Cultivation of black truffles in Western Australia

Introduction

Truffles are the fruiting bodies of soil fungi that live in a symbiotic relationship with the feeder roots of trees. The combination of the fungus and the root is called a mycorrhiza (plural: mycorrhizae). There are some 5000 species of truffle-producing fungi though only 70 of those produce truffles that are edible by humans.

The truffle contains the spores (analogous to seeds) of the fungus and when the spores are ripe the truffle develops characteristic aromas that attract animals to dig them up and eat them. The spores pass through the animal's digestive tract and are deposited in the scat where they can germinate and begin a new the symbiotic relationship with appropriate host trees. Each species of truffle-producing fungi will form a relationship with a limited number of tree species, however trees are less selective in their hosting of different soil fungi on their roots.

Traditionally truffles are harvested from woodlands, however several species can now be cultivated. Host tree seedlings are inoculated with truffle spores in the nursery before planting out in the field, and production and harvest begins five to seven years later. Specially trained dogs are used to locate the truffles which are found in the top 30cm of the soil profile.

The genus Tuber produces the most highly prized truffles in Europe, and several species are sold in local markets at various times of the year. Two species of truffles are particularly highly prized - the Italian white truffle (Tuber magnatum) commands the highest prices but has not yet been successfully cultivated, while the French black truffle (T. melanosporum) is both highly valued and has been cultivated. Table 1 lists some of the commonly used truffles from around the world.

It is possible to confuse different truffle species in the market. Testing to determine the type can be done using DNA analysis or microscopic inspection.

Care must be taken when inoculating trees to ensure the correct truffle spores are used. Different species of truffle, and indeed other fungi, can compete in the soil resulting in reduced yield of the desired truffle. Tuber brumale, in particular, is very competitive.

While truffles have been eaten and written about for centuries, it is only since the late 1960s that systematic research has been undertaken on the conditions needed for growing truffles in cultivated orchards. Hence our knowledge is still developing. Presented here is the best available information at present for the conditions required in Manjimup, Pemberton and Northcliffe where black truffles are being cultivated. Much of the information may be applicable to other areas and growers are advised to consult with local agronomists and horticultural personnel who have a specific interest in truffle production, as well as undertaking their own research before deciding on a course of action.

The life cycle of the truffle is somewhat different to that of the more familiar above-ground mushrooms. Whereas mushrooms develop rapidly after rain events in the appropriate season, truffles are initiated in summer and early autumn and continue to grow before maturing in the cold winter months. Hence, by the time a truffle is mature (and aromatic) enough to attract the attention of a well-trained dog, it will have been developing underground for up to eight months.

If truffles are harvested or even disturbed before they have developed the quintessential aromas associated with maturity, they will not continue to mature or develop and are likely to rot. Current evidence suggests that as the truffles grow in late summer and autumn, they continue to derive all their nutrients from the tree via mycorrhizae.

Western Australian truffle industry

The Western Australian truffle industry is based on the French black truffle or Périgord truffle (Tuber melanosporum) grown in association with oak trees (mainly Quercus robur and Q. ilex) and hazelnut trees (Corylus avellana). Seedlings are inoculated with fungal spores from mature truffles before being planted out into orchards (or trufferies). Truffle production usually begins five to seven years after planting.

The first commercial plantings of truffles in Western Australia occurred in 1997, with the first harvest in 2003. While black truffles are successfully cultivated around Manjimup and Pemberton, it is not known whether production will be successful in other areas. Inoculated oak and hazelnut trees have been planted in the Perth Hills and at Toodyay, north-east of Perth, as well as Margaret River, Busselton, Nannup, Donnybrook, Bridgetown, Walpole and Denmark.

Planning a truffle orchard

Site selection

The primary considerations for site selection include climatic conditions, soil texture and structure, slope, past use, aspect and availability of water for irrigation.

There are number of different factors to take into account when selecting a site for a truffle orchard, including soil texture, slope and availability of water for irrigation.

Climate

Truffles and their host trees are native to south-west Europe, namely France, Italy, Spain and parts of the former Yugoslavia. The climate in that region is continental to Mediterranean, with cold winters, warm to hot summers and rainfall throughout the year.

Where trees are not irrigated, sufficient summer rain is needed to maintain some degree of moisture in the root-zone for the developing truffle. It appears that declining rainfall, especially in the summer months, is a factor in the observed decline in European production of truffles in the last 80 years.

Truffles grow best in regions with warm summers and mild to cold winters. The warm summer temperatures are needed to initiate truffle formation while the cold winter temperatures appear to be necessary to trigger final maturation of the truffle and development of the aromas.

