Notes on Apple Diseases
Fire blight
Identification
The bacterium that causes fire blight (Erwinia amylovora)
attacks the blossoms, shoots, fruit and rootstock of apple trees.
Blossom blight develops when warm moist conditions occur during
bloom. Blossoms infected with fire blight appear water soaked and
the petals, sepals and receptacle wilt and turn brown or black.
Fire blight affects new shoot growth throughout the spring and summer.
Bacteria enter the shoots after they have been damaged by wind,
hail or sand. Infected shoots become discoloured and shrivel. Eventually
the leaves turn brown or black and begin to ooze. Infected shoots
droop at the tip, to create a symptom often referred to as a shepherd's
crook.
Fruit damaged by hail, high wind, rain or insects can readily be infected
by Erwinia amylovora. Initially, infected fruit appears grey, green
or water soaked and eventually the fruit turns dark brown and mummified.
Some rootstocks such as M.9 and M.26 are susceptible to fire blight infection.
Fire blight symptoms in rootstocks are similar to phytophthora collar
rot. Infections initially appear dark, water soaked and purplish, with
indefinite margins. Later the margins become more definite and are marked
by a crack or crevice. The removal of bark in the infected area will reveal
red-brown streaks in the internal tissues.
Period of activity
Fire blight blossom infections occur during bloom; however shoot infections
and trauma blight can occur throughout the growing season. Once terminals
have hardened off during the summer they are less susceptible to fire
blight infections.
Thresholds
Maryblyt and Cougarblight are two software programs that determine the
need and timing of sprays to control fire blight during bloom. These programs
build models to predict fire blight infections through the use of daily
minimum and maximum temperatures, rainfall amounts, leaf wetness from
heavy dew and weather forecasts to predict the potential of fire blight
infection. Advanced warning of one to four days for fire blight infection
is provided in the critical susceptible growth stages. The most precise
prediction for your farm can be obtained by using your own data. Copies
of the Maryblyt program are available from Gary Lightner, 293 Tuscawilla
Hills, Charles Town, WV 25414, Tel: 540-869-8372, e-mail:gary.lightner@maryblyt.com.
For information on Cougarblight, see www.ncw.wsu.edu/treefruit/fireblight/2000f.htm.
Management notes
Warm temperatures of 18°C or greater and moisture from rain or heavy
dew, creates a favourable environment for infection of blossoms
in spring. When these conditions are predicted and if they persist
through the blossom period, protect the blossoms with Streptomycin
17. Apply in slow drying conditions. Spray is most effective when
applied in high volumes of water prior to a wetting period. For
best results use Streptomycin 17 alone in the tank.
Streptomycin 17 is sensitive to UV light and is effective for only two
to three days. If warm, wet weather conditions prevail, two to four
sprays in the bloom period may be necessary toprotect newly opened
blossoms. Excessive use of Streptomycin 17 may lead to fire blight
resistance.
In non-bearing orchards, copper sprays may be substituted for Streptomycin
17; however, copper is less effective. Mix copper sprays with lime
according to label directions.
Cultural management is very important to reduce the incidence of
fire blight. For more information on the use of cultural practices
to manage fire blight, refer to Fire
blight of pear.
Recently Apogee, a plant growth regulator, has received registration
for suppression of fire blight. Apogee reduces terminal growth and
makes the terminals less susceptible to fire blight infection. Apogee
is ineffective for control of blossom blight infections and has
no activity against fire blight bacteria. Apogee should be applied
at a rate of 1350 g/ha, based on a dilute water volume of 3000 L/ha,
in late bloom or early petal fall when the shoots are 2.5-5.5 cm
long. Make subsequent application(s) at 14-21 days if fire blight
risk remains high based on predictive models and field scouting
of fire blight in the orchard. Apogee must be applied preventatively
for effective management of fire blight because Apogee is a strong
growth regulator and will limit shoot extension. When you use the
high rate for fireblight suppression, shoot extension may be reduced
more than is desired in non-vigorous plantings.
For a list of susceptible varieties, see Examples
of Apple Cultivars Susceptible to Key Diseases. For more information,
see OMAFRA Infosheet on Fire
Blight.
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