With a resurgent pathogen threatening Oregon orchards, a new hazelnut breeder is facing a remarkably similar problem as his predecessor did four decades ago.

Though history isn’t exactly repeating itself, it certainly seems to be rhyming as a mutant strain of Eastern Filbert Blight infects trees across the Willamette Valley.

The new strain has overcome resistance to EFB incorporated into more than 20 hazelnut cultivars released by longtime Oregon State University breeder Shawn Mehlenbacher, again forcing growers to battle its spread with fungicides and pruning.

Upon his retirement, however, Mehlenbacher will leave behind additional genetic defenses and promising tree selections that will give his replacement, Gaurab Bhattarai, a leg up in fighting the new strain through breeding.

“We don’t have to go back to the beginning and start over. There’s a lot of good stuff in the pipeline,” said Mehlenbacher, who was hired in 1986, when little was known about the disease and its life cycle.

Not starting from zero

As Bhattarai takes the helm of Oregon State University’s hazelnut breeding program, he’ll be able to build upon decades of research with new technologies that speed up the traditional tree crossing and selection process.

“There are tremendous opportunities to develop not only new varieties but new knowledge,” Mehlenbacher said. “We’re not starting from zero, we’re starting pretty far along the path. We just need to be patient.”

Several new sources of resistance to EFB have already been discovered in hazelnut trees from around the world, and it’s now possible to quickly identify whether their progeny have inherited those traits.

Rather than watching for visible signs of whether the young trees are susceptible to the disease or not, they can be tested for genetic markers associated with resistance to EFB.

“You have to continue doing research, especially genetic research, to find the markers that can tag the resistance,” said Bhattarai.

Of course, modern tools cannot accelerate every aspect of hazelnut breeding. Over the next year, Mehlenbacher will be familiarizing his successor “row by row and tree by tree” with hundreds of selections planted at OSU’s research farm near Corvallis, Ore.

He’ll also be bringing Bhattarai up-to-date on ongoing research projects that he can finish, allowing allow him to publish studies in scientific journals — an important consideration in obtaining academic tenure.

“I’m trying to set him up to be successful,” Mehlenbacher said.

The original source of genetic resistance to EFB, a variety named Gasaway that served as a pollinizer in orchards, grew small nuts that weren’t suitable for commercial production.

Now that the pathogen has defeated the Gasaway gene, the goal will be to incorporate other genetic resistance into new varieties. But like Gasaway, the trees containing those traits generally don’t produce nuts with the requisite size and quality sought by processors.

“Most of our sources of resistance don’t have much else going for them,” Mehlenbacher said.

Fortunately, commercially desirable qualities can be combined with disease resistance within just a few generations, he said. For example, the highly productive Yamhill variety is the grandchild of Gasaway, which is among the least productive.

Hazelnut trees produce nuts five years after they’re planted as seeds, which necessarily prolongs the selection process. Mehlenbacher and Bhattarai must also wait until late summer to scrutinize the nuts as they approach maturity.

“Most of the traits are nut traits, and the harvest is when you see the difference,” Mehlenbacher said.

These evaluations will involve a degree of subjectivity, but the nuts are also measured against objective standards, such as the prevalence of defects and the percentage of kernel within the shell, Bhattarai said.

“Those parameters you can quantify,” he said.

Another key factor in the selection process is yield efficiency, which is basically the tree’s yield adjusted for its size. This benchmark gauges a variety’s productivity per acre, regardless of its growth rate or the size it achieves.

“I don’t get rid of vigorous selections because they are vigorous, I don’t get rid of small selections because they are less vigorous,” Mehlenbacher said.

Mutating fungus

The speed with which EFB was able to mutate and conquer existing sources of resistance was surprising, indicating such adaptations will be a long-term challenge for the hazelnut industry, he said.

“The fungus will change again, no doubt,” Mehlenbacher said.

The industry and OSU also face a conundrum about what to do with Gasaway-dependent selections that seemed like up-and-comers until the new mutant strain was confirmed over a year ago.

“What is the point of releasing something that could be just as susceptible?” he asked.

Some selections may prove worthy of release anyway based on their nut quality, since some farmers still plant old cultivars with no resistance to the disease, Mehlenbacher said. The cost of pruning and spraying can be worthwhile if the nut commands enough of a premium.

“Maybe that applies to some of these with Gasaway resistance,” he said.

On the whole, though, the breeding program is likely to emphasize the development of varieties that have additional sources of resistance, he said. One such cultivar is already slated to be released in June, demonstrating OSU’s ability to pivot on short notice.

With Mehlenbacher retiring at the end of the year, decisions about the program’s direction will be Bhattarai’s to make. He won’t face the burden alone, though, as an advisory committee of growers will help evaluate new plant material.

“I think it’s also driven by the industry,” Bhattarai said.

Born in Nepal to a family of subsistence farmers, Bhattarai originally studied agriculture to help such small growers adopt modern farming practices.

“When you are relying on subsistence farming, you don’t have the resources to buy hybrid seeds or improved varieties,” he said.

After obtaining an undergraduate degree in agricultural science in his native country, Bhattarai worked at a nonprofit assisting Nepalese farmers recovering from a major earthquake.

His interest in advanced plant research soon drew Bhattarai to continue his education in the U.S., where he studied the genetics of wild grapes at Missouri State University. Upon completing his master’s degree, he went on to get a doctorate from the University of Georgia in pecan breeding.

Bhattarai stood out among the candidates applying to replace Mehlenbacher at OSU due to his experience with applied breeding and lab research in genetics.

“That narrowed the field considerably,” Mehlenbacher said.

Typically, candidates specialize in either one or the other, while Bhattarai was proficient in both, he said. He was hired by OSU and joined the university at the beginning of 2025.

“My number one choice is sitting right here,” Mehlenbacher said.