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Chart version of thinner sensitivity ratings
Weather data for Belchertown MA.  Forecast values begin August 3, 2013
Estimated Apple Sensitivity to Chemical Thinners
     Other factors may outweigh the weather effects use for these estimates.  Other influences include: fruit diameter, fruit exposure to temperatures >=75 F, pollination, this year's cropload, last year's cropload, warm February-early April temperatures, frost and prolonged cool and cloudy weather during bloom, previous fall weather, foliage condition last summer)
     Ratings below are for the influence of weather on fruit sensitivity to chemical thinning agents.  Ratings assume fruit are less than 13mm diameter.  Dates in bottom rows are beyond 18mm diameter for most cultivars, and are intended only for late cultivars.
  Look at your fruit!  
Rough guess McIntosh King fruit diameter dates: 
12mm - May 25, Saturday         15mm - May 30, Thursday          18mm - May 31, Friday
Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F:  Tuesday,  May 21
     Very warm night temperatures (average > 69F = 19 heat units in table) can lead to "EXCESSIVE THINNING RISK", especially if combined with cloudy weather, or if combined with use of NAA.

Warm nights + Cloudy days  ----->  Increased sensitivity, maximum thinning.
Warm nights + Sunny days   -----> 
Intermediate senstivity, moderate thinning.
Cool nights + Cloudy days    ----->  Reduced sensitivity
Cool nights + Sunny days     -----> 
Reduced sensitivity, minimum thinning. 
THINNER APPLICATION DATE
Estimates assume morning application.
For afternoon application use rating for next day.
Average
% CLOUD SHADING
on application day and the next two days.


< 20% = "sunny blue sky" 
> 40% = "cloudy"
Cloudy weather on 3 days after application increases thinning.
Average
NIGHT HEAT UNITS

= degrees above 50 F for 7pm--7am
for 3 nights after application. 
Example:  If average night time temperature is 55F, then value
= 5. 
   
Values above 10 for three nights after application increases sensitivity to thinning.
SUMMARY
I
nfluence of weather on sensitivity of fruit to thinning agents for 3 days following a morning application

Ratings assume that fruit
are within diameter range
of susceptibility for thinning.
McIntosh
95% Petal Fall
Sat, May 18   
22% 4 Intermediate
100% Petal Fall Sun, May 19      53% 6 Intermediate
 Mon, May 20    30% 14 Increased sensitivity
 Tue, May 21    22% 6 Increased sensitivity
Fruit Set Wed, May 22      45% 12 Increased sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Thu, May 23      62% 15 Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Fri, May 24      63% 0 Stronger influence for reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Sat, May 25      73% 0 Stronger influence for reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, May 26      58% 0 Reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Mon, May 27    sunny  1% 2 Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Tue, May 28    sunny  9% 8 Increased sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Wed, May 29      48% 14 Stronger influence for Increased sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Thu, May 30    sunny  12% 19 Stronger influence for Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Fri, May 31    sunny  5% 21 Stronger influence for Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sat, June 1    sunny  2% 22 Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, June 2    sunny  8% 18 Intermediate -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Mon, June 3      45% 8 Intermediate -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Tue, June 4    sunny  9% 5 Reduced sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F