Manjimup has an average maximum summer temperature of 26.4°C and an average minimum winter temperature of 6.8°C. Rainfall averages 1000-1100mm annually, mainly between April and November. The Pemberton climate is very similar.

Truffle orchards in Western Australia (WA) require irrigation from late spring for optimum production.

Soils

Key soil considerations are good aeration, good drainage after rain or irrigation, 'fluffy' friable structure to allow the developing truffle to grow, and a high pH with abundant calcium. While pH is amenable to adjustment, drainage and structure can be more difficult to significantly alter and sites should be considered with these two features in mind. Drainage is of paramount importance. Rainfall in WA is predominantly in the autumn-spring period when truffles are growing and maturing. Waterlogging during this period will lead to increased risk of rot and reduction of saleable truffle.

Further, both hazelnuts and oaks are susceptible to root rots if their roots are in very moist or wet soils for any period of time, especially when actively growing in spring and summer. Some sites may be engineered to reduce waterlogging but this is an expensive option. Slopes are likely to provide good drainage and such sites are preferable, providing machinery access is safe.

The best soils appear to be the sandy loams to clay loams with 10-30% clay content. Very sandy soils will drain rapidly and provide little water-holding capacity, while heavy clay soils will remain wet for long periods and may inhibit the growth and proper maturation of the truffle. In their native areas and in WA, truffle production occurs in soils with some stones or gravels which assist with drainage, contribute to aeration and may provide protection against soil compaction. Many of the soils in the Manjimup area contain pea gravel which provides good drainage but this can affect the shape of the truffles. Truffles obtained from soils with less gravel may be rounder and have better shape.

In Europe, truffles grow on well-drained alkaline loams with a relatively high level of calcium. The acidic soils in the Manjimup/Pemberton area and most of southern Western Australia require considerable quantities of lime, usually in the form of agricultural lime, to increase pH to greater than 7.5 (measured in water). Dolomite lime supplying magnesium as well as calcium can also be used if magnesium is deficient. Lime may need to be added periodically for the life of the orchard.

A soil test should be carried out on each different soil type in the proposed orchard to determine the soil depth, the pH and the levels of macro and micro-nutrients. Truffle production is best in soils with low to moderate levels of all plant nutrients. Excess nutrients as a result of previous intensive farming activities may adversely affect maintenance of the mycorrhizae.

Planting source

Truffles are grown in association with a host tree, most commonly hazelnuts (Corylus avellana), or oaks such as English oaks (Quercus robur) or Holm oaks (Quercus ilex). In orchards with hazelnut trees, the hazelnuts are generally not collected commercially.

Inoculated hazelnut and oak seedlings for commercial growers are available from specialised nurseries. If planting stock is being brought from interstate, Quarantine WA biosecurity requirements must be met.

Inoculation techniques are theoretically quite straight forward, however problems occur due to contamination with unwanted competitive species, or poor colonisation of the roots. Growers are advised to obtain inoculated plants from a commercial nursery. Commercial production methods are still closely held secrets. Growers are strongly advised to have their plants independently assessed for tree quality, for percentage of roots successfully colonised and for purity of the colonising species. An independent tree evaluation/certification process is operated through the Australian Truffle Growers Association. Several truffle orchards in eastern Australia have been found to be contaminated with Tuber brumale, an inferior truffle species that may outcompete T. melanosporum. It is incumbent upon growers to ensure their plants have been correctly inoculated before they plant them as it is virtually impossible to remove contaminants such as T. brumale from the field. Further, it is not feasible to attempt to re-inoculate mature trees if they lack the truffle fungus at planting out.

Managing the truffle orchard

Planting

Soil preparation including weed removal, pH amelioration and deep ripping should begin well in advance of planting. It is also recommended that irrigation infrastructure is completed before planting. This ensures that no further heavy machinery is taken into the truffle orchard where it can cause damage to trees and root systems or soil compaction. Soils should be ripped before planting to break up any hardpan or compacted layers and improve drainage.

Tree spacing varies widely. Some orchards planting predominantly hazelnuts may plant to a 3 x 5m grid. Hazelnuts are relatively easy to maintain as medium-sized trees although the removal of suckers can be very time consuming. Oaks need considerably more space and may be planted out to 6 x 6m. Oaks eventually grow into large trees and even this spacing will see the canopy close within 10 years if not rigorously pruned or hedged.

