Fire blight is a type of bacterial infection caused by Erwinia amylovora. This bacteria is most
often found in apple, pear and mountain ash varities in our area but can also be found in any of
the rose family, cotoneasters, hawthorns, serviceberries and even blackberries and raspberries. It
is important to know about these other hosts especially if you keep finding repeat infections. You
may have removed it from your trees only to have them get re-infected from one of the
Trees can show symptoms within days of being infected. Symptoms will show up at the site of
infection but will quickly spread if the weather is ideal. One of the things they all have in
common is an overall appearance of having been scorched by fire, which is where it got its
name, fire blight.
These are one of the most vulnerable places for infection to set in. They are open to the world
and have a direct link into the “veins” of the tree. Affected flowers will look water soaked and
have a dull washed out, gray/ green color which will eventually shrivel and turn brown/ black.
Leaves can be infected through natural openings such as the stomata or wounds inflicted by wind
or even hail damage. They will first develop the same off color water soaked look as the flowers
then wilt and turn brown or black. A key factor is that they will remain on the tree instead of
falling away like they would if naturally browning out.
Shoots get infected through any small wound such as those caused by branch rubbing, wind
whipping, insect or animal feeding etc. Shoots will quickly wilt and bend forming the iconic
shepherds crook. If there are several infected shoots in a grouping that part of the tree can take
on a scorched appearance.
Large Branches or Trunk:
A branch or main stem will be infected by either a wound or through the xylem/ phloem coming
from an infection site into the trunk or larger branch. Infected wood will dry, turn black and can
even have a shrunken appearance. Root and trunk suckers are highly susceptible and give easy
access to the main stem.
Erwinia amylovora overwinters on a branch or stem canker. Once the weather gets above 70
degrees the bacteria’s growth rate goes into overdrive and the canker will start to push out a
bacterial ooze that is sweet and very attractive to many insects such as ants, bees, flies and
aphids. This ooze will start out as white but will turn an amber color as it ages. These insects will
than bring the bacteria to any wounds they run across or flowers through pollination. You will be
seeing symptoms within days to weeks after the initial infection depending on outside
temperature, humidity and growth rate of your tree. If the infection reaches the main stem of the
plant it will die.
Once this disease has taken hold there is no cure. The only thing we can do is try to manage the
disease by: planting resistant varieties, use cultural practices that will strengthen the plant
without strengthening the pathogen, pruning out infected tissue(amputation) and some chemical
sprays. Planting resistant varieties is your best bet with quickly removing infected tissue as your
second. Chemical spraying is not a viable option for most homeowners as timing is critical to
success and incredibly tricky. It is also no good as a curative, it can only help prevent infection
from spreading or taking root.
The main goal of our cultural practices is to minimize the rapid growth of vulnerable tissue when
fire blight is most active. To do this we recommend reducing the amount of nitrogen fertilizer
applied to help reduce the number of water sprouts and adventitious shoots. It is also helpful to
prune less out at any given time as well as to keep the cuts on larger limbs to a minimum. Young,
vigorously growing trees are naturally more susceptible than older more established stands.
When you have spotted fire blight in your trees it is best to prune it out during the off season
when the bacteria is dormant and there is no risk of dislodging spores or opening new infection
sites. Keeping that in mind there are times when it is necessary to prune during the summer
season. This is when the infection is in a young tree (the bacteria can quickly spread and girdle a
main stem if you wait until the off season), when it is found in a highly susceptible variety or
when the number of infections in an older tree is limited and can be removed with ease.
Once you have decided that pruning is necessary it is important to remove all signs of the blight
8-12 inches below the site of infection. It is better to remove a little more now than to find out
later that your tree is going to die because you were too timid to get it out. If you are doing this
during the summer it is CRITICAL to sterilize your tools between EVERY cut. Fire blight is an
active bacteria and can spread as easily as strep throat in a preschool. This can be done by
dipping or spraying your tools with household bleach or ethyl alcohol. Winter removal is a
simpler process as the bacteria will be dormant and you will not need to sterilize your tools in
between pruning cuts. Once you have finished it is best to remove and destroy (preferably burn)
all infected brush. Do not send it to the landfill as that could easily spread the infection
throughout the county.
http://extension.colostate.edu/topic-areas/yard- garden/fire-blight- 2-907/