The science of making mushrooms

MU researchers work to make shiitake, morels
and truffles viable crops for Missouri
Friday, October 10, 2003 | 12:00 a.m. CDT; updated 9:50 a.m. CDT, Monday, June 30, 2008

A thousand sunlight-dappled logs rest beneath the canopy of trees on a cool, wooded slope in New Franklin. Protected from the steady wind, dozens of shiitake mushrooms sprout from several logs under observation in the MU Horticulture and Agroforestry Research Center’s research experiments.

The agroforestry center is in the midst of three research projects involving the shiitake, morel and truffle — three types of edible fungi. Johann Bruhn heads the MU gourmet mushroom research project. Now three years into the research, Bruhn says that the most successful experiments have been with the shiitake.

When a flush of shiitake occurs at the center, the job of harvesting the mushrooms falls to researcher James Wetteroff. He labels bags with each log’s tag number, then deftly snaps each mature mushroom from the log and bags it.

Shiitake mushrooms cost relatively little to produce, and they can grow in wooded areas that are not suited for traditional crops or are too expensive to clear for that purpose. By growing mushrooms on presently unused land, farmers can gain extra income. “It’s a matter of diversifying opportunities,” Bruhn says.

Bruhn, his wife, research assistant Jeanne Mihail, and Wetteroff are conducting four experiments involving shiitake. The first experiment was started in 1999, with the additional projects launched in 2000 and 2002.

This winter, Bruhn will start pulling together data from the shiitake experiments to see what strain of the fungus grows best in mid-Missouri. The research studies what logs best grow shiitake, what inoculation methods work well and whether the temperature of water used to trigger growth of the mushrooms makes a difference in production.

The agroforestry center plans to use Bruhn’s research to teach farmers which methods work best. “The emphasis is on the small family farms,” Bruhn says.

To start the experiments, oak and sugar maple logs are inoculated with one of three strains of shiitake fungus. Oaks and sugar maples have less resistance to shiitake fungus than do other trees, Bruhn says. “The shiitake is a wood-decay fungus, and if the tree is a developed hardwood, it has a strategy to resist it,” he says.

Sugar maples both have less resistance and also rot more completely than do oak logs.

“We think that we can get more complete and faster utilization of a sugar maple log than we can of a red oak or white oak log of the same weight and size,” Bruhn says.

Bruhn wants to analyze production from different strains because each strain fruits during a different season, and one may grow better in Missouri’s climate than another. A cold weather strain fruits in the winter; a warm weather strain fruits in spring, summer and autumn; and a wide range of strains fruit in all but the frozen months of winter.

Along with the different strains, researchers test different types of spawn by inoculating the logs to see if one works better than the others, Bruhn says.

The final variable is what water temperature will encourage the fungi to grow. In nature, the shiitake fungus will rest dormant in a log until after a good rain. To produce shiitake, growers use a method known as “forcing” to prompt the fungi to fruit.

To force logs, growers submerge the logs in a watering tank for 24 hours. Then they remove the logs and leave them outside.

Depending on weather temperature and humidity, the logs develop shiitake with caps 4 inches in diameter within a week. In warm, humid temperatures, shiitake can grow to that size in as little as one day.

If water temperature used for forcing makes a difference, farmers will want to take that information into account in their own production, Bruhn says.

“For instance, people forcing their logs with pond water would be using warmer water than those using stream water or well water,” Bruhn says. At the New Franklin center, researchers force half of the logs using water from the farm pond at the normal temperature. For the other half of the logs, they add ice to the pond water to bring the temperature down to 50 degrees Fahrenheit.

There are commercial shiitake growers in Missouri, but the vast majority of farmers are hobbyists, says Joe Krawczyk, production manager of Field and Forest Products in Peshtigo, Wis. Krawczyk says his company has sent orders to 330 people in Missouri at one time or another, “and that includes everyone from commercial scale growers to people with five logs in their back yard. We probably can count on one hand the commercial growers.”

Krawczyk says that within the state, “the bulk of our sales come from the southern two-thirds of the state of Missouri.” The wood available in the Ozarks is well suited to shiitake growth, he says. “Shiitake fits so well into woodland management that instead of making firewood out of the small logs, you can make a mushroom crop as either a hobby or source of supplemental income.”

Field and Forest provides the spawn used in the agroforestry center’s experiments at low cost. As large-scale sellers of shiitake spawn, the company could benefit from the results of Bruhn’s work.

“The information has really just started to come in,” Krawczyk says, but preliminary data indicates that growing shiitake can be done faster than he originally thought possible.

Area grower Ron Bonar has already experienced some success cultivating shiitake in Missouri. He sells small baskets of the mushrooms at the Columbia Farmers Market for $3 each. He also sells to the Root Cellar in Columbia and to restaurants in Kansas City.

Bonar started inoculating logs in 1989 after learning that shiitake contain many nutrients and potential cancer-inhibiting agents.

“I’ve always liked mushrooms,” Bonar says. But his mushroom venture didn’t come without patience. “It took three years before I harvested any logs,” Bonar says.

Bonar has 6,000 white and red oak logs, each 40 inches long, propped vertically against one another on his land near Versailles. He forces 150 to 200 logs a week for a constant crop of mushrooms.

This year, Bonar estimated his logs yielded 1,200 to 1,300 pounds of mushrooms. After a dry summer, Bonar says, this yield is “not as good as it was three years ago.” A good year yields 3,000 to 5,000 pounds of mushrooms, he says.