Whatever the planting density, the number of trees per hectare can easily be calculated by dividing 10 000 by the two planting distances. For example the spacing 3 x 5m will give 667 trees per hectare, a 6 x 6m spacing will need 278 trees per hectare.

Many growers in Manjimup/Pemberton region grow a combination of hazelnuts and oaks, with plans to remove the hazelnuts when the oaks grow very large. There are no clear guidelines available as yet as to the best spacing or planting combination and this may depend on local factors.

Many truffle orchards have a combination of oak and hazelnut trees.

Growth and irrigation

The truffle fungus has a symbiotic (mutually beneficial) association with the host tree by means of mycorrhizae, which are the combinations of the tree feeder roots with the truffle fungus hyphae.

The fungus supplies the tree with nutrients from the soil and may make soil water more available to the tree, while the tree provides the fungus with a place to live and supplies carbohydrates (sugars) for growth.

Once established, both oaks and hazelnuts grow quite well even under semi-drought conditions. Similarly, they will both struggle through competition with weeds. Studies from Spanish orchards indicate that the level of mycorrhization is best maintained when the tree is provided with adequate water, and weeds are removed from the zone where the feeder roots are actively growing. Weed management is thus particularly important in the first years after planting. Weeds may be manually removed or weed-matting used.

Irrigation is emerging as an important factor in determining the level of truffle production in an orchard. The truffles are formed in the heat of summer and autumn and continue to grow for up to eight months. During that time they require moist soil, or are liable to dry out and die. In their native environment, rainfall is often fairly consistent throughout the year. Summer and autumn rainfall is in the form of thunderstorms with heavy falls of short duration. A study of European black truffle production has shown that between 1970 and 2006 the decline in truffle production was correlated with a decline in the amount of summer rain.

The southwest of WA, has a strongly winter-dominant rainfall. Hence, irrigation is crucial for truffle production. In the establishment phase, trees may need watering every second or third day, depending on conditions and soil type. Micro-sprinklers delivering 40-70L/hr over a large area (3-4m radius) are commonly used. Most such sprinklers have a tab to limit the area of watering in the first year. This will assist weed management through the hot months.

It is important to regularlly inspect your irrigation system to ensure it is in good order and functioning properly.

As trees mature, growers generally replace at least 50% of evapotranspiration every 7-20 days. Rates of evapotranspiration can be obtained from Department of Agriculture and Food, Western Australia (DAFWA) weather stations. This area of truffle production is the subject of ongoing research. Some growers water more frequently. The outcomes of these variations will become apparent as the industry matures. There is already some evidence to suggest that frequent watering encourages the production of truffles close to, or at, the soil surface. Such truffles are more prone to rot and in most instances should be discouraged.

Estimated irrigation water required for mature truffle orchards in the Manjimup/Pemberton region is 2000-4000kL/ha/year. Water should be of a high quality with low levels of soluble salts. As a general guide, fruit and nut trees exhibit some salt damage when irrigated with water containing greater than 600ppm total soluble salts (EC dS/m >1.0 ). As both groundwater and surface catchment water is highly variable in the South-West of WA, potential irrigation water should be tested as part of the site selection process.

Fertiliser

The use of fertilisers in truffle orchards is the subject of ongoing research. One of the advantages of the mycorrhizae for the tree is that the fungus can explore the rhizosphere (soil in the root zone) and make nutrients available to the tree that would normally be locked up. For this to occur, the nutrients need to be present, at least in small amounts. In general, trees will require moderate levels of all plant nutrients, ensuring that none are in excess as this may lead to decreased levels of mycorrhization on growing feeder roots. The high pH needed for truffle production may lead to reduced availability of nutrients such as iron, manganese, copper and zinc, especially in younger trees.

Canopy Management

Hazelnut trees are commonly grown to a vase shape with one main trunk. They will require constant de-suckering to prevent the natural tendency to form several stems and a bushy canopy. Several growers are allowing two or three 'trunks' to develop as insurance against the single trunk being damaged and leading to death of the entire tree.

Hazlenut trees are commonly pruned to a vase shape and all new suckers removed to maintain one main trunk.

Oak trees are grown with one main trunk and are gradually pruned so as to provide access for truffle dogs and people harvesting the ground beneath the canopy. Some oaks show a definite weeping habit in their branches and will require constant pruning to prevent foliage laying on the ground. Oaks can grow very large, greater than 20 metres, and with considerable spread. Traditionally, they may be coppiced to manage their size, though this has not yet been attempted in truffle orchards in Australia.

