Download 4-25-12 Fire Blight of Pear

Fire Blight of Pear
by Keith C. Hansen, Extension Horticulturist
In early April, many people began noticing dead twigs in their flowering pear trees. The twigs
look like they have been scorched with a blow torch. The ends of tender new growth curls,
looking like the crook of a shepherd's staff. The blackened leaves cling to the blighted twigs.
These are the classic symptoms of fire blight.
Fire blight is a bacterial disease, and is one of the worst diseases of pears and apples, since it is
capable of killing trees of susceptible varieties. Many edible pears and apples are very
susceptible to fire blight and easily contract the disease, and is thus one of the factors limiting
their production in our area.
Most ornamental pears are resistant, but they are not immune to fire blight. Certain varieties, like
Aristocrat, are more susceptible than Bradford, one of the more common varieties of ornamental
pear. Most years, fire blight is not a problem on ornamental pear.
Fire blight, like all diseases, requires a certain set of circumstances to occur in order for the
disease to take place. This includes a warm period prior to and during bloom, and rain during
bloom. When conditions are favorable for bacterial growth, populations can develop rapidly. At
70 degrees, bacterial numbers double every 20 minutes, becoming billions in less than a day.
Each bacterium is capable of causing an infection!
It appears environmental conditions were just right this year during the pear blooming season.
However, I've seen lots of ornamental pears around town that appear to be fine.
The bacterium overwinters in cankers, or dead areas, on twigs and limbs. In the spring, the
bacteria in the cankers become active and an ooze containing bacteria will emerge which attracts
various types of insects. These insects carry the bacterium to blooms, and then honeybees
visiting the flowers pick up the bacterium and pass it to other blooms, continuing the spread of
the disease.
The bacterium can also be spread by hard, driving rain, and it will infect plants through wounds
caused by hail, insects, and fresh pruning cuts.
Besides rainfall, other environmental conditions and cultural practices that can promote fire
blight development include excess moisture, high rates of nitrogen, and excessive pruning.
Anything that promotes rapid and soft, succulent growth will make that tissue more susceptible
to infection.
So, what can you do if your trees are showing symptoms? Pruning is one method of reducing the
source of future infections. Whether to prune now or later partly depends on the age and size of
the tree. If you have a very small, young tree showing symptoms, further spread of the disease
could seriously damage the tree. So, removal of infected growth should be done now.
For older, larger trees, or plants with only a few infections, it may be better to wait until later in
summer when all the new growth has hardened, and stimulating new growth through pruning is
not as likely.
It is very important when pruning to remove the affected portions several inches below any signs
of canker on the shoots. Look carefully on the affected shoots for cankers which will appear
sunken, shriveled, and are often elongated, progressing downward from the dead shoot tissue.
Pruning equipment should be dipped in a 10 percent bleach solution for 2 seconds to prevent
spreading the bacterium from one cut to another. To prevent the bleach from corroding your
tools, clean and wipe them with light oil at the end of the day.
Affected trees can be treated with a copper fungicide (Kocide 101) or antibiotics, but ONLY
DURING THE BLOOMING PERIOD. More than one treatment needed, especially if blooming
is prolonged. Sprays at other times of the year other than blooming are ineffective.