Fire Blight Control, Nature's Way
Plant pathologists Virginia Stockwell (ARS) and
Oregon State University's Kenneth Johnson inspect Bosc pear trees and prune out
Apples and pears are big business for both large and small fruit growers in
the Pacific Northwest. Washington, Oregon, and California produce more than 6
million tons of the two fruits annually, worth more than $2 billion.
Research by Agricultural Research
Service and university scientists is helping the growers manage one of the
industry's major disease challenges, fire blight. In addition to the standard
chemical controls, growers are beginning to have access to another tool:
The bacterium Erwinia amylovora causes the blight. Wind, rain, and
insects carry the bacteria to fruit blossoms. Warm, wet weather helps the
bacteria reproduce. Once a large population builds up, the bacteria can infect
the blossom and spread internally through the stem. Fire blight isn't a problem
every year, but when it does flare up, growers can spend thousands of dollars
removing infected tree limbs.
In the 1950's, the antibiotic streptomycin became the treatment of choice.
Copper and other chemicals joined the arsenal later.
But the favored streptomycin began losing its effectiveness in the 1970's
because E. amylovora was showing resistanceat least in California.
In 1988, ARS plant pathologists Joyce E. Loper and Rodney G. Roberts
spearheaded an effort that showed streptomycin resistance was taking hold in
Washington. Since then, ARS plant pathologist Virginia O. Stockwell has found
streptomycin resistance in Oregon.
"Unfortunately, streptomycin sprays for fire blight control are less
reliable than in the past," Stockwell says. She and Loper work at the
Horticultural Crops Research Laboratory in Corvallis, Oregon. Roberts is at the
Tree Fruit Research Laboratory in Wenatchee, Washington.
To help growers manage the streptomycin resistance, the ARS scientists
intensified their workand helped with university research already under
wayon biological control options.
The first commercial biological control product went on the market in 1995
after years of testing. Blight Ban, produced by Plant Health Technologies of
Boise, Idaho, uses a beneficial bacterium, Pseudomonas fluorescens
Pf-A506. This strain of Pseudomonas competes with the fire blight
organism for nutrients on blossoms, keeping the numbers of Erwinia low
enough to avoid severe infection. The key is to give the biocontrol organisms a
head start so they can build up a large population before the pathogen arrives.
Steven E. Lindow, plant pathologist at the University of
California-Berkeley, discovered Pf-A506 in 1979 as beneficial for reducing fire
blight and frost damage. To confirm its effectiveness, ARS scientists
contributed 5 years of testing to Lindow's 16 years. Working with Oregon State
University plant pathologist Kenneth Johnson, Stockwell and Loper also found
that Pf-A506 maintains optimum effectiveness if freeze-dried.
Another important finding: The biocontrol can be used safely with
antibioticsbut not with copper, which is also used for fire blight
control. Now they're testing Pf-A506 in combination with other biocontrol
organisms, such as E. herbicola C9-2. Although this bacterium is related
to the pathogen that causes fire blight, it is a different species and does not
hurt the fruit trees. In fact, C9-2 produces antibiotics that inhibit the
growth of E. amylovora. Plant Health Technologies hopes to use C9-2,
once registration with the U.S. Environmental Protection Agency is complete.
More Biocontrols on the Way
A new technique developed by ARS plant pathologist Larry Pusey in Wenatchee
is speeding up the search for effective biocontrols. The technique allows
efficient, year-round screening for naturally occurring organisms with the
potential to suppress E. amylovora.
Pusey's method uses crab apple nursery stock. These trees are cheap and
readily available because they are used by growers to help pollinate apple
Pusey gets the small trees from nurseries in December or January. He uses
some and holds others dormant until needed. By stripping leaves off trees that
have already provided blossoms and adding a growth hormone, he can induce
individual trees to bloom twice in 12 months. Currently, he gets new stock
every year, but he thinks he can manipulate the trees to provide blooms twice a
year for 2 or 3 years before new ones are needed.
Pusey developed a laboratory assay that uses live blossoms plucked from
greenhouse trees to test for effectiveness of an organism in controlling fire
blight. He is now in a position to pre-screen thousands of new organisms and
conduct in-depth studies with those that are especially promising. Previously,
researchers had to assess the value of potential biocontrols by growing them on
sliced pear fruit, which is very different from the flower tissue the slices
are meant to represent.
Plant pathologists Virginia Stockwell (left) and
Joyce Loper examine genetically engineered bacteria on a colony counter.
"Our lab results closely mirror what happens in actual field studies,
or what would happen assuming that field studies could be done. Fire blight is
not a problem every year. If the right weather conditions aren't present, we
can't successfully field-test new organisms," says Pusey.
Pusey has one natural bacterial strain that seems very promising. He has
applied for a patent and private industry has shown interest in further
Because biological controls are living organisms, they don't always perform
consistently over a wide range of conditions. Loper and Stockwell in Corvallis
are trying to resolve that problem at a genetic level.
"Variability is a huge obstacle to using biocontrol in
agriculture," Loper says. "The goal of our work is to identify
sources of variability so that we can use biocontrol organisms more
When sprayed on trees, Pf-A506 often establishes well on pear or apple
blossoms. But it sometimes dies off rapidly and does not persist long enough to
suppress the fire blight pathogen. On the plant surface, bacteria face drying
out (desiccation) and temperature changes, Loper says. Her goal is to
manipulate beneficial bacteria genetically to help them thrive under adverse
In Pseudomonas fluorescens Pf-5, a strain related to the active
bacterial agent in Blight Ban, Loper identified a gene called rpoS that
governs how bacteria respond to environmental stresses like desiccation.
Stockwell and Loper are now doing field tests to see if rpoS influences
the capacity of Pf-A506 to survive on blossoms or to suppress fire blight.
ARS' commitment to fire blight control doesn't stop at the orchard fence.
Plant pathologist Roberts has worked for the last 11 years on international
plant quarantine issues related to fire blight. The West Coast fruit industry
depends heavily on exports to maintain profitability and good domestic prices,
but the presence of fire blight in the United States has limited development of
new markets in Asia, Europe, and Australia.
Roberts' work showed that commercially produced apples from Washington do
not pose a significant risk of carrying the fire blight pathogen to countries
where fire blight does not occur. Roberts also led a team that confirmed the
effectiveness of a postharvest chlorine treatment now used for all apples
exported to Japan.
Similar research with pears done collaboratively by Roberts and Pusey kept
open the Brazilian export marketcurrently valued at $35 million
annuallyand might allow future exports to Chile, should U.S. growers
attempt to develop a pear market there.By Kathryn Barry Stelljes
and Dennis Senft (retired). To reach the authors, contact
Marcia Wood, Agricultural Research
Service Information Staff, 800 Buchanan St., Albany, CA 94710; phone (510)
Joyce E. Loper and
Virginia O. Stockwell are at the
cultural Crops Research Laboratory, 3420 N.W. Orchard Ave., Corvallis, OR
97330; phone (541) 750-8760, fax (541) 750-8764.
Larry Pusey and
Rodney G. Roberts are at the
USDA-ARS Tree Fruit Research
Laboratory, 1104 North Western Ave., Wenatchee, WA 98801; phone (509)
664-2280, fax (509) 664-2287.
"Fire Blight Control, Nature's Way " was published in the
January 1998 issue of Agricultural Research magazine. Click
here to see this issue's table of contents.