Weather data for Belchertown MA. Forecast values begin May 17, 2012

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 16, Wednesday 15mm - May 19, Saturday 18mm - May 22, Tuesday

Date after which fruit have exposure to 2 days of daily high temperature of at least 75 F: Sunday, May 13

     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
Tue, May 1

66%

Reduced sensitivity

100% Petal Fall Wed, May 2

66%

Reduced sensitivity

Thu, May 3

67%

Intermediate

Fri, May 4

58%

Intermediate

Sat, May 5

54%

Reduced sensitivity

Sun, May 6

38%

Reduced sensitivity

Fruit Set Mon, May 7

sunny 6%

Intermediate

Tue, May 8

61%

Intermediate

Wed, May 9

65%

Reduced sensitivity

Thu, May 10

52%

Reduced sensitivity

Fri, May 11

21%

Reduced sensitivity

Sat, May 12

sunny 0%

Intermediate

Sun, May 13

sunny 9%

Increased sensitivity

Mon, May 14

46%

Increased sensitivity-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F

Tue, May 15

57%

Intermediate-- sensitivity may be less because of fruit exposure to 2+ days with high temps. > 75F

Wed, May 16

55%

Reduced sensitivity -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F

Thu, May 17

sunny 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

Fri, May 18

sunny 0%

Intermediate -- fruit >12mm probably less sensitive to thinning due to size and exposure to 2+ days with high temps. > 75F

Sat, May 19

sunny 0%

Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F

Sun, May 20

sunny 16%

Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F

Mon, May 21

34%

Stronger influence for Increased sensitivity -- fruit >15mm may have minimal sensitivity due to size and exposure to 2+ days with high temps. > 75F

Tue, May 22

33%

Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F

Wed, May 23

34%

Increased sensitivity -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F

Thu, May 24

sunny 17%

Intermediate -- fruit > 18mm are probably beyond predictable chemical thinning due to size and exposure to 2+ days with high temps. > 75F