I've enjoyed reading the postings and opinions concerning how to
manage fire blight in Alex's Ernst Bosch apple tree. Those of us who
have worked with or experienced fire blight know that this disease is
very difficult to understand and manage. What works on one cultivar,
in one year, or in one location may fail to work on another
cultivar/year/location and what is cost-effective for one apple
grower may be totally implausible in other economic models. Thus,
I'm not going to debate Alex's observations.
HOWEVER, I feel obligated to point out that within the boundaries of
the United States (and I believe that still includes California),
application of an antibiotic paste to pruning cuts is a violation of
federal law. Any product used in that manner would require a federal
pesticide label from EPA. That requirement applies to homeowners and
hobby farmers as well as commercial growers. So far as I know, no
antibiotic pastes are labeled for use on tree fruits to control fire
blight. There are good reasons for these regulations, but I won't
take time to discuss them. The bottom line is that using human or
veterinary antibiotics on fruit trees or other crop plants is both
illegal and potentially dangerous.
I posted a while back on an Ernst Bosch Apple tree that came down
with fireblight. I wanted to share with the group what had happened,
although I must say that after reading the latest Illinois Fruits
and Vegetable Crop news, I am grateful not to have to deal with the
fireblight outbreak that the Midwest is experiencing this year.
We are not in a fireblight prone area due to our cool, dry
conditions, but this year, fireblight has broken out in a number of
local orchards thanks to a warm and humid, rainy and drizzly week in
May. So this has been a learning experience for me.
I finally figured out how to deal with the fireblight infection,
here is what I did:
Since only one of my 200 trees (of which 80 are apple trees on
MM111) came down with fireblight, my focus was to eliminate any
further disease vectors, so removal of the tree is the only
solution. However, I am a scientist by training, and my curiosity
got me to play with the tree for a bit to try to understand what is
going on.
Here are some of the observations:
Step one was to cut, but to my dismay, the infection returned, even
after several cuts. Disinfecting the shears seemed irrelevant. The
morning dew collecting on the cuts would often turn orange, and the
drops would carry the infection to lower limbs.
Step two was to try to figure out if the re-infection vector was
external, or the bacteria was already systemic in the cambium. So
for the next sets of cuts, my approach was to coat the cuts with a
oily antibiotic paste with a bactrin base (bactrin is a wide
spectrum antibiotic). The coating would prevent re-infection from
external sources, but not if the bacteria is systemic in the
cambium. Sure enough, another set of cuts lead to more infection,
the ooze coming from under the paste. It only took 24 hours for the
wood to turn orange under the paste, proof that the infection came
from inside the tree, not from external sources or from the pruning
sheers.
Step 3 was to prune back radically, past the node of the infected
branch. Upon close inspection, I discovered that the cambium had a
darker shade, Cutting into the darker shaded areas revealed healthy
wood, but the cut turned orange within 24 hours and would start to
ooze. However, cutting several inches below any slightly darkened
areas did not cause the wood to turn orange, and no re-infection
took place.
Step 4 was to douse the tree with antibiotic to make sure there is
not any residual fireblight bacteria on the tree. I used a
combination of bactrin and retromycin sulfate to treat the tree and
surrounding mulch.
At this point, it's safe to say the fireblight active infection is
gone, although I believe the bacteria is still in the tree, but
dormant for now even though there are no visible cankers. Since the
tree is so young, I plan on removing and replacing it after the
harvest this October. But before I do, I also plan on feeding the
tree some heavy nitrogen to get it to grow and see if the new growth
will show signs of re-infection.
My conclusion for controlling fireblight is the following: the
bacteria travels much, much further than 6-12 inches, and one must
cut past any infected ambium: Chances are trees could carry
fireblight even without showing any active signs of infection.
1) Always coat the cuts with an antibiotic oil-based paste to
prevent re-infection from outside vectors.
2) The wood must be placed into a plastic bag immediately and not
allowed to touch anything else. So it's a good idea to cover any
oozing branch into a plastic bag and break off the branch, then do
the pruning so no ooze droplet hits any other part of the tree.
3) Always disinfect the sheers in case the previous cut was into
still infected wood. Preferred disinfection is a field heat
treatement disinfector, which should be used anyway to prevent any
sort of disease spread. You wouldn't want the doctor not to
disinfect his surgical instruments before treating you, a plant is
no different.
4) Always observe for orange discoloration under the paste for the
next 24 to 72 hours. If the orange discoloration returns, the cut
was made into more infected tissue, and you have to cut further.
5) After any infected tissue is removed and discarded, the tree
needs to be treated externally with an antibiotic to remove any
residual bacteria.
I cannot vouch that the above would work outside of Santa Cruz
county; when I see the pictures of fireblight in the midwest, it
makes me wonder how one would even deal with fireblight once it
broke out, the only reasonable approach seems radical removal, and
preventative spraying in the Spring.
--
**************************************************************
Dave Rosenberger
Professor of Plant Pathology Office: 845-691-7231
Cornell University's Hudson Valley Lab Fax: 845-691-2719
P.O. Box 727, Highland, NY 12528 Cell: 845-594-3060
http://www.nysaes.cornell.edu/pp/faculty/rosenberger/
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