A few more comments concerning my observations on fire blight
that may help to answer some questions (or perhaps to muddy the
water). As I indicated in my first post, it is VERY, VERY difficult
to make any broad generalizations about fire blight that apply to all
cultivars/years/locations. The following comments are based on my
observations in eastern NY. If the observations/conclusions below
conflict with what others know to be true, I'd be happy to take the
corrections.
1. So far as I know, there is still open debate about
whether or not fire blight bacteria can actually over-winter in
symptomless tissue and then initiate disease again the following
year. There are situations where I believe that scenario must have
happened (e.g., nursery trees that come down with blight the year
that they are planted into the orchard). However, infection of newly
planted trees occurs infrequently. When it does occur, it often
results from infections of the blossoms that developed later in the
season than flowers on established trees due to the transplant shock
in the newly planted trees. Finally, even where there is no blossom
blight in newly planted trees that develop blight, there is always
the possibility that the blight actually survived in small cankers
that were overlooked during grading of nursery trees and subsequent
planting of the orchard.
2. We know that fire blight bacteria can move through trunks
of young trees and initiate rootstock blight in susceptible
rootstocks such as M.9 and M.26. However, I'm not aware of any
evidence that the bacteria can actually overwinter in the symptomless
trunk tissue and then cause disease the next spring.
3. Even where visible limb cankers exist, the literature
indicates that bacteria survive winter in only a very small
proportion of the cankers (without looking it up, was it about 10-15%
of cankers?). Thus, not every infection, even when it results in a
canker that is left in the tree, results in overwintering inoculum,
at least under conditions in NY and other cold-winter states.
4. I tend to agree that sterilizing clippers between cuts is
not important, especially if cuts are made during dry weather.
However, when blight showed up in my orchards the past two years (see
below), we still sterilized clippers between cuts whenever feasible.
5. I think that Dr. Paul Steiner's recommendation of "ugly
cuts" has merit. That is, when cutting back a blighted branch, the
cuts should be made between nodes on straight sections of branches or
at least 3-4 inches out from subtending limbs (if possible) so that
the end of the cut that is left on the tree will have no growing
points and therefore will dry out and die back. These cuts are
horticulturally incorrect cuts, but they ensure that bacteria left on
the pruning stub will end up in tissue that is not conducive for
bacterial growth.
6. The answer to the question of how far below the
infections one must cut to remove infected tissue is dependent on
several other variables. Last summer, we purposely inoculated 210
two-yr old trees of Lady Apple (very blight susceptible) on MM.111
rootstock to study factors that might affect spread of fire blight to
new shoots during summer. We inoculated shoots by simply misting a
suspension of bacteria over the surface of the shoots on a warm rainy
day (4 June 2008). The test trees had been pruned back to about 30
inches in height and had been purposely managed so that they had a
forest of upright shoots (about 25/tree) that were about 10 inches
long at the time of inoculation. We added high rates of nitrogen
periodically through early July to keep the trees growing vigorously.
The first symptoms of shoot blight (wilted shoot tips with slight
orange discoloration in the leaf veins) was observed on 9 June. By 16
June roughly 60% of the shoots had developed infections. We pruned
these shoots out as soon as we noted the infections (daily at first,
then just Mon-Wed-Fri). Thus infections were removed just as the top
2-3 leaves were beginning to wilt and/or develop an orange mid-vein.
In most cases we removed just 6 to 8 inches of the terminal shoot
below the symptoms. We had very few cases where the cuts turned
orange (as Alex observed) and where the branches therefore had to be
cut a second time. In the course of the season, we pruned out a
total of 2,707 blighted shoots from these 210 trees. In only about
three of the 210 trees did the infection progress into the main
trunk. CONCLUSIONS: If blighted shoots are removed during early
phases of symptom expression, one can usually eliminate the infection
by removing as little as 8 inches of shoot below the infection point,
and even highly susceptible trees did not become systemically
infected under conditions of our 2008 trial . Most growers do not
detect blight until infections have progress much further than we
allowed in our trial, so the general recommendation of attempting to
cut at least 18 inches below visibly damaged tissue is probably wise.
We also noted that the probability of having blight show up on old
cuts (i.e., we got orange stubs and had to make a second cut a week
or two after the first cut) seemed to increase as the season
progressed, but I can't be certain about that.
7. How far below visible infections a cut must be made will
depend on how long the infection has existed in the tree, on the
vigor and cultivar of the tree, and on the time that cuts are being
made. Blight bacterial will travel downward further and faster in
immature and vigorous trees than in mature trees with a full crop
load. Secondary infections that show up on shoots in late June when
terminals shoots have almost stopped growing will not progress very
far downward where as infections that occur earlier can very easily
travel backward 6 to 8 feet in a vigorous tree if the infection is
not removed in the early stages.
