by Matt Milkovich

A 4-year-old cherry tree on Mahaleb rootstock infected with armillaria root rot. No known cherry rootstocks are resistant to the fungus. (Courtesy Pratima Devkota/Michigan State University)
A 4-year-old cherry tree on Mahaleb rootstock infected with armillaria root rot. No known cherry rootstocks are resistant to the fungus. (Courtesy Pratima Devkota/Michigan State University)

After four years of work, a multistate research team is still tackling armillaria root rot “from every possible angle,” said Clemson University peach breeder Ksenija Gasic.

They’ve located genes responsible for tolerance to the wood decay fungus in the genus Armillaria. They’ve convinced Southeast peach growers to plant new trees on berms. And they’ve developed a screening technique that can detect Armillaria resistance faster and more reliably than field inoculations. 

Gasic, leader of the four-year, $5 million project funded by the U.S. Department of Agriculture’s Specialty Crop Research Initiative, said the project recently received a one-year, no-cost extension and will now wrap up its work in August 2025. 

In future projects, the team plans to continue the fight against armillaria root rot — a wood decay that poses a major economic threat to Southeast peaches, Michigan cherries and California almonds and peaches. There’s still plenty to discover. For one: They haven’t yet found Armillaria-resistant rootstocks that are graft-compatible with cherries. 

In Michigan, home to most of the nation’s Montmorency tart cherries, cherry trees are planted on Mahaleb or Mazzard rootstocks, both of which are susceptible to Armillaria. And there are no known chemical or biological controls for the fungus, said Michigan State University researcher and project participant Pratima Devkota.

Devkota developed an in vitro assay that can quickly and reliably screen for Armillaria resistance in rootstocks and germplasm. Previously, researchers had to inoculate specimens in the field, then wait to see if they became infected — a time-consuming and labor-intensive process with no guaranteed results. 

Her technique: Cut an undamaged segment from a root, clean the segment’s surface, seal it at both ends and place it on top of the fungus in a lab culture box. Then, wait to see if the fungus penetrates the outer layers of the root. Defenses among root specimens vary, likely related to the chemical compounds in their bark as well as anatomical differences. Using this method, each genotype can be screened for resistance in less than six weeks, she said. 

Devkota has found Prunus rootstocks with Armillaria tolerance, but still no cherry rootstocks. She tried grafting cherries (with an interstem) to MP-29 and Krymsk 86, both known for their Armillaria tolerance, but they were graft-incompatible. She and other researchers are crossing cherry rootstocks with plum material, hoping resistance can be transferred to a hybrid. She’s also collecting cherry and plum species in forests and woodlots near Armillaria-infected orchards to identify potential sources of resistance from the wild. 

She works with MSU molecular physiologist Courtney Hollender, who plans to take a genetic approach. Hollender said Devkota’s findings suggest that increasing the natural wounding responses in roots may slow or prevent Armillaria infection. And the Clemson team identified gene expression changes associated with armillaria root rot susceptibility and resistance in peach. Hollender’s lab will use this information to increase the expression of genes that promote wound responses and resistance in cherry rootstocks, she said.

Hollender’s lab recently sequenced the Montmorency genome, which will aid their cherry work. 

“The availability of the Montmorency genome will help us identify the exact DNA sequences for the genes we are interested in,” Hollender said. “Without it, we would be in the dark about what regions to target.” 

As a mitigation technique, Clemson researchers recommended Southeast peach growers grow trees on berms or raised beds, because the armillaria root rot fungus doesn’t grow above the soil line. This can extend orchard life by two or three years. At this point, all Southeast peach growers plant trees on berms, Gasic said. 

Project researchers have combed through USDA’s Prunus and peach germplasm repository in Davis, California, searching for Armillaria-resistant germplasm or material. They’ve found some and are crossing them with popular commercial rootstocks, Gasic said.

After rounds of testing, they’ve found one selection that has commercial promise, which they call 14-4. A plum seedling from the Davis repository, 14-4 also reduces tree size, another commercial benefit. Other promising selections were too susceptible to bacterial canker or suckering, showed scion incompatibility or had other problems, Gasic said.