Both hazelnuts and oaks are best pruned in autumn or spring. Pruning should be undertaken in dry weather where possible to reduce the risk of fungal infections to the pruning wounds.

Some evidence from Europe suggests that in old plantations and natural woodlands the production of truffles "retreats to the edge of the forest". The reasons for this are not clear but may include lack of sunlight or warmth on the soil or even excessive mulch (leaf litter) build-up on the soil under trees. Until we have answers to this issue it seems prudent to maintain an open canopy where possible.

Pests and diseases - trees

Young trees are susceptible to several pests, in particular African black beetle, weevils, spring beetles, wingless grasshoppers and snails.

African black beetle damage is mainly caused by the adult beetles and occurs at or just below the soil surface, effectively ringbarking the tree. The impact can be minimised by leaving the area bare for as long as possible, ripping along the planting rows, removing any grasses, especially kikuyu, and with the use of grow guards that are set about 5cm into the soil around the tree. This prevents the beetle accessing the new tree stem. Searching for and removing any larvae or beetles from the planting soil, followed by use of a tree guard can be very effective.

Weevils, beetles and grasshoppers can decimate the foliage of young trees and larvae can chew roots leading to stunting and stressed trees. Monitoring numbers in the field prior to planting can give an indication of the problem and informed management options.

Birds such as ducks, guinea fowl and chickens at a stocking rate of 50 per hectare can be very effective at beetle control and have the added benefit of controlling other insect and snail pests. Guinea fowl and chickens do dig when they dust-bath which may not be beneficial for the developing truffles, however ducks do not tend to do this.

Snails may chew young leaves of seedlings and are more likely to damage hazelnut seedlings than oak seedlings. Trapping and physical removal of snails is likely to be effective at this stage in the orchard life cycle. Maintaining weed-free areas around new trees will also minimise damage. Small pointed snails can interfere with sprinklers. Snails and slugs are potentially a problem for the growing truffle as discussed below.

Mature trees are less susceptible to damage by pests, however in some areas fruit tree borers can severely damage trees. These are best managed by completely removing any affected trees and burning the wood.

Oak powdery mildew (Erysiphe alphitoides) is often observed on trees in spring. This mildew is a different fungus to that seen in vineyards (grapevine powdery mildew - Erysiphe necator) and there is no cross-infection between vines and trees. Warm damp spring conditions favour formation of mildews in many horticultural crops. Spring infections with powdery mildew can be severe in trees already stressed for other reasons, such as mineral deficiency or water-logging. Late summer and autumn infections often occur on senescing leaves and are unlikely to be a problem. In general, no treatment is required for powdery mildew.

Pests and diseases - truffles

Snails and slugs appear to be the major pests of growing truffles, followed by springtails (Collembola spp.), fungus gnats and truffle beetles. Encouraging truffles to form below the soil surface appears to be the single most effective defence against all pests at present.

Mammals including small marsupials, rabbits and pigs can be troublesome by digging up both ripe and unripe truffles. Good fencing will prevent most of these animals entering the orchard. If feral pigs are present, electric fencing will be needed as pigs are particularly fond of truffles. The presence of pigs should be reported to the local DAFWA office. Kangaroos may come into the truffle orchard to eat nuts and acorns but are not known to eat the truffles. They can cause damage to fences so monitoring their activity is important.

Weed control

During the establishment phase trees will benefit from good weed management. Research indicates that weed minimisation will lead to increased mycorrhization of the new feeder roots, which in turn will improve production. Weeds may be manually removed. Spanish research has found that weed-matting is effective. Deep mulching is not recommended as this may encourage shallow feeder roots which in turn leads to more truffles being formed at the soil surface.

Hunting and harvesting

The truffle is the fruiting body of the fungus. It is located up to 30cm below ground and weighs between 30 and 300g, but can be as heavy as 1.2kg. The shape of truffles varies wildly with some being a uniform round shape while others may have numerous crevices and lobes.

The shape appears to be partially influenced by the soil. Truffles formed in soft soils with fewer stones tend to be rounder while truffles formed in hard stony soils may be knobblier in appearance. While the shape does not seem to confer any particular aromatic characteristics and ugly knobbly truffles are just as likely to have the same aromatic characteristics as round truffles, restaurateurs and retail outlets may favour the more evenly-shaped truffle.

Harvesting commences in the Manjimup/Pemberton area in late May and continues until early September. Dogs and their handlers usually begin hunting in the early morning. High winds can impair the dogs ability to locate the ripe truffles. Dogs of any breed can be used provided they have a good sense of smell and are capable of being trained. They will also need to have some endurance as detection requires lots of walking. Common breeds include spaniels and Labradors.