Another example: in my previous 32 years on this research
farm, we had only three previous instances where a small amount of
blight occurred in our orchards. One was an introduction on nursery
stock in 1986. One was a just three blighted twigs in one location
about 1989, and the third incident was just two trees in the corner
of a block in about 2004. We resolved the first two cases by just
pruning out infections and the latter case by removing the two
infected trees because they were full of blossom blight. In all 3
cases, we took preventive action the following spring (copper sprays
at green tip followed by strep sprays applied according to MaryBlyt
during bloom), and we never had any further problems. Incidentally,
our orchards are set up with multi-tree plots for pesticide trials,
and our cultivar mixes include such blight-susceptible cultivars as
Ginger Gold, Jerseymac, Redcort, Honeycrisp, and Cameo, all on M.9 or
M.26 rootstocks or M.9/MM.111 interstems. This spring we were not
too concerned about the blight risk from last year's trial because
that small research block of Lady Apples was in an isolated corner of
the farm, we had pruned out all of the blight, and weather conditions
during early bloom, while warm enough for blight, did not include
rain or dew. Therefore we did not apply strep during early bloom.
To my surprise, we ended up with a moderate amount of blossom blight
(perhaps 2 to 10 strikes per tree) spread across every planting on
our 20 acre research farm except for the Lady Apple block which had
no few blossoms. There was no apparent gradient of infection, and we
have no way of knowing if our blight this year originated in some
unknown way from the inoculum that we applied last year or whether it
originated from some other independent source. The latter seems
unlikely, however, given that we never had wide-spread blight on the
farm in 32 previous years, and given that in many previous years we
applied no strep at all even when MaryBlyt suggested environmental
conditions favored infections because we believed (correctly up until
this year) that no inoculum was present in our area.
Because of the long bloom period we had in 2009, the fact
that our plots have both early-bloom and late-blooming cultivars, and
the fact that initial infections occurred early during bloom of the
earliest cultivars, we were able to see the first blossom blight
symptoms about the time the late-blooming cultivars (e.g., Golden
Delicious) were at petal fall. At that time the early cultivars
still had some bloom on first year wood. We immediately treated the
whole farm with strep and include Apogee in some blocks. We also cut
blight 3 times/week for about 4 weeks and applied one more strep
spray ahead of a prediction for a hail-bearing thunderstorm in early
June. Ultimately, we had to remove major limbs in only a few trees
on the farm. Thus, pruning out blight (with help from foliar sprays
of strep and Apogee) was effective for arresting a blight epidemic
that, had we ignored it, might have destroyed many trees in our
orchards. And it is worth noting that we had the wettest June on
record with about 9 inches of rain to spread the bacteria to new
shoots.
CONCLUSION: early intervention via pruning (and perhaps
Apogee if it can be applied near petal fall) can arrest blight on
trees that had just a few strikes per tree. However, I agree with
Debbie Breth's earlier post that trees with extensive blossom blight
will either need to be removed (if there is just a few trees in a
block) or ignored until winter pruning.
Finally, I close this overly long discussion of blight by
re-stating that fire blight control strategies must be fine-tuned for
every individual situation. It is very difficult to provide blanket
recommendations that can be applied across diverse geographic regions.
Alex wrote:
My focus is on breeding, not commercial production, so I have about
80 apple trees with 450 varieties of apples. Because of our unique
climate (largely freeze free yet high chill) we can grow a very
large number of apples from low chill to high chill, and create
interesting crosses.
I am deeply concerned about fireblight in my orchard, given the
large # of varieties, I have to be very careful and try to keep the
orchard free of any potential vectors. We are not in a firebight
prone area, since our late Springs are very cool and dry with night
time temperatures well below 55F all the way into late June, this
year was an exception, and a number of local growers experienced
fireblight outbreaks due to a single week of warm and drizzly
weather with night time temperatures in the 60's.
The main thing I have learned from various off-line discussions is
that the antibiotic coating is a waste of time, of course, except
that it was useful for my experiment in that it demonstrated to me
that the re-infection was coming from inside the tree. So now I
understand that once fireblight has infected a tree, the fireblight
bacteria is systemic in the cambium even if the tree shows no more
signs of infection. Some of the articles I read suggest that the
bacteria can travel some 5-6 feet below visible infection sites.
So this leaves me with a number of questions:
1) How is it then that fireblight doesn't re-appear the next year
given that the only control is to cut out visible infection in the
Summer, cankers in the Winter, and blossom sprays + growth
inhibitors in the Spring?
2) Doesn't cambium with high carbohydrate content inhibit bacteria growth?
3) what is the scientific method for detecting the bacteria? Microscope?
--
**************************************************************
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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