Quote:. . . fire blight bacteria can move downward inside apparently healthy branches . . . into
the rootstock.

Effect of Pruning Out Fire Blight Infections on Incidence of Rootstock Blight

John L. Norelli1and Alison DeMarree2

1Department of Plant Pathology, New York State Agricultural Experiment Station, Cornell
University, Geneva, NY 14456
2Cornell Cooperative Extension, Newark, NY

Presented at the 42n dAnnual IDFTA Conference, February 20-24, 1999, Hamilton, Ontario,
Canada.

SHOOT INFECTIONS AND ROOTSTOCK BLIGHT
Fire blight (Erwinia amylovora) can kill dwarf apple trees by girdling susceptible rootstocks or
interstems, especially M.9 and M.26. We now know that these infections can originate by
internal movement of bacteria from infection in the scion and through infected root suckers. In a
field experiment, about equal numbers of trees became infected via suckers, and via some other
entry path, including internal movement of bacteria within the plant, from blossoms and shoot
infections, and direct infection of rootstocks through growth cracks or wounds caused by
mechanical, insect or freeze injury.

We have determined in greenhouse experiments that fire blight bacteria can move downward
inside apparently healthy branches and the trunk from infections in shoots into the rootstock.
Movement of bacteria to the rootstock occurs rapidly.Bacteria were detected in the M.26
rootstock of Empire trees 21 days after inoculation of the shoot tip and 41 days after inoculation
in Golden Delicious trees.To determine the effect of growth stage of the tree on the movement
of the bacteria through the scion, Empire trees on M.26 were inoculated in the greenhouse at
different times during the growing season.Surprisingly, it was found that most internal spread
occurred in more mature shoots (10 weeks after bud break) rather than in younger, more
vigorously growing shoots. Therefore we think that late-season infections may be particularly
dangerous for the rootstock. When several resistant and susceptible scion varieties were
inoculated withE. amylovorain the field and later tested for movement of bacteria into
rootstock, we showed that the bacteria moved into the rootstock from both resistant and
susceptible scion varieties. Our preliminary results show that age of the tree may affect
susceptibility to rootstock blight.

ECONOMICS OF SHOOT INFECTIONS
It is often recommended that fire blight infections be pruned out of young apple trees during the
growing season to prevent rootstock infection. We evaluated how effective pruning was on
Empire, Jonamac, and Liberty trees on M.26 rootstock and trees of Empire, Liberty, and Mutsu
on M.9 rootstock. Pruning out scion infections 3 weeks after blossom inoculation and then
repeatedly during the growing season gave variable results in 3 years of experiments.In 1995
and 1997 pruning had no beneficial effect on eventual death of the rootstock, but in 1996 pruning
reduced rootstock death.However, 2 to 3 consecutive years of pruning reduced fruit yield.In
comparison to non-inoculated trees, fruit yield on M.9 trees was reduced by 6% on unpruned
inoculated trees, but by 62% on pruned inoculated trees (2 years of pruning).On M.26 trees

yields were reduced by 8% on unpruned inoculated trees, but by 79% on pruned inoculated trees
(3 years of pruning).Despite these high yield reductions, an economic analysis (conducted with
Alison DeMarree, Cornell Cooperative Extension) indicated that pruning out fire blight
infections was cost effective.Accumulated net present value (NPV) 20 years after planting for
an M.26 planting (272 trees /acre, central leader) without fire blight was $4,684, compared with -
$47 for unpruned inoculated trees and $521 for pruned inoculated trees.The improved
profitability in the pruned treatment was a result of the reduced tree loss in 1996.The analysis
also indicated that replanting, rather than pruning fire blight out of infected trees, may be cost
effective if severe fire blight occurred in trees in their 1st or 2nd leaf, but losses from replanting
greatly increased for trees in the 3rd, 4th, or 5th leaf.In the trial M.26 trees were in their 3rd,
4th, and 5th leaf in 1995, 1996, and 1997, respectively.For example, if a fire blight epidemic
required replanting 50% of the trees in the 2nd year and 3rd year, accumulated NPV was reduced
to $3,191 and $2,001, respectively, whereas replanting in the 5th year or 6th year reduced NPV
to -$6,360 and -$9,290, respectively.