The elusive morel

The sets of shiitake logs at the New Franklin research center are interspersed with bright orange stakes that represent a more challenging line of research: how to grow morels.

“We are physically mapping the location of every morel that fruits in a particular area — about an acre or so,” Bruhn says.

Bruhn and his coworkers have found that understanding morels is difficult.

“We’re studying purely wild morels,” Bruhn says. He wants to determine what provokes morel growth, with the goal being to establish and market natural populations of the mushroom.

Morels produce spores, and the spores fall away from the mushroom caps and establish themselves in the forest floor, where they eventually develop into a walnut-sized growth called a sclerotia, Bruhn says.

“It appears that those sclerotia are what ultimately go on to produce the fruit by spring,” Bruhn says. The researchers do not know what signals the sclerotia to fruit, but they have a few theories.

People have reported seeing morels fruiting around piles of walnut hulls and beneath abandoned apple trees, Bruhn says.

“It would indicate, perhaps, that as the walnut hull decomposes and degrades, either that it produces natural chemicals that stimulate the sclerotium to go ahead and fruit, or that the decomposed hull itself is an energy source for the fungus to go ahead and fruit,” he says.

“They like to grow under dying elms,” says Maxine Stone, president of the Missouri Mycological Society, a group dedicated to the study and enjoyment of mushrooms. “That’s the best place to find them. Not dead and not alive, but dying.”

Bruhn says his crew surveys for morels every spring, “and every place we find a morel, we collect the top of the morel to get a culture.”

Part way through this process, Bruhn says, the research team realized they would have to take DNA samples of the morels in order to tell if all the fruiting in a particular area comes from one morel spore or if each sclerotia below ground only produces one morel mushroom.

The researchers will analyze the DNA to determine the molecular biology of the morels and see whether the mushrooms will require growers to re-seed them on a yearly basis or if morels will propagate on their own once they colonize the soil.

So far, MU researchers have managed to grow sclerotia in the lab, and a few have been planted, but until spring the researchers won’t know the results. Bruhn also wants to know if a sclerotium will re-fruit if a morel is cut from it and the sclerotium is left in the ground.

“It’s been tried so many times,” Stone says of attempts to grow morels. “I know someone else who tried very hard to do it, and he managed to get them to grow, but the flavor just wasn’t the same.”

Even if morels can be grown for mass marketing, Stone doesn’t see commercial production as a substitute for hunting wild morels. “There’s something about the hunt. It just gets your heart beating,” she says.

Truffles across time

Across a gravel road from the shiitake and morel experiments, a field of upturned dirt waits to receive trees infected with the tuber uncinatum fungus, commonly known as the burgundy truffle.

“This is a silty soil, silt loam — and very deep, well drained, with a natural pH of six or so, so it’s slightly acid,” Bruhn says. Heavy liming has sweetened the soil by bringing it to a pH of 7.5, the minimum needed to grow truffles.

To the right, young white oak and hazel trees rustle in the wind. These trees were inoculated in 1998 with tuber melanosporum, the highly valued black truffle — also called a perigord truffle — in 1998.

To inoculate a tree, a truffle of the desired variety is ground into a slurry — or a thin, watery paste. An acorn is then planted in the presence of that fungus. Once the tree is well established, it is planted outside. Then researchers wait.

It takes seven to eight years before the first truffle appears, Bruhn says. Full production won’t come for another 12 to 15 years. Many growers are hesitant to get involved with truffles for this reason, Bruhn says.

“This is why it hasn’t been done substantially around the world — because until recently, people didn’t know how to produce bona fide infected trees, and some people weren’t willing to take the risk in planting an acre or two of hazels, and wait for eight years to find that the trees weren’t properly infected,” Bruhn says.

Aside from the eight years of uncertainty, truffles are fairly easy to grow, he says. Bruhn has high hopes for the burgundy truffle, which might grow better in Missouri because it fruits in the autumn rather than the winter. Winter frosts can destroy truffles, which grow just below ground surface, Bruhn says.

“Truffles grow in a wide range of soils as long as the pH can be raised,” Bruhn says. The pH is raised by liming the top 8 inches of soil where growers wish to grow truffles.

Previous vegetation makes a difference too, Bruhn says. The ideal place to try to grow truffles is in an old field that had crops planted in it for a long time and is now unused. The plot should have good drainage and the proper pH, and it should not be downhill from trees that might wash competing fungi into the plot during rain.

The trees that work best with truffles are oaks and hazels because these trees are more prone to infection by the fungus, Bruhn says. The truffles and trees establish a symbiotic relationship with one another. The tree photosynthesizes sunlight and provides needed nutrients to the fungus below the surface. The fungus, in turn, gathers chemicals the tree needs and provides them to the tree.

Truffles can turn quite a profit when they reach full production. Bruhn reports yields from France in the neighborhood of 40 pounds per acre. Depending on the year, black truffles are worth $400 a pound, and burgundy truffles are worth $200 a pound.

“We’re waiting for him to do the research before we hop into it,” mushroom marketer Krawczyk says of Bruhn’s work with truffles. Krawczyk is enthusiastic about the prospects of expanding his company’s market to truffles, “as long as we have proven technology that we can replicate here.”

“I think it’s going to be a doable thing in southern Missouri, maybe with a given species of truffle,” Krawczyk says.

But Bruhn says not to expect market changes too soon. “I think we’re well on the way,” Bruhn says, “but I think we’re three years out on answering the questions.”

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