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Chart version of thinner sensitivity ratings
Weather data for Belchertown MA.  Forecast values begin August 1, 2014
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 31, Saturday         15mm - June 3, Tuesday          18mm - June 5, Thursday
Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F:  Tuesday,  May 27
     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
Wed, May 21   
sunny  10% 7 Intermediate
100% Petal Fall Thu, May 22      63% 5 Intermediate
 Fri, May 23      64% 3 Intermediate
 Sat, May 24      55% 3 Intermediate
 Sun, May 25    25% 7 Intermediate
Fruit Set Mon, May 26    24% 15 Intermediate
 Tue, May 27    32% 2 Stronger influence for reduced sensitivity
 Wed, May 28      67% 0 Stronger influence for reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Thu, May 29    sunny  7% 2 Stronger influence for reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Fri, May 30    30% 3 Reduced sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Sat, May 31      40% 2 Reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, June 1    sunny  4% 6 Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Mon, June 2    sunny  1% 11 Increased sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Tue, June 3    sunny  7% 12 Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Wed, June 4      50% 11 Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Thu, June 5      61% 8 Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Fri, June 6    39% 9 Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sat, June 7    sunny  9% 12 Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, June 8    sunny  10% 15 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
 Mon, June 9      48% 13 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
 Tue, June 10      54% 13 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