Managing the truffle orchard
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.
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.
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.
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.
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.
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.
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.