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Chart version of thinner sensitivity ratings
Weather data for Easthampton MA.  Forecast values begin May 25, 2018
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! 
  
 The rating for each day accounts for conditions on that day AND the two following days, i.e. each rating is for a three day period starting on the listed day.  The rated value for a day indicates relative efficacy of a thinner applied on that day based on weather conditions in the 72 hours following application.  For evening applications, use the rating for the next calendar day.
Rough guess McIntosh King fruit diameter dates: 
12mm - May 26, Saturday         15mm - May 28, Monday          18mm - May 31, Thursday
Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F:  Monday,  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
Thu, May 17   
25% 11 Intermediate
McIntosh 100% Petal Fall,  Fri, May 18    sunny  9% 1 Intermediate
 Sat, May 19      63% 6 Intermediate
 Sun, May 20      61% 7 Intermediate
 Mon, May 21    sunny  3% 6 Intermediate
McIntosh Fruit Set,  Tue, May 22      54% 5 Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Wed, May 23    sunny  14% 10 Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Thu, May 24    sunny  1% 6 Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Fri, May 25    sunny  3% 18 Increased sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
 Sat, May 26    sunny  9% 17 Increased sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Sun, May 27      77% 5 Increased sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
 Mon, May 28    40% 14 Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Tue, May 29    sunny  17% 16 Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Wed, May 30    sunny  17% 13 Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F
 Thu, May 31    sunny  18% 16 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
 Fri, June 1    34% 15 Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F