Oct 19, 2020
Clemson researcher leads national effort to combat Armillaria root rot

Stone fruits and nut crops are vital to human health, communities and economies across the United States but a fungal disease threatens to devastate these perennial tree crops and pose a national agricultural security threat.

Armillaria root rot (ARR), or “oak root rot,” is caused by the Armillaria/Desarmillaria fungus and affects more than 500 woody species including stone fruit and nut trees. No controls exist for the fungus, which costs U.S. farmers millions of dollars in crop losses each year.

A multi-state team of scientists led by Ksenija Gasic, a Clemson University peach breeder and geneticist, has received a $5 million grant from the United States Department of Agriculture National Institute of Food and Agriculture (USDA-NIFA) for a 4-year study that involves development of new ARR-resistant rootstocks for Prunus species, specifically almond, cherry and peach.

“Armillaria root rot is a problem with peach trees in the Southeast, cherry trees in Michigan and peach, cherry and almond trees in California,” Gasic said. “Growers with infected orchards face a persistent problem because both the host and the disease are long-lived.”

Joining Gasic and other Clemson scientists in this study are researchers from the University of California-Davis, University of Georgia, University of Kentucky, Michigan State University and the USDA-ARS in Davis, California. The researchers believe this study will provide building blocks needed to support on-going Prunus programs, as well as encourage the use of cultural practices to increase tree longevity on replant sites. Cooperative Extension Service agents will relay research information to the public.

Resistant rootstocks

ARR occurs naturally in much of the United States. Because it is so widespread, ARR poses a threat of national importance. Objectives of the study are to discover pathogen-resistant rootstock sources, develop effective breeding solutions for ARR-resistance and/or tolerance, and implement short- and long-term management solutions.

“Without practices to eradicate the pathogens or methods to counteract reduced productivity, infected sites are unsuitable for replanting,” Gasic said. “Without solutions, this will not only have devastating impacts on the affected producers and rural communities but it also will contribute to a reduction in the supply of these products and potentially increased costs for consumers.”

Trials to evaluate potential rootstocks will be conducted at sites throughout the country. Germplasm (genetic material) will be provided by the USDA-ARS National Clonal Germplasm Repository in Davis, California, participating commercial nurseries and researchers involved in the project.

“We will be primarily providing Prunus — almond, apricot, cherry, peach, plum –wild relative germplasm to support this study,” said John Preece, research leader and horticulturist at the Repository. “Many plants that grow in the wild in eastern forests have the potential to have resistance to Armillaria. We hope germplasm we provide will help yield Armillaria-resistant Prunus rootstock that will be adopted by the industry.”

More than 2 billion pounds of almonds and 1 million pounds of peaches are produced in California annually, but ARR limits production and reduces grower returns.

“Control of oak root fungus is complex at both the genetic and environmental level,” said Thomas Gradziel from the University of California-Davis. “Consequently, a holistic, multi-institution and multi-disciplinary approach is the most promising path to success.”

Amy Iezzoni of Michigan State Unviersity agrees.

“Our efforts complement each other so what we can accomplish as a group is greater than the sum of its parts,” Iezzoni said. “Sharing expertise and ideas is extremely important so we can make as much progress as possible during these 4 years.”

Additional objectives of the study include determining the cost-benefits of cultural practices and economic impacts of ARR.

Cultural practices

Cultural practices can extend long-term orchard productivity on replant sites. Researchers will implement Above Ground Root Collar Excavation for peach and cherry and explore using sectorial chimera rootstock in almond as strategies to increase tree longevity until ARR-resistant rootstocks are developed.

Above Ground Root Collar Excavation for peach production was developed by Clemson plant pathologist Guido Schnabel. In this practice, a tree is planted and grown on a berm before soil is removed from around a tree’s root collar, exposing the primary roots and fungus to heat, causing the fungus to dry and die.

Phillip Brannen, Dario Chavez and Jeff Cook from the University of Georgia will determine how using cultural practices, such as Root Collar Excavation, can benefit Georgia peach growers.

“Our main goal is to obtain a deliverable management protocol to be shared with Georgia peach growers for production in Armillaria infested soils,” Chavez said. “This could be from using resistance sources such as MP-29 as a rootstock and/or Root Collar Excavation with Guardian rootstocks. The idea is to have a plan that can be quickly deployed by growers.”

Economic impacts and outreach

The economics team, led by Michael Vassalos, a Clemson associate professor of agribusiness, will design surveys to help determine economic effects of ARR and impacts that proposed solutions may have in affected regions.

“ARR can have a devastating impact on the profitability of peaches, cherries and almonds,” said Tyler Mark, an agricultural economist with the University of Kentucky who also is on the economics team. “As ARR spreads through an orchard, it reduces the yield and, eventually, it gets to the point the orchard is no longer profitable. If the ARR infection is bad enough, the field may no longer be an acceptable location to replant an orchard.”

Researchers from across the United States are joining forces to attack the Armillaria root rot fungus.

Researchers from across the United States are joining forces to attack the Armillaria root rot fungus.
Image Credit: Clemson Public Service and Agriculture

Results of the study will be shared with the public at field days, meetings, in scientific and trade journals, as well as from county agents, Cooperative Extension bulletins and posted on various websites. Tree-fruit nurseries nationwide will deliver research results to producers.

Consumers and producers will benefit from this cross-country collaboration.

“The industry will see an immediate benefit with the change in cultural practices and direct utilization of new sources of resistance in nurseries,” Gasic said. “Consumers will benefit from sustainable stone fruit and nut production, which will help increase consumption and improve human health and physical well-being.”

Fruits and nuts are key players in U.S. agriculture. Figures from the USDA-National Agricultural Statistics Service (USDA-NASS) show 645,500 tons of peaches and 334,000 tons of sweet cherries were produced in the U.S. in 2020. California is the only place in North America where almonds are commercially grown. The latest figures for almonds from the USDA-NASS Pacific Regional Office shows 1,275,000 tons were produced in California in 2019.


Researchers participating in the study include: Clemson: Ksenjia Gasic, Gregory Reighard, Guido Schnabel, Chris Saski, Jeff Adelberg, Nishanth Tharayil, Michael Vassalos and Bulent Koc; University of Georgia: Phillip Brannen, Dario Chavez and Jeff Cook; University of California-Davis: Tom Gradziel; USDA-ARS Davis, California: Kendra Baumgartner and John Preece; Michigan State University: Ray Hammerschmidt and Amy Iezzoni; and University of Kentucky: Tyler Mark.

This study is funded by a United States Department of Agriculture National Institute of Food and Agriculture (USDA-NIFA) Grant 2020-51181-32142. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the view of USDA-NIFA.


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