Hi Art
If a flower opened at 7am on Friday May 19, and you had sprayed strep
just before that, then that flower would not be protected. Using Sanford
temperatures (not too far from you), there were 193 Cougarblight heat units
between Friday morning 7am and 5pm on Friday.
After Friday 5pm, heat unit accumulation fell off very sharply. Only an
additional 25 units accumulated between Friday 5pm and a rain on Monday May
22 (and no new heat units on Monday). The total number of heat units from
Friday 7am to Monday morning would be 221. The Cougarblight threshold for
a category II orchard (fire blight within 1 mile within the last two years)
to move from Caution to High infection risk is 200 units.
So if that new flower was contaminated right as it opened, then presumably
it would have had just above the threshold of heat units to call for
another streptomycin application before a Monday rain. That scenario seems
unlikely for several reasons. First, almost all of the flowers except
perhaps very late blooming cultivars were already open by the time of a
Friday morning streptomycin application. Second, the chance that a flower
opens at 7am and is immediately contaminated with fire blight bacteria
seems low.
If that hypothetical flower had opened at noon on Friday, May 19, then it
would have accumulated only 120 new heat units between opening and a Monday
rain (no additional heat units on Monday, only 12 new heat units on Tuesday
May 23).
All the above is for the assumption that you applied streptomycin after the
Thursday night / Friday morning shower, after daily high temperatures of 90
and 93 on Wed. and Thur May 17-18.
If streptomycin was not applied on Friday, then unprotected flowers that
are still open during a Monday rain (i.e. not too old for infection) would
be at Exceptionally high infection risk because many of those heat units
are still considered applicable for a Monday rain.
Note to other Maine growers: Temperatures on Friday were much cooler in
Monmouth than Sanford on Friday May 19. By Friday at 5pm in the scenario
described above, only 52 Cougarblight heat units had accumulated in
Monmouth (vs. .193 in Sanford). While Monmouth has more bloom remaining
for a Monday rain, the heat units since Friday just aren't there.
But if you did NOT apple strep on Friday May 19, then a Monday rain would
not only been in the Exceptional risk rating, that risk would coincide with
most apple trees sitll being in full bloom.
- Glen
Glen Koehler
University of Maine Cooperative Extension
Pest Management Office
Voice: Office 207-581-3882, Cell 207-485-0918
491 College Avenue, Orono, ME 04473
UMaine Apple IPM https://extension.umaine.edu/ipm/programs/apple/
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On Sun, May 21, 2017 at 3:52 PM, Arthur Kelly <kellyorcha...@gmail.com>
wrote:
> Thanks, I was more wondering how close to try and get to the infection
> period due sometime tomorrow morning. We are mostly
> in bloom with some varieties nearly complete petal fall and others
> (honeycrisp) at full bloom. There are very few flowers still to open and I
> expect that this will be the only strep spray necessary. The forecast is
> slight chance of showers daily going forward.
>
> Sent from my iPhone
>
> On May 21, 2017, at 2:13 PM, Vincent Philion <vincent.phil...@.qc.ca>
> wrote:
>
> Hi all,
>
> My two cents:
>
> Although streptomycin is degraded by light, this doesn’t really matter:
> For two to four days, the antibiotic keeps bacterial population at low
> levels on sprayed flowers.
>
> Once it’s degraded, the flower is also 2 to 4 days older and there is
> simply not enough time left for the bacteria to multiply back to
> detrimental levels and infect.
>
> Pusey demonstrated quite well that as flower age, they carry less bacteria
> and become increasingly more difficult to infect.
>
> Bottom line: Opened flowers that are sprayed stay protected for the life
> of that flower.
>
> As Quan underlined, you should mostly concentrate on flowers unopened at
> spraying time:
>
> 1) How many flowers weren’t open on the last strep spray?
>
> 2) Will the weather for these flowers be conducive for bacteria
> multiplication and infection?
>
> If so, then you need to consider additional sprays for unsprayed flowers.
>
> Trapman and myself developed RIMpro-Erwinia to help manage which flowers
> are at risk and at need for a spray. This model is very different from
> Cougar and Maryblyt and is proving more reliable.
>
>
> Vincent Philion, agr., M.Sc.
> Microbiologiste/Phytopathologiste (pomiculture)
>
> Institut de recherche et de développement en agro-environnement
> Research and Development Institute for the Agri-Environment
>
> www.irda.qc.ca
>
>
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>
>
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