No resistance from bloom sprays is good news. I have had no control
problems with copper prior to green tip, strep timed by Maryblyt,
Apogee to minimize pruning/harden shoots to possible shoot blight,
and control of aphids/leafhoppers at petal fall/1st cover. A few
years ago, however, we had a long, extended bloom with Pink Lady.
Maryblyt called for a total of 6 sprays (predicted EIP over 100),
which exceeds most recommendations of 4 max. I went with the 6
applications and came out OK, but have always wondered if this might
trigger resistance. I will stick with strep during bloom and leave
the Kasumin for those that need it. I did just put in a block of
CrimsonCrisp and haven't thought about putting copper on them. Good
advice. Thanks.
On 3/21/2015 10:36 AM, David A.
Rosenberger wrote:
While Kasugamycin works about as well as streptomycin,
oxytetracycline is generally a bit less effective and has the
disadvantage of preventing bacterial multiplication without
killing off all of the bacteria contacted by the spray. Of the
three antibiotics, it is my understanding that only strep is
absorbed into apple tissue, thereby giving it a bit of an edge
over both of the other products, especially in cases where a few
infections might have been initiated a few hours before the
product is applied. Kasugamycin, like strep, kills bacterial
cells that it contacts, but it has the disadvantage of being
considerably more expensive than strep.
Some of my pathologist colleagues may disagree with
me, but I see no reason to pay the extra price for kasugamycin
in established orchards that have no history of strep
resistance. (An exception would be in countries like Canada
where the strep labels allow a maximum of 3 applications/yr.)
In eastern New York and New England, we have used strep
exclusively for fire blight control for more than 60 years
without encountering resistance. Resistance to strep has only
appeared in regions where nurseries or fruit growers have used
it repeatedly during summer (as many as 12 times/yr) to prevent
shoot blight. Thus, there is an abundance of observational
evidence that repeated applications of strep after bloom
DEFINITELY WILL result in strep-resistant Erwinia amylovora (Ea)
whereas, so far as I know, there is absolutely no evidence that
multiple applications during bloom have ever resulted in strep
resistance. Thus, I would argue that strep is still the
cheapest, most effective, and most proven product for
controlling blossom blight, and I see no reason to use other
products except where strep resistance has been documented or is
suspected due to failure of well-timed strep sprays. In fact,
alternating with biologicals or with oxytet may actually be
counter-productive because they may allow more bacteria to
survive, thereby leaving larger populations to be controlled by
strep and/or allowing some infections to become established and
thus carry the disease through until the next year.
Given that there is increasing evidence that fire
blight is sometimes present in symptomless nursery trees, one
could argue that strep-resistance may show up anywhere as a
result of distribution via nursery trees. This is a very real
and valid concern. To diminish the likelihood that
strep-resistance might be introduced with nursery stock, we in
NY have been recommending that all newly planted trees be
sprayed with copper shortly after they break bud and then with
copper plus strep during bloom. The basis for this
recommendation is that copper should knock out strep-resistant
Ea on plant surfaces whereas strep will still be more effective
for preventing local sources of Ea from infecting flowers on
newly planted trees. Using several sprays of Kasugamycin on
newly planted trees when they produce flowers during the first
year of planting might be even better than copper plus strep for
preventing establishment of strep-resistance brought in with
nursery stock. Finally, it should be obvious that all new apple
plantings should be observed very carefully for evidence of fire
blight symptoms for several months after the trees begin to
grow, and any diseased trees should be removed immediately.
We probably had more fire blight in newly planted
trees in 2014 than in any prior year, but it is not a new
phenomenon. I ended up with some blight-infested nursery trees
in 1986 when I was establishing one of my research orchards.
Both in that 1986 situation, rapid removal of diseased trees as
they showed up during the year of planting prevented blight
introduced in nursery trees from becoming established in my
research blocks, and the remaining trees were completely
disease-free in subsequent years. However, last year some
growers opted to remove all newly planted trees when they found
significant percentages of the trees were developing fire blight
because it was unclear whether they could successfully identify
all of the trees that were carrying the disease. Hopefully there
will be less blight in nursery stock in 2015.
********************************************
Dave Rosenberger, Plant Pathologist,
Hudson Valley Lab, P.O. Box 727, Highland, NY 12528
Cell: 845-594-3060
http://blogs.cornell.edu/plantpathhvl/blog-2014/
********************************************
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