Growing Wheat in Apple Orchards:
A Possible Aid in Preventing Replant Disease

Growers of the nation's largest crop of apples—in Washington State—may benefit by planting another of the Pacific Northwest's top commodities: wheat.

But the wheat in this case wouldn't be a typical for-profit crop. Grown instead as a rotation crop or cover crop in the orchard, it might help prevent replant disease, a crippling condition that strikes young orchards. And it could serve as an alternative to methyl bromide and other soil fumigants typically used to sterilize old orchards before planting new ones.

The idea comes from Mark Mazzola, a plant pathologist at the Agricultural Research Service's Tree Fruit Research Laboratory in Wenatchee, Washington. He's discovered that in the Pacific Northwest, replant disease seems to be caused by buildup of a complex of four soilborne fungi: Cylindrocarpon, Phytophthora, Pythium, and Rhizoctonia.

"Apple trees seem to change the soil in a way that favors these pathogens," Mazzola says. "Some wheat varieties, on the other hand, modify the soil environment to the benefit of different microorganisms."

Specifically, he found a bacterium, Pseudomonas putida, in some wheat soils that can protect young apple roots from the fungi. ARS has patented use of a strain of the bacterium to prevent replant disease.

Searching for Root Causes

When nothing is done between taking out an old orchard and putting in a new one, the young trees are often stunted and have small, decayed root systems. For years, scientists have debated whether the cause was biological—a disease or organism—or a result of abiotic factors, such as soil chemistry.

"Since methyl bromide and fungicides that suppress microbes seem to improve apple trees' health, it now looks like the cause is something biological," says Mazzola. "That cause may vary from place to place. In the Northeast, for example, nematodes may cause a problem. Here in Washington, specific groups of fungi seem to be the primary culprits," he says.

Methyl bromide is due to be phased out by 2005, and other common pesticides used in Northwest orchards may also be taken off the shelf because of environmental concerns. For that reason, and to support the region's organic growers, Mazzola is looking for a nonchemical approach to prevent replant disease.

The search is becoming more urgent all the time. "Years ago, growers might have left an orchard in for several decades," Mazzola says. "But to meet market demands, some growers now pull out old orchards and plant new varieties much sooner."

Growers produce at least a dozen commercial varieties in Washington's billion-dollar-a-year apple industry. About half of the nation's apples are produced there on about 180,000 acres. About 10,000 acres of apples are replanted each year.

A Bigger Problem Over Time

If a new orchard is planted on ground that was previously used for something else, the detrimental fungi don't build up fast enough to hurt the trees before they get established. But if trees are planted into an existing or previous apple orchard, the fungal population prevents the young, new trees from growing well.

Scientists agree that chemicals exuded from plants affect the surrounding soil, thereby favoring specific populations of microorganisms. Although the specific chemicals have not been identified, Mazzola found that after an orchard has been in place about 3 years, apples promote a fungal population that can cause replant disease. He's studied 18 orchards in Washington, collaborating with Wenatchee Valley College and David Granatstein, director of Washington State University's Center for Sustainable Agriculture and Natural Resources in Wenatchee.

Now Mazzola is looking at how long wheat would have to be grown as a rotation crop to change the soil microbial community enough to stave off replant disease. Alternatively, he'll look at whether growing wheat as a cover crop in existing orchards can reduce fungal populations sufficiently to allow new trees to grow well. He doesn't anticipate that apple growers would harvest the wheat as a crop, but says that would be up to the individual.

Theoretically, if young trees are given a good start, they'll be able to grow—despite the deleterious fungi—when they're older. But Mazzola speculates that continuing to keep populations of these fungi low might improve yield, even in mature trees. Although greenhouse tests have indicated the strategy has merit, he doesn't advocate that growers abandon fungicides and rely on wheat until he has conclusive evidence in a field situation.

"Washington has a progressive apple industry, and they're really interested in this work," Mazzola says.

Next he'll try to identify why some wheat cultivars work while others have little or no effect. So far, he hasn't found any commonality among wheat types—such as hard red or soft white—only that some varieties provide a good environment for P. putida while others don't.

Last Updated: July 24, 2000