The USDA won’t impose biotech regulations on walnuts genetically engineered to withstand crown gall disease, an important step toward their commercialization and similar modifications to other crops.

The agency has decided it has “no authority” to regulate the cultivar, which has been engineered with soil bacteria genes to avoid expressing the disease, because the “walnut is unlikely to pose an increased plant pest risk.”

Though the USDA’s decision was a key hurdle, the walnut variety must still obtain similar clearances from the U.S. Environmental Protection Agency and Food and Drug Administration before nurseries can propagate the rootstocks for sale to farmers, said Abhaya Dandekar, a professor at the University of California-Davis involved in its development.

Crown gall disease, caused by microbes that create tumor-like growths on plants, has long been the top pathogen affecting walnut trees, Dandekar said.

“Essentially, the bacteria is hijacking the plant resources to make a home for itself in the plant,” he said.

The biotech walnut has been altered with genes from the same soil bacteria responsible for the disease — agrobacterium tumefaciens — to block the hormones that cause the tumors.

“We are providing the trigger to suppress the tumor formation,” Dandekar said. “We’ve essentially stopped that process.”

Messenger ribonucleic acid, a type of genetic material commonly called mRNA, prevents the tumor’s formation similar to the way a plant naturally fights off viruses and bacteria.

“The mechanism itself is already present,” Dandekar said. “It does not have to be engineered.”

The approach confers durable resistance to crown gall disease because the bacteria won’t develop tolerance to the mRNA mechanism, he said.

Since it doesn’t kill off the bacteria, the mechanism does not select for individuals that are impervious to its effects, Dandekar said.

The bacteria will persist in the walnut tree’s environment, just not at high enough levels to cause problems with crown gall disease, he said.

Only the walnut’s rootstock possesses the genetically engineered trait, while the rest of the tree is grafted onto it, he said.

“It’s not what people will eat,” Dandekar said. “All this stays below the graft union.”

If the cultivar is fully cleared by the relevant federal agencies, farmers could gradually replace existing walnut acreage with the disease-resistant rootstocks, he said.

“It will ultimately be their choice,” he said. “It will take some time before you see an impact.”

However, the same genetic alteration can render winegrapes and other susceptible plants impervious to the disease, he said.

Once the walnut cultivar has blazed the regulatory trail, federal clearances will likely be easier to obtain for these other crops, Dandekar said.

“There are many plants for which this mechanism can be used,” he said.