| Fireblight is caused by the bacterium, Erwinia
amylovora. It is a serious disease that affects new
leaves, fruit, flowers, and stems of over 75 species of
trees and shrubs in the rose family including: apple,
crabapple, hawthorn, pear, pyracantha, cotoneaster,
spirea, flowering quince, and mountain-ash.
Temperature, humidity, insect vectors, and
wounding can all affect the severity of infection. If
not controlled, it can cause flower and fruit blight,
twig and branch dieback, or even kill the plant.
Symptoms and Diagnosis
Symptoms begin as dark green, water-soaked spots
on tissue where it is penetrated by bacteria.
Common entry points are through wounds,
blossoms, and natural openings such as stomatas
and nectaries. As the disease progresses, leaves and
twigs take on a black shriveled appearance as if
scorched by fire. A characteristic shepherd's crook
often develops as tender growth rapidly wilts and
dies. As the twigs die back, dead sunken areas on
the stem called cankers develop. These can girdle
the stem causing even more dieback. Young trees
can be killed in one season, but it generally takes
several years of continuous dieback for death
to occur.
Life Cycle
Fireblight bacteria overwinter in cankers larger than
one inch in diameter and in mummified fruits. In
spring, the cankers produce a white or amber ooze
which contains the bacteria. The ooze dries and is
carried to new sites by insects, wind, rain, or
pruning equipment where it causes new infections.
Hail damage is often followed by a rapid spread of
the disease. When terminal buds set and tissue
hardens, the disease stops progressing.
Integrated Pest Management Strategies
1.Prune out diseased wood. In late summer or winter, prune out
all diseased wood at least 10 to 15 inches below the
infection and destroy. Disinfect tools between cuts by
dipping in a 10% chlorine bleach solution. Oil tools
after use. Avoid pruning in spring
when bacterial ooze is prevalent. Also, clean up and
dispose of any dead twigs and mummified fruit on
the ground. These may harbor the bacteria.
2. Minimize succulent vegetative growth. Ensure good
drainage and avoid heavy fertilizing in spring that
leads to an abundance of succulent growth. If plant
vigor is lacking, fertilize lightly in the fall. Avoid
excessive pruning which can also cause lush growth,
but do remove suckers at the base of the tree and
watersprouts which arise from the trunk or branches.
3. Remove cankers. If cankers are present on large
branches and trunks, remove them in winter.
Include 1 inch of healthy bark on each side and
3–4 inches on the ends. Disinfect all tools
between use.
4. Eliminate old pear, apple, or quince trees in the
area that may harbor the disease.
5. Replace diseased plants with resistant varieties.
Many resistant varieties are now available. Ask at
your nursery or garden center for fireblightresistant
varieties.
6. Chemical sprays are not very effective.
streptomycin or Bordeaux sprayed during bloom,
then every 3–5 days when the daily high is 65
degrees F and humidity is about 65% can
minimize infection. Most homeowners would be
better served by replacing a problem tree than
going to the expense and bother of spraying. |
