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Beneficial Bacteria Help Thwart Apple
Disease By Jan
August 14, 2002
Instead of chemical fumigants, beneficial bacteria may be called
on to fight fungi that cause replant disease, so-named because it strikes young
apple trees planted in old orchards.
The Pseudomonas putida bacteria are part of a
bio-rational approach now being field tested by plant pathologist
Mark Mazzola, with the
Agricultural Research Services
Tree Fruit Research Laboratory in
Wenatchee, Wash., and
Granatstein, director of Washington State
Universitys Center for Sustaining
Agriculture and Natural Resources at Wenatchee.
In Washington state, where half the U.S. apple crop is grown,
replant disease is primarily caused by certain species of Pythium,
Rhizoctonia, Cylindrocarpon and Phytophthora fungi.
Unchecked, they can diminish a growers crop returns by $40,000 per acre
over 10 years, an orchards average production life.
Marketing demands often force growers to replant old orchards
with new varieties sooner than they might like. Fumigating orchards with
chemicals like methyl bromide offers a fast, effective way of sterilizing soils
before planting time. But methyl bromide is expensive to use, and will be
prohibited in 2005.
P. putida bacteria offer an appealing alternative because
they often occur naturally around apple roots and secrete antibiotics that
check infection by Rhizoctonia fungi. Mazzola, who identified the
bacteria, doesnt apply them, so much as augment their populations in
orchards. This is done by planting a cover crop of certain soft white wheat
cultivars whose root secretions create a favorable soil environment.
In a 2001 trial with Gala apples, this resulted in fruit yields
of about 46 pounds per tree versus 60 pounds where methyl bromide treatments
were used. Field tests begun this May may show whether applying dried rapeseed
meal after the wheat crop can synergize, or improve, the bacterias
Apple orchards often harbor many different replant disease
fungi. So the Wenatchee researchers are testing other bio-rational approaches
as well. These include using various cultural practices--planting trees in
drive aisles rather than rows, for example-- fungicides, and hardier apple
ARS, the U.S. Department of
Agriculture's main scientific research agency, has patented the beneficial