Exposing peach tree roots shields them from deadly fungus
CLEMSON, South Carolina — Matt Forrest, a fourth-generation peach grower in South Carolina, is a formidable force in the peach industry. His family farm, Dixie Belle Orchards, spreads across 2,700 rich, rolling acres and three counties between Columbia, South Carolina and Augusta, Georgia.
But Forrest and other peach growers have been powerless in fighting an insidious, interminable threat that lurks in the soil of nearly every peach orchard and forest in the southeastern United States: a fungus called Armillaria tabescens that feeds on roots and slowly kills trees. It has a particular penchant for peach trees, shortening their lives from 15 years to four or five.
For the past several years, Dixie Belle has been one of four farms in South Carolina participating in Clemson University research that’s testing an elegant, and so far very successful, answer to Armillaria. They’re planting the trees shallow on berms — earthen rows built 12 to 15 inches high and 36 inches wide. After a few years the berms are either eroded or they’re removed, leaving the top several inches of the root crown above ground.
After five years of testing in heavily infested soil, 40 percent to 50 percent of the trees planted according to the standard method, with all their roots below the soil, have died; 90 percent of the berm-planted trees are still living, said Guido Schnabel, a plant pathologist in the College of Agriculture, Forestry and Life Sciences at Clemson University who came up with the idea and implemented the research.
Armillaria survives in the soil by feeding on dead roots. It latches onto new, healthy roots and grows up the root to the bottom of the tree trunk, where it circles the root crown and infects other roots.
But the fungus can’t grow out of the soil. Raising the root crown above the soil level halts Armillaria’s march. A tree might lose one root to the fungus, but others can compensate. The tree might eventually succumb to Armillaria, but it will have many more productive, profitable years, Schnabel said.
The proposed solution was eagerly welcomed by some farmers in South Carolina, the nation’s second-leading peach producer behind California, where Armillaria tabescens doesn’t grow. Georgia, despite its nickname, produces half as many peaches as the Palmetto State.
Forrest took a little convincing before he agreed to test the method. His berm-planted trees are only three years old, but he hasn’t lost any of them to Armillaria.
“We’re hoping this will be a big help for us,” Forrest said.
Saving peach trees is more than nostalgia for a family farm. From May through August each year, Dixie Belle sells 700,000 half-bushels of peaches every year throughout the eastern United States and Canada. “Yellow flesh peaches, mostly, but also white peaches, all kinds. We have 36 varieties right now.”
“It’s a big investment to get the trees planted and established and up to production. And if you get trees four or five years old, after you’ve invested money growing the trees, and they start dying or they’re not producing, obviously that’s a huge problem. It’s huge for the whole industry.”
Armillaria has puzzled scientists for decades. “The fungus isn’t susceptible to chemicals or fumigants, and we have yet to come up with a commercially available peach tree resistant to Armillaria,” Schnabel said. Injecting roots with fungicide is effective but cost prohibitive.
Schnabel had read research studies from the early 1900s about citrus trees that were planted with their roots exposed to survive water stress. More recently, grape vines in California were temporarily relieved of a different Armillaria fungus when soil was removed from the top roots, but the excavated roots were soon covered again with soil and the problem persisted.
One day about eight years ago, during his lunchtime run around Clemson’s campus, the idea struck him. “I thought, ‘Oh, man. I can probably combine these two: plant peach trees on berms to help relieve water stress and excavate the roots permanently above ground to avoid Armillaria tabescens,” Schnabel said. “I get some of my craziest ideas while running.”
Forrest isn’t planting all of his new trees on berms, but he says he likes what he sees so far.
“We know it’ll help with the oak root rot but the question is whether there’s any other problems you may encounter with it that maybe you haven’t thought of,” he said.
Planting trees on berms is more labor-intensive and orchard management practices need to be adjusted, Schnabel said, and trees might need to be staked to prevent strong winds from blowing them over. If the berms don’t erode naturally they have to be removed manually, a process that might be accomplished with a blast of air from an air spade or a mechanized brush that another Clemson researcher is working on.
But if those adjustments extend the lives and productivity of the trees, the growers working with Schnabel say they’ll be worthwhile.
“If everything continues to look promising, I’m sure more and more acres will be going to this kind of planting,” Forrest said.
Schnabel’s research was funded by the U.S. Department of Agriculture.