SHOOT BLIGHT
Shoot blight can be an extremely damaging phase of fire blight.Unfortunately, its biology is not
well understood and it is difficult to control.In general, shoot blight is more likely to occur in
orchards with blossom blight or a previous history of fire blight, after hail or high wind
thunderstorms, or when insect vectors with sucking or piercing mouth parts are present in the
orchard.Numerous experiments and field observations have indicated the importance of wounds
as avenues for entry of fire blight bacteria.Types of injuries vary from small insect punctures
and stem abrasions to large wounds caused by severe wind, hail, or frost.

The following are recommendations for the control of shoot blight:
•Fertilization programs should be designed to:
1)discourage late vegetative shoot growth
2)provide proper balance of the major nutrients and especially avoid an excess of nitrogen.
•Pruning out infected shoots to limit the spread of shoot blight is of doubtful benefit on large
trees but is recommended on young small trees, particularly those on M.9 or M.26 rootstocks
or interstems.To effectively limit damage, strikes should be pruned out as soon as they
appear throughout the terminal growth period; begin checking for symptoms about 90 to 100
degree days (base 55ºF) after an expected infection event such as rain during bloom or a
summer hailstorm.During periods of active lesion extension, pruning cuts should be made
6-12 inches or more below visible lesions, because large numbers of bacteria are present in
tissues that do not yet appear to be symptomatic.
•An application of streptomycin (Agristrep 17WP 1/2 lb/100 gal dilute basis) is strongly
recommended following a hailstorm in fire blight-affected orchards. This application may be
critical if even moderate amounts of blight were present before the storm.Sprays should be
complete within 24 hr after the start of hail.Growers in the U.S. should be aware that on
apple streptomycin can be applied until 50 days before harvest.
•To reduce the chance of developing resistance to streptomycin, the routine use of
streptomycin to control the spread of shoot infections isnotrecommended.
•Should blight develop, it is also important to maintain control of insects with piercing-
sucking mouth parts (aphids, leafhoppers) in order to reduce further spread.
•Summer prune during dry weather, especially in orchards with a previous history of fire
blight or when fire blight is present in nearby blocks.Caution is needed especially for

susceptible varieties (Crispin, Fuji, Gala, Gingergold, Idared, and others).Summer pruning
should be avoided entirely in orchards that are severely affected with fire blight.
The grass sod or cover crop should be well mowed early in the season and then allowed to
grow in midsummer to check tree growth.

•

The above points are based upon recommendations found in Cornell Cooperative ExtensionPest
Management Recommendations for Commercial Tree-Fruit Productionand van der Zwet, T. and
S.V.Beer. 1995.Fire Blight—Its Nature, Prevention, and Control: A Practical Guide to
Integrated Disease Management. U.S. Department of Agriculture. Agriculture Information
Bulletin No. 631, 97 pp.Copies of a revised edition of the latter publication should be available
July 1999 from the Government Printing Office on the web at: www.access.gpo.gov, phone: 1-
202-512-1800, fax: 1-202-512-2250, or by mail: Superintendent of Documents, P.O.Box
371954, Pittsburgh, PA 15250-7954.

SHOOT BLIGHT CONTROL
Rapidly growing, succulent apple shoots are more susceptible to fire blight infection than are
slow- or non-growing shoots.Prohexadione calcium (formulated as BAS 125 or Apogee, BASF
Corporation) is a plant growth regulator that reduces the vegetative growth of apple trees.
Recently, Apogee has been shown not only to suppress apple shoot growth but also to reduce the
incidence of shoot blight infections and suppress the extension of lesions.In tests we conducted
in New York last year, treatment of Idared trees with Apogee resulted in less shoot growth and an
86% reduction in the severity of shoot blight.The effect of Apogee on fire blight severity was
far greater than that of streptomycin, which resulted in a 39% reduction in disease severity.

Although Apogee is not yet registered for fire blight control in New York, it has great potential as
a tool to control the shoot blight phase of fire blight in the future.However, due to its growth
effects, its use in orchards will need to be integrated into other orchard practices.Application
will need to be timed carefully to have the desired, but not excessive, growth reduction effects.
Interaction with thinning sprays will need to be evaluated.In addition, the effect of treating
young trees (less than 4 years old) on yield will need to be evaluated.