It is important to train dogs to detect ripe healthy truffles to prevent disturbance of immature truffles. In the early part of the season, dogs may detect slightly immature truffles. If the truffle is not disturbed it may continue to mature in the following days. Handlers and harvesters will carry a small sharp knife to nick a small piece of skin from the truffle to determine whether it is fully mature. As the season progresses this problem diminishes.

Dogs are generally trained to 'mark' the site where they smell the truffle by lightly scratching the ground. The handler will smell the soil, usually by taking a handful or trowel-full of earth. The aromas of a ripe truffle permeate the local soil creating a rich sweet earthy smell. If this is present, the dog is rewarded and the truffle is dug up either by hand or with a small trowel. It is often more practical to have a second person to dig up the truffles after the dog handler has marked the sites where ripe truffles have been detected.

Trained dogs are used to locate mature truffles

Female pigs can be used, as one of the principal truffle aromas is very similar to a pheromone produced in the saliva of male pigs. However, pigs will eat the truffles if they can and it can be very difficult to prevent a 100kg pig from eating truffles.

Grading

Harvested truffles are thoroughly washed and may require scrubbing with a small brush to remove all dirt. Large producers may utilise a modified vegetable washing assembly to speed up the process. Once truffles are washed and dried they are then individually graded.

The aromas of healthy ripe truffles vary markedly, even in truffles harvested from the same orchard. Nonetheless, damaged truffles will emit very unattractive odours, while unripe truffles will lack appreciable aromas. Grading involves assessment of the aroma and then the shape and health of the truffle. Each truffle is smelled by an experienced grader to detect any off-notes that may indicate a sub-standard truffle or internal damage. The truffle is then visually assessed and weighed.

Truffles are cleaned and trimmed before being graded.

Grading in WA is based on a modified version of the European UNEC Grading Standard (2010).

Marketing

Production of truffles in Australia is counter-seasonal to that of Europe and most production is exported. Current exports go to Europe, North America and Asia. The acceptance of Australian grown black truffle has taken considerable effort.

Using truffles

Fresh truffles are best stored wrapped in paper towel in a glass jar (with lid on) in the fridge. The paper towel should be changed every couple of days. The glass jar is necessary to prevent the entire contents of the refrigerator becoming infused with truffle aromas. A healthy truffle may keep for up to two weeks if stored in this way. Truffles are traditionally sliced very thinly using truffle planes, but a very fine grater can also be used. Truffles can be snap-frozen, though the aroma profile will change. Frozen truffle is best planed or grated while still frozen as it will become mushy when thawed.

Truffles added to recipes, generally at the end of cooking, will add earthy, blue-cheese, mushroom, garlic and chocolate notes, depending on the quantity used and the other ingredients. Truffles may also be added to recipes to act as a general flavour enhancer, without adding a strong truffle character to the dish. This latter method works well in meaty casseroles or pies. Truffle lovers can of course boost flavours by adding grated or planed truffle at the table.

A delicious way to try truffle is to grate or plane very thin slices of fresh truffle over foods such as scrambled/poached eggs just as the cooking is finished. This also works very well with steaks. Grated or planed truffles can be added to mashed potato and left to infuse for a couple of minutes prior to serving. Creamy pasta dishes also benefit from the addition of truffle at the end of cooking. Usually, 5-10g per person is used in a dish.  Eggs stored with truffles will be infused with the truffle aroma which is retained when lightly cooked.

Further reading

Bonet et al 2009, Cultivation methods of the black truffle, the most profitable Mediterranean non-wood forest product; a state of the art review. EFI Proceedings no.57. Modelling, Valuing and Managing Mediterranean Forest Ecosystems for Non-Timber Goods and Services. Marc Palahi, Yves Birot, Felipe Bravo and Elena Gorriz (eds).

UNECE STANDARD FFV-53 concerning the marketing and commercial quality control of truffles, 2010 edition.

Gareth Renowden, The Truffle Book. Limestone Hills Publishing 2005.

Ian Hall, Gordon Brown and Alessandra Zambonelli, Taming the truffle: the history lore and science of the ultimate mushroom'. Timber press, 2008, Portland OR/US.

Acknowledgments

Christine Fischer, Forest Sciences Centre of Catalonia Spain; Mel Booth, Truffle Dogs WA; Al Blakers, Manjimup Truffles; Anne Mitchell, Manjimup Underground and Celeste Linde, Australian National University.