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/ Ag-Radar https://extension.umaine.edu/ipm/ag-radar-apple-sites *Our Changing Climate: It's Real, It's Us, It's Bad, Experts Agree. T**here's Hope* 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 > > > _______________________________________________ > apple-crop mailing list > firstname.lastname@example.org > http://virtualorchard.com/mailman/listinfo/apple-crop > >
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