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Chart version of thinner sensitivity ratings
Weather data for Belchertown MA.  Forecast values begin May 19, 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 24, Friday         15mm - May 28, Tuesday          18mm - May 31, Friday
Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F:  Monday,  May 20
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 Influence 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 15	34%		2			Reduced sensitivity
100% Petal Fall Thu, May 16	sunny  5%		8			Reduced sensitivity
Fri, May 17	sunny  9%		2			Reduced sensitivity
Sat, May 18	22%		4			Intermediate
Sun, May 19	58%		6			Stronger influence for Increased sensitivity
Fruit Set Mon, May 20	55%		13			Stronger influence for Increased sensitivity
Tue, May 21	77%		13			Stronger influence for Increased sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
Wed, May 22	41%		16			Increased sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
Thu, May 23	31%		16			Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F
Fri, May 24	30%		7			Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
Sat, May 25	sunny  16%		5			Reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F
Sun, May 26	sunny  17%		6			Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F