Sex, money and absent fathers: it’s truffle season

FROM VISIONS OF RUSTIC Frenchmen stalking through misty forests in search of arboreal delicacies to the drama of haute cuisine, there’s a romance to truffles, the pungent fungi.

But when it comes to understanding its mysterious mating habits, the romance is less Alexandre Dumas and more Mills & Boon, featuring secret sexual encounters and a wide range of daddy issues.

For the next few months, truffles are back on the menu in Australia. Most sought-after is the French, or Perigord, black truffle (Tuber melanosporum) with its musky, garlicky flavour and extravagant price – truffles are being sold lately in Victorian farmers’ markets for $2.50–$3 a gram, or $2,500–$3,000 per kilo.

But for such a lucrative crop, scientists and truffle growers still aren’t completely sure how they reproduce. One truffle, the even-more prized Alba white (Tuber magnatum pico), was only cultivated for the first time last year following nine years of research.

Furthermore, a key detail in how truffle “fruit” was only unearthed in 2010. This is because some truffle types need to have sex first, but researchers are mystified as to where the “fathers” go once they’ve done their duty.

“Growing truffles is a very, very new field,” says Dr Alexis Guerin, a French scientist and mycologist – fungi specialist – now transplanted to New Zealand. “We’re talking 44 years since the first cultivation of a truffle.

“Compare this with the cultivation of wheat, rice or fruit trees, where we have thousands of years of experience. So I like to say we are at the start of the journey.”

Let’s talk about sex

A truffle is the fruit of a mycorrhizal, or symbiotic, fungi that lives, loves and dies underground.

Mycelium, the vegetative network of tiny filaments that make up the body of the subterranean fungi, colonise the roots of their chosen tree. The Perigord likes a French oak (Quercus ilex) but will happily reside with an English (Q. robus) or Cork oak (Q. suber), for example, and with hazelnut trees (Corylus avellana). The mycelium gives the tree access to extra nutrients and water, partly by massively expanding the surface area of its root system, while the tree shares photosynthesised sugars.

“Compare this with the cultivation of wheat, rice or fruit trees, where we have thousands of years of experience. So I like to say we are at the start of the journey.”

If the conditions are right, thousands of tiny sporophytes, or the beginnings of a spore-containing truffle, emerge in spring, of which a fraction may go on to ripen. A truffle grown in Australia is fully formed by about March and is ripe around July.

The right conditions include not too much and not too little rainfall, a soil with a high pH (around eight), not too much competition from other fungi in the early days when a seedling is planted, the right soil temperature (about 23–25°C), and less than optimally nutritious soil – if it’s too rich the tree won’t need the mycelium to fix nitrogen, Guerin says.

And even if they supply all of these conditions, growers may still find they have precisely zero truffles.

Scientists don’t know for sure what triggers a truffle to grow. But DNA testing of spores from a Perigord truffle in 2010 gave them a key piece of the puzzle: Perigords have two “parents”, meaning a truffle fungi needs two mating types, MAT1 and MAT2, to fruit. And these need to be opposite: a maternal MAT1 needs a paternal MAT2, and vice versa.

But when scientists at the Institute of Biosciences and Bioresources in Perugia, Italy, ventured into the wild to find these mating types, they were in for a surprise: all they could find was DNA for maternal MAT1 and MAT2 types in the truffle networks around tree roots.

Furthermore, each tree was dominated by one type, either MAT1 or MAT2. What they discovered was that while both types could be seen initially on colonised roots of nursery seedlings, over time one would wipe out the other mating type.

Which posed the question: if only one maternal mating type could be found around a cluster of truffle-producing trees, where were they sourcing the paternal type to produce fruit? Where were these missing males, which have never been found on plant roots surrounding a truffle-producing tree?

Theories abound among prominent European truffle researchers. Marc André Selosse, from the National Museum of Natural History in Paris, summed the evidence up last year.

He speculates that paternal spores are “ephemeral” germinators, which in biology means they live fast and die young. High levels of inbreeding in truffles suggests the maternal types breed with any offspring that haven’t been harvested.

“In T. melanosporum truffle grounds, more than 40% of fruitbodies are not removed,” Selosse wrote in the journal Current Biology. “We do not know whether these fruitbodies go undetected or never produce the required fragrance. However, their decay allows truffles, like most other fungi with subterranean fruitbodies, to establish a spore bank in the soil.”

Rescued by a Spanish Armada

Back in Australia, the mating problem is being tentatively solved with an innovation imported from Spain, the global powerhouse of truffle production.

“You’ll have a tree that looks like it should produce, but it may grow for years and never produce,” explains Colin Carter, horticulturist and founder of nursery Trufficulture in Victoria. “So the Spanish now do a thing called Spanish trenches or Spanish wells, where they dig a trench and put truffle spore into those trenches. So if a tree’s only got one mating type, you’re introducing the missing one.

“We’ve been to Spain three times. On the last trip we saw them doing these trenches, so we came back and did some experimental work straight away. It took two years. [My son] Nathan does the hunting and we found a lot of truffles in these trenches. So we know it works.”

Because all cultivated truffles in Australia – the Perigord, the Burgundy or summer truffle (Tuber aestivum), and the Italian white or bianchetto truffle (Tuber borchii) – are all introduced from Europe and grown in harvested truffières (truffle orchards), natural truffle inoculation in the soil is absent. Trees must be manually inoculated with truffle spores, and the specifics of the process are secrets closely guarded by nurseries.

“So the Spanish now do a thing called Spanish trenches or Spanish wells, where they dig a trench and put truffle spore into those trenches.

Trees can be inoculated two ways. Some growers use a purified culture of mycelium, which allows them to prevent cross-contamination with more aggressive fungi like the Italian bianchetto, which doesn’t need two mating types to fruit, but only provides one mating type. (French researchers abandoned a trial on mycelium pure-culture inoculations after the 2010 mating type discovery – until the trees began producing truffles. They weren’t able to work out quite where the opposing mating type for fertilisation came from.)

Other growers inoculate trees using spore, a method nurseries have perfected by dipping roots in pureed ripe truffle.

“But the problem with just mashing up a truffle – and if you read the literature from overseas that’s what they were doing – is that you get very variable results,” says Carter. “Some truffle doesn’t break dormancy.”

Dormancy is another truffle mystery. Carter’s only explanation for it? “It’s nature,” he says.

“Even in French forests, where clearly all the trees are inoculated, not every tree reproduces every year. It’s only a small percentage and that can change and move through the forest. Some trees will go back into dormancy and other trees will start producing.”

The first truffières in Australia were farmed in Tasmania, but it’s Western Australia’s iron-rich, competitor-free soils that now do the heavy lifting in what was estimated to be a $7 million industry in 2018–19. Australia’s golf-ball-like spheres, which come about from growing in loose soils rather than around rocks and tree roots as they do in Europe, are just one element of the local industry leaving northern specialists shaking their heads at what they still don’t know about the enigmatic truffle.

“How long does it take to fruit? What are the effects of different treatments to trigger fruiting? What is the yield of a tree? How long will the production last? All these questions seem very basic but are key, and for most of them we don’t have an answer,” Guerin says.

Even with its daytime soap tendencies, our romance with truffles will remain very much a French liaison amoureuse.