FIREBLIGHT (Alias: "ERWIN")
by Jerry Parsons, Ph.D.
Horticulture Specialist, Texas Agricultural Extension Service
in San Antonio
"An ounce of prevention is worth a pound
of cure" is an old saying which has much application
in the world of growing plants. The idea of prevention rather
than cure is especially applicable when diseases caused by
fungi and bacteria are considered.
the beauty of spring blooms burst forth, stimulated by abnormally
warm temperatures, a concealed bacterial beast which we shall
refer to as "Erwin" will infect the most beautiful
of the spring bloomers--the pear.
Erwin, known to the scientific world as Erwinia
amylovora, is the bacterial organism which causes fire blight
of pears, apples and kumquats. Erwin's fire blight becomes
painfully apparent to those who have planted pear varieties
which are highly susceptible to its ravages. Such varieties
include the well-known Bartlett and any of the French-named
varieties. These susceptible varieties can be killed in one
year by Erwin's blackening destruction. The blackening of
leaves, twigs and, in severe cases, limbs resembles fire damage
which would be caused if the tree or tree parts affected were
burned. Thus the term fire blight. Damaged branches are usually
widespread throughout the tree but most apparent in the top,
new growth. All varieties are susceptible, but some, such
as Kieffer, Monterrey, Orient and Fan-Stil are more tolerant.
Fire blight is not much of a problem if temperatures
remain generally below 68
degrees F. during bloom and if there's little soil moisture
and the nutritional level of the tree is low. Homeowners can
regulate soil moisture and nutritional levels, but temperature
is the factor they must be concerned with since it cannot
When Erwin gets the warm, moist, well-fertilized
environment it likes, a single cell is capable of miraculous
feats--it can double itself every hour. That may not sound
like much at first, but at the end of a 24-hour period, that
one cell has become over 15 million cells.
Experiments have shown that a single cell can
reproduce enough to infect a tree 2-3 percent of the time.
If there are 100 bacterial cells at work in an ideal environment,
those 100 are enough to insure practically 100 percent infection.
So now, consider the penalties for delaying control measures
too long-- each random cell of Erwin suddenly has reproduced
its 16 million cells.
Thus, the key to successful control measures
is that they must go into effect before the pathogen begins
to multiply rapidly. A preventive attitude must be adopted.
A homeowner may not think he has even a single pathogen overwintering
on his tree and may buttress his confidence because he can't
see any evidence to the contrary. But appearances are deceiving,
and even trained
pathologists have trouble locating the pathogen during dormancy.
There may be stray bacterial cells of Erwin
overwintering in cankers, or they may be on a neighbor's tree.
You may ask, if Erwin is on a neighbor's tree, how is he going
to get into my yard? Not on the wings of a snow white dove--that's
But what about on the feet of a small brown
bee. When bees are actively pollinating blooms, they transmit
Erwin from infected trees to previously non-infected trees.
Obviously, with the number of industrious bees
on the loose and the probability of contamination which exists,
pear growers should take measures to curb Erwin's destructive
potentials. Antibiotics, such as streptomycin (Agri-Strep),
and copper-containing products, such as Kocide 101, will control
Erwin if applied at the proper time. For these products to
be maximally effective, the first spray should be applied
at green tip. (Green tip refers to first bud activity on the
tips of branches before bloom occurs.) The objective is to
get the level of these products on and in the tissue as high
as possible as soon as possible. Apply each product according
to label recommendations. Applications should be made in a
sequence of 4 or 5 sprays at 7- day intervals, or less if
rain occurs, from the time of green tip through petal fall
(when the petals fall from the flower). Applications before,
during and after bloom are necessary since all pear blooms
do not open simultaneously and to insure that when our dirty-footed
bee friends visit, Erwin will be killed. It couldn't happen
to a more deserving pathogen! These products will not hurt
the bee, just sanitize her! Even though bees carry Erwin from
tree to tree, take care not to kill them with insect sprays
during the bloom period. Honey bees and other insects are
necessary to pollinate the bloom. If pollination does not
occur, the bloom will fall off and no fruit will develop.
Your apple and pear trees will prosper from Erwin-prevention
action now and produce an abundance later. But prepare now
before you enjoy the first pear blooms of spring.
More information can be found at: