanosporum, in space widely open to the
contaminations with other fungi. Tuber melanosporum naturally finds its place in the dynamic of
the environment as far as this one is weakly artificialized (no frequent tillage which reduces the
biodiversity). In agricultural landscapes, without oaks hedge and woodland, truffle production is
easier and earlier.
From these observations, we can recommend to the truffle growers certain
precautions[14] summarized as following : 1) choose top-quality host trees (tree species suited to
the environment, well mycorrhized) ; 2) limestone soil, aerated, well draining, good biological
activity; 3) adapted cultivation techniques (tilling or plantation cleaning depending on the depth
of the soil and physicochemical characteristics, truffle trees irrigation, pruning ) ; 4) manage a
favourable fauna and flora without excess to limit feeding competition but sufficient to maintain
the necessary biodiversity ; 5) improve mycorrhizal potential by bringing truffle spores ; 6) take
precautions with oak woodlands on the edge of plantations (cut oak roots from the edge with
chisel, put mycorrhized trees at a good distance from the edge of the woodland creating a health
perimeter) ; 7) thin the plantation in order to maintain a sufficient conquest space for the truffle
(T. melanosporum is a pioneer fungi who needs space in its limestone environment).
A digression can be done about the truffle soil characteristics after the paragraph above.
Discussion on this topic is still in progress and there are many advanced researches on it. In the
traditional truffle areas, the physicochemical characteristics of truffle soils have not changed
during one century. Of course, the consequence of a frequent tilling is a fast evolution of the
organic matter. Soils with a high level of clay become more compact when they loose organic
matter. Their cohesion and stability diminish. If the chemical fertilisation was vulgarized in the
20 th century, it has not really affected the traditional truffle region with shallow soils. Moreover,
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we observe very good results in truffle plantations established in large plains where there are
cereals and vineyards cultivations without any oaks in hedge or woodlands near the plantations.
Chemical fertilizers and soil fertility are not main factors to explain why some plantations cannot
begin to trigger truffle production.
After at least one century of agricultural abandonment, we observe many oak woodland
surrounding truffle plantations. They have grown naturally and progressively. Nowadays, they
are penalizing the truffle production. Their impact depends of the « contaminating reach » of the
oaks (10 meters in average) and of the « power of contamination » of the oaks on the edge. If the
oaks on the edge are grouped in woodlands, their power of contamination is stronger than if they
are in hedge [15]. This situation underlines the necessity to build « truffle bastion » in order to
keep all T. melanosporum strength (power = strength x number of producing truffle trees). In the
heart of big truffle plantations, there is a “truffle bastion” protected against fungi coming from
outside.
The strength of the truffle is stated trough the concepts of « truffle virulence » or « truffle
aggressiveness » which can be observed with “brûlés” spreading or gaining ground in good
truffle plantations. In South-West of France, default of T. melanosporum “virulence” or
“aggressiveness” is generally concomitant of 4 facts: 1) T. brumale is harvested instead of T.
melanosporum, 2) T. melanosporum does not resist well during the drought, 3) truffle production
has not durability in time (tree years in average in wild “truffières”), 4) harvest starts later in
plantation after many years (as if there was an inertia to fruiting).
After the characterization of the lack of the truffle virulence, we can propose the opposite
arguments to define positive truffle virulence. Many experiences or trials are done at the Station
of experimentation on the truffle at Cahors-Le Montat and in some truffle areas in France and
abroad. For these trials, we study another factor which could be the impact of the domestic
animals like sheep and horses. We had observations which involve these animals in exceptional
truffle production fields. We can suggest an analogy with other mushrooms production: Agaricus
campestris L. and Pleurotus eryngii (De Cand.:Fr.) Quélet need respectively in their grassland
habitat cows and sheeps.
We humbly recognize some explanations are still lacking. Why in 2006 (a good climatic
year), the Lot district did produce 3 tons of truffles whereas its production was more that 300
tons in 1906? This 100 factor seems to summarize this lost of “virulence” or “strength” of the
black truffle. This decline is usually explained by the deterioration of the T. melanosporum
ecosystem, consequences of changes in human activities, mainly agriculture, animal farming and
forestry. With less traditional activity, oaks woodlands are more and more present with their
fungi of forest ecology.
Why is the black truffle less dominant than in the past, why does it defend less against its
potential aggressors? This is the double question asked in the introduction. Does sick black
truffle like other mushrooms suffer pathogen attacks like virus? This hypothesis is supported by
some scientists [16] to explain the difficulty of truffle production. In this article, we favour that,
during the abundance period, there were practices of mixed farming with organic matter
restitution, biodiversity management. The rural world was entirely exploited and cultivated.
Population density in the country was higher than today. Woodlands were rare and firewood was
precious and collected. In limestone areas, landscapes were very open for the pioneer fungi like
T. melanosporum: its “strength” or “virulence” was at the optimum. Furthermore, the harvesting
pressure was lesser than nowadays, leaving in the soil a lot of truffle spores useful as natural
inoculum. There were no inoculated controlled mycorrhized plants to sell and buy; nevertheless,
there was an abundance of truffles.
Section:
Mycorrhizal mushrooms
527

Proceedings of the 7 th International Conference on Mushroom Biology and Mushroom Products (ICMBMP7) 2011
ACKNOWLEDGMENTS
These results were obtained with the financial support of the trials and research programmes by
FranceAgriMer (ONIFLHOR, VINIFLHOR), Région Midi-Pyrénées, Conseil général du Lot,
Ministère de l’Agriculture, de la ¨Pêche et de l’Alimentation, Europe (FEOGA). Europe
participates to the results vulgarisation with the support of FEADER (mesure 111B).
We thank: Truffles growers who have allowed investigations in their plantations, Laurent
Génola, truffle technician, who did many mycological surveys and made comments on truffle
soils, William Saenz, truffle technician, for his constructive remarks about cultural techniques,
Jean-Marc Olivier, coordinator of the truffle experimentation in France, for his encouragements
and all his advice and corrections on this work and many others, Rodrigo Donoso who counsels
me for the English language.
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