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Impressed by early research results, Titan Farms planted 200 acres of peaches on ridges to try out this new approach to Armillaria root rot.

Impressed by early research results, Titan Farms planted 200 acres of peaches on ridges to try out this new approach to Armillaria root rot.

Richard Lehnert

Since coming to Clemson University in 2000, Dr. Guido Schnabel has been taking on the two big disease problems that plague peach growers in the Southeast and many stone fruit growers elsewhere.

He’s had some success.

Working with other plant pathologists at the University of Georgia, he developed a simple and fast test kit, trademarked Profile, that will tell a peach grower if the brown rot organisms in an orchard are resistant to fungicides—and which fungicides will still work (see “Fast, easy test reveals fungicide resistance”). He and others in his laboratory are sharing an $850,000 grant to develop these resistance management kits for other diseases in other crops, such as gray mold of strawberry.

Now, he believes, he’s onto a method to greatly slow down Armillaria root rot, which growers usually call oak root rot. It can knock years off the productive life of a peach orchard. The new method, which looked good in preliminary tests, is so clever in concept that growers are already trying it in their orchards.

The idea is to plant peach trees on ridges, then pull back the soil a couple of years after establishment, exposing the tops of the roots. The trees look strange, as if they’re standing on tiptoes, but Armillaria cannot reach the crown to kill the tree. “The fungus does not like to grow above the soil line—that’s the key,” Schnabel said.

Commercial scale

When Chalmers Carr III, at Titan Farms, saw the initial test results, he planted 200 acres and is the first to try the method on a commercial scale (see “Titan Farms,” page 16). Oak root rot costs peach growers in South Carolina an estimated $4 million a year in direct costs, not counting lost income from sites that can no longer be used because of persistence of the pathogen in the soil.

The procedure aims at a weak spot in the Armillaria armor, Schnabel said. The disease, once it affects a peach root, will ultimately kill the root. Moreover, the fungus will move up the root to the crown of the tree. It may kill the tree quickly by shutting off its supply of water and nutrients, or it may move down into other roots, infecting them. The disease infects many kinds of forest trees and produces a type of attractive, edible mushroom called honey fungus, that grows at the base of dead trees. While the fungus does not spread easily by spores, it will grow on living trees and on dead and decaying woody material. It kills its hosts by invading the root collar and girdling the trees, or by causing extensive root death.

After a tree dies, a new tree planted near that site is doomed.

“Armillaria will last in the roots for years—decades—and even soil fumigation with methyl bromide doesn’t help much,” Schnabel said. “Chemical management has met with limited success because of the protected nature of ­Armillaria inoculum, being encased underneath the bark of roots and ­surrounded by soil.”

Growers need to replant their peach orchards every 20 years or more often, he said. “The more they use the land, the more infection centers there are.”

Peach growers use tillage to bring up the old tree roots and root rakes to sweep up all the roots they can. But peach roots are deep, three feet and more. When a new tree’s roots touch a dead, infected root, “the fungus jumps from the old root to the new root as soon as the new root approaches the dead root,” Schnabel said. “The fungus moves up the secondary root to the primary root, then into the crown of the tree. The fungus will grow around the crown of the tree and shut off its water supply. That’s when the tree dies.” In one of his test plantings on a replant site, four-year-old trees were already dying.

“Planting on a mound will keep the roots shallow for a year or two, but as they roots go deeper, they contact old, infected roots and become infected themselves,” he said.

Ridge

But that bit of knowledge, combined with the knowledge that Armillaria does not like to grow above the soil line, spawned an idea. How about planting on a ridge and then blasting the soil away at the tree base? Schnabel thinks growers can do that using an air blaster.

“The fungus may kill a root,” Schnabel said, “but it can’t infect the collar and kill the canopy. Other, uninfected roots will take over more responsibility with new flushes of roots. Eventually, the tree will die if too many roots are infected. But you can buy yourself four or five years. Growers say they need eight to ten years for an orchard to make money.”

Schnabel started his tests with trees in open-bottom, fabric pots put into the ground in infected sites known to have heavy disease pressure. “After five years, nearly all the trees planted according to grower standard were dead,” he said. “But only 10 percent of the trees planted in pots initially and later with their roots exposed died. We think we can significantly extend the life of orchards on Armillaria root rot-infested replant sites.”

On the road to this discovery, Schnabel tried other things. He found, for example, that injecting infected trees with the fungicide Alamo (propiconazole) would kill the fungus and halt the expansion of the fungus from the infection center. This product and the injection method are used by ­professional arborists to protect valuable landscape trees.

“This may work for someone with a small orchard,” he said, “but it’s too labor intensive for most growers. A smaller grower can protect the peach site by injecting infected trees and actually stop the root-to-root spread. Infection centers could be kept small and isolated.”

Schnabel also tested a biological control agent, Trichoderma, which has some uses in controlling pathogenic fungi in soil. After five years of testing on two grower sites, he found it had no effect on Armillaria. Other researchers have also found this method needs careful refinement to match the strain to the specific disease. Schnabel believes Armillaria is too well protected inside dead roots for either Trichoderma, fumigation, or soil drenches to be effective.

Armillaria is a problem in cherry orchards in Michigan and on grapes in California.