But methyl bromide is due to be
phased out by 2005. Other common pesticides used in the Northwest's 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.
And his 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-per-year apple industry. About half of the nation's apples are
produced here on about 180,000 acres. About 10,000 acres of apples are
replanted each year.
The Problem Intensifies Over Time
If a new orchard is planted on ground that was previously used for something
else, the harmful 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
It is accepted among scientists that chemicals exuded from plants affect the
soil that surrounds them, favoring specific populations of microorganisms.
Although the specific selective chemicals have not been identified, Mazzola has
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, in collaboration 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 the 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
growdespite the harmful fungiwhen 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
typessuch as hard red or soft whiteonly that some varieties provide
a good environment for P. putida while others don't.By
Kathryn Barry Stelljes,
Agricultural Research Service Information Staff.
This research is part of Plant Disease (#303) and Methyl Bromide
Alternatives (#308), two ARS National Programs described on the World Wide Web
Mark Mazzola is with the
USDA-ARS Tree Fruit Research
Laboratory, 1104 N. Western Ave., Wenatchee, WA 98801; phone (509)
664-2280, fax (509) 664-2287.