Fire Blight Under Wraps
This Rome Beauty apple tree inside an arborsphere at the Appalachian Fruit
Research Station in Kearneysville, West Virginia, has been pruned to remove all
It has now been well over a century since Louis Pasteur proved that bacteria
can cause disease in animals.
A few years after that well-known French scientist published his work, an
American professor, T.J. Burrill of Illinois, began working with an unknown
disease that was devastating apple and pear orchards in the Midwest. In 1880,
he discovered that the disease now known as fire blight was also associated
with a bacterium. And by 1885, Joseph Arthur performed the experiment at
Cornell University that proved the
suspect bacterium was truly responsible for the disease, earning him the first
Doctor of Science degree granted in America.
Like Pasteur, Burrill and Arthur faced scorn and derision from distinguished
scientists of their day. Eventually, a U.S.
Department of Agriculture scientist named Erwin F. Smith and colleagues
overcame the opposition and carried on additional research in the early 1900s
that proved that bacteria cause diseases in many plants. The fire blight
bacterium, Erwinia amylovora, was later named for him.
Today, yet another USDA scientist is investigating uncharted territory of
this bacterial disease. Plant pathologist Tom van der Zwet has been working
with fire blight at Beltsville, Maryland, and
Virginia, for 30 years.
"Fire blight attacks young fruit treesespecially pear and
applecausing leaves and fruit to shrivel and blacken, as though scorched
by fire," says van der Zwet. "Once it strikes, there isn't much that
growers can do.
"Despite all the research that'd been done on fire blight, we still
didn't know until recently if the bacterium resides in the large scaffold limbs
of an infected tree's vascular system, or if it actually reinfects the tree
each season," he says.
To find out, last year van der Zwet and soil scientist D. Michael Glenn,
entomologist Mark Brown, and technician Craig Cavin set up an aseptic,
whole-tree arborspherea kind of plastic growth chamberexperiment at
the ARS Appalachian Fruit Research Station in Kearneysville.
"From this research, we discovered that the bacterium that causes fire
blight doesn't live in a tree's older vascular system in numbers sufficient to
cause disease," van der Zwet reports.
Fire blight infects terminal leaves on an unpruned and uncovered apple
For the 6-month experiment, they used four severely blighted, 12-year-old
Rome Beauty apple trees that had suffered severe fire blight nearly every year
for the past 10.
In the fall of 1995, they heavily pruned the trees and removed any cankers
or damaged bark that might house bacteria. Then in March 1996, they covered the
trees with dormant insecticidal oil to kill any insect eggs. That treatment
ensured no insects would hatch that could wound the trees' succulent new
growth. Two weeks later, two of the trees were carefully hand-painted with
TennCop, a copper compound, to eliminate any surface bacteria.
On April 25, 1996, van der Zwet and colleagues covered two of the
treesone copper-treated and one notwith 14- by 14-foot, clear
plastic-and-pipe frames 16 feet high, creating two arborspheres. They equipped
them with air supply systems and bacterial filters. This confinement was
necessary because the bacteria could be carried by insects, rain, wind, or
perhaps even orchard tools, to attack new growth in the spring.
On June 20th, the scientists first checked for the fire blight bacteria by
lowering four open petri dishes containing a selective growth medium and sticky
traps into the arborspheres. They left them there for 4 days.
"Then, twice in early July, we placed similar dishes for 2 hours on the
outflow boxes where air exited the arborspheres," van der Zwet explains.
"On July 23, we collected 10 shoots, free of disease symptoms, from
several locations on the control trees that weren't under the
To check for the presence of fire blight bacteria, the plates from the
arborspheres and the control shoots were incubated in the lab at 80oF.
Plant pathologist Tom Van Der Zwet (center) and technicians John Walter (left)
and Larry Crim examine trees outside of the arborspheres for fire
There was no sign of bacteria on the samples taken from the arborspheres,
and sticky traps caught only a couple of white apple leafhoppers.
Neither was there any evidence of fire blight from a colony of adult potato
leafhoppers released into one of the arborspheres. But the control trees, not
protected by the sterile atmosphere of the arborspheres, were heavily infected
with fire blight.
"The unprotected trees began to show the first signs of blight on June
4, about 3 weeks after new growth emerged from the severe pruning we had done
in the previous fall, but no fire blight symptoms ever appeared on the trees in
the arborspheres," says van der Zwet.
The day after hurricane Fran destroyed the arborspheres on September 16,
1996, 20 shoots were collected from the protected trees and closely examined
and plated in the lab for the bacterianone were found.
"For the first time, we've shown that E. amylovora is not
present as a systemic pathogen in large scaffold limbs of trees known to have
been infected for the previous 10 years," says van der Zwet.
Results from the arborsphere experiment can help growers, according to van
der Zwet. Extremely heavy pruning results in an overabundance of new, tender
shoots that are more susceptible to fire blight infection. Therefore, when
trees are dormant, growers should remove only the blighted shoots and large
cankers caused by bacteria. However, proper pruning should also ensure that
there is adequate light penetration into the tree canopy to maintain good tree
"Our next step is to look at the presence of bacteria in younger shoot
tissues, internally and externally, and to study the role of cankers in primary
infection of the fire blight syndrome," van der Zwet says. By
Doris Stanley, ARS.
Small Cause, Big Effect
Although microscopic25,000 laid side by
side or 12,000 laid end to end would not measure more than an inchfire
blight bacteria can cause big problems. One reason is that each bacterial cell
is completely independent. So, under favorable conditions, cells multiply by
dividing at a phenomenal rate, reaching 10 billion in 72 hours.
Fire Blight Bulletin
Available from the U.S. Government Printing Office:
"Fire BlightIts Nature, Prevention, and Control: A Practical
Guide to Integrated Disease Management" (Agriculture Information Bulletin
631), published in 1995 by the U.S. Department of Agriculture, Agricultural
Research Service. GPO Stock No. 001-000-04617-9; price $7.00 in United States,
To order from Superintendent of Documents, phone (202) 512-1800, fax (202)
512-2250. On the World Wide Web, go to http://www.gpo.gov