Pilar 6: Instrumentos de concientización e información

The greatest limitation to trufficulture in south-west WA is most likely to be insufficient annual rainfall and the availability of good quality water for irrigation during the summer months. The increased risk of surface and groundwater salinity in the lower rainfall zones (<1000 mm) will most likely limit the establishment of truffières particularly given that this can be compounded by irrigation practices (National Heritage Trust 2001). Evaluating potential truffle growing areas involves consideration of two very different entities – the truffle fungus and the host plant. Suitable soil moisture and temperature conditions are presumed to be of utmost importance for optimum plant growth but also mycelial growth, formation of primordia, growth and maturation of the ascocarp but to this point in time such conditions have not been sufficiently defined.

Soil temperature comparisons of productive sites are limited given there are few available datasets, however, approximations of ideal soil temperatures exist in the literature. Callot (1999) describes a good truffle producing year as that where soil temperatures (c. 15 cm depth) generally exceed 15°C by the beginning of spring (October) and are >23°C during the summer months (December-March). The significance of warm spring temperatures was demonstrated by the development of immature truffles when soil temperatures in late spring (November) were >15°C (Kulifaj 1994 as cited by Callot 1999). Low temperatures (5-10°C) are considered important for maturation of the ascocarp in the winter months. Generally there is a marked oscillation (amplitude) in soil temperature at good truffle producing sites. Hence, it has been suggested that potential truffle growing sites should have a mean annual soil temperature of ~15°C with a minimum yearly amplitude of >8°C (c. 15 cm depth) (Callot 1999). A poor truffle year would constitute prolonged low soil temperatures (<15°C) during spring and temperatures not regularly exceeding 23°C in summer. However, many productive sites in the SH do not comply with these recommendations from Callot (1999) as many have lower mean annual, lower maximum monthly temperatures and lower annual amplitudes then the le Montat site (Figure 2.6, Table 2.2). It is acknowledged that a decline in production at the Gisborne,

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NZ site in recent years is associated with a decline in annual soil temperature and annual amplitude due to canopy closure (I. Hall pers. comm.).

The significance of cool soil temperature for the maturation of the ascocarp during the winter months is unknown. The warmer conditions of the productive Hazel Hill site would suggest it is not necessary although a truffière needs to be at full production to confidently confirm this. Nonetheless, if there is a requirement for cool winter soil temperatures it may not be possible to establish productive truffières in northern and coastal parts of the south-west where air temperatures are moderated as indicated by the increasing cumulative degree days with decreasing latitude, impacting on soil temperatures as a consequence.

Determining a suitable climatic range for the host species is less problematic given the relative importance of both major host species for other commercial purposes. In the more southern regions of the southwest, VPDs are lower which are likely to favour the use of more temperate host species (e.g. hazel, hornbeam). Northern parts of the region where VPDs and cumulative degree days are higher, and are less significant for defining developmental stages in certain plant species, could favour the use of oak species that tolerate warmer conditions (Bolstad et al. 2003; Tretiach 1993) and have more extensive root systems to capture available moisture deeper in the soil profile.

It is well documented that yield of black truffle improves with irrigation to supplement summer moisture levels and hence it is encouraged in artificial truffières (Hall et al. 2001; Mamoun and Olivier 1993a; Mamoun and Olivier 1993b; Sourzat 1994; Sourzat 2002). The literature suggests irrigation is only for the benefit of the truffle ascocarp and not for maintenance of host plant growth. As an example, in late summer on the rendosols of the le Montat site in SW France, the brûlé is irrigated with 20 mm at 20-26 day intervals (Sourzat 2002) to maintain growth of the ascocarps. At the Hazel Hill site, which is situated on deep duplex kandosols, there was a change in the drying cycle as indicated by soil moisture in response to the altered watering regime in 2003/04. Without further experimentation it is impossible to determine whether such a regime is appropriate to maintain adequate growth of the ascocarp in these soils. It is

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inappropriate to suggest the increased production at the site in the following winter (increased by 4.2 kg) is the result of this irrigation regime without rigorous experimentation (more likely the result of maturation of the fungus in the soil). It is not clear if humidity of soil air spaces is more appropriate than complete saturation of the soil to maintain ascocarp growth and development. The primary difficulty in interpreting the effectiveness of changes in truffle management is locating ascocarps early enough to monitor the impact of various irrigation regimes in a statistically sound manner.

A further complicating factor in evaluating truffle production is soil moisture conditions during critical initiation and growth stages. Callot (1999) found that a good truffle season should consist of heavy rain periods during summer with periods of dryness not exceeding 21 days. The author found conjecture as to whether significant rainfall events in spring or in autumn cause greater harm to truffle yield. Callot (1999) suggests soil type and underlying geology is likely to play a major role in determining soil moisture properties in any given region.

Soil type has not limited trufficulture in many areas outside Europe given the diverse range of soils of productive truffières documented in this chapter. Similarly, soil type need not limit the expansion of trufficulture in the south-west other than in soils that have poor structure, drainage and inappropriate chemistry. This would exclude much of the Swan coastal plain where naturally occurring alkaline sandy soils exist. Truffle production in WA has, so far, occurred in deep loamy soils that are considered prime soil for horticultural pursuits. These soils occur extensively in the >800 mm rainfall zone, often as dissected laterites with variable soil depth. The existing pH of these soils is in the range of 5.1 to 6.8 and lime amendment of these soils and its incorporation to sufficient depth is necessary to raise the pH to between 7.9 and 8.5. Lime requirement of individual soils is dependant on its physical and chemical structure and consideration should be given to past and future fertilizer practices given the tendency for sub-surface acidification in these soils.

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The climatic boundaries of traditional trufficulture have, to some extent, been redefined by trufficulture in the SH. Until recently, the potential climatic range of T.

melanosporum production in the SH had not been thoroughly explored. Our

understanding of the important climatic factors is limited and is based on restricted climatic datasets and, often, anecdotal records of production. There is a need for comparison of climatic conditions from a range of sites in both hemispheres (particularly European sites) in conjunction with concise production records to make informed assessments of suitable new sites for trufficulture. In addition, there needs to be rigorous experimentation in manipulating soil moisture and temperature conditions to attempt to understand more about the interaction of climate, soil and fungi that will improve management practices and yield in truffières.

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