Adventures in Apple Chemical
Thinning, 2016
JIM SCHUPP
FRUIT RESEARCH AND EXTENSION CENTER
PENN STATE UNIVERSITY
BIGLERVILLE, PA
May in March - March in April…
Minimum temperature (°F)
Biglerville
Piney Mtn
York Springs
Date
4/3
4/5
4/6
elev. 732 ft.
29.6
24.1
25.3
elev. 1217 ft.
26.6
21.9
19.6
elev 740 ft.
28.0
22.9
24.6
4/10
24.7
21.4
22.2
Apple, Live & Dead
Apple flower viability following multiple cold events in April 2016
%
viable
%
full crop
No. blossoms
examined
Piney Mtn C. Gala
Honeycrisp
Golden Delic
Fuji BC2
72
74
48
64
936
675
581
730
391
374
380
445
Biglerville Buckeye Gala
Honeycrisp
Red Delic
Golden Delic
Brak Fuji
Pink Lady
Y Springs Buckeye Gala
Honeycrisp
51
81
51
82
62
77
62
61
767
1140
472
1172
745
1080
753
739
460
562
586
592
404
515
341
501
Golden Delic
Fuji BC2
54
26
667
275
448
409
Site
Variety
2016 Lessons:
 Critical temperature charts are not the final word for
apple flower mortality

Sweet cherry – unfortunately, yes…
 Early freezes “pre-thinned” the spurs – reduced
competition for surviving flowers
 Perception: scarcity
 Reality: strong set
2016 Lessons:
 Focus on what survived – not what is dead
 Avoid “scarcity thinking”
 Excessive crops require more resources, especially labor.
 Excessive crops produce fruits of reduced fruit size (and $)
 Excessive crops could reduce return bloom in coming season
Initial Set
Year in, year out:
the single most
reliable predictor
of final yield.
How many are there?
Are they growing?
Growing Fruit Are Setting Fruit
April in May: Chemical Thinning Season
80
Daytime
high,°F90
SupDemBal
TmaxF
60
80
40
Carbohydrate Balance
70
20
0
60
-20
50
-40
40
-60
-80
30
2016 Lessons:
 Carbon deficit does not mean that thinner will work
 Carbon balance only shows if the tree is susceptible
to thinning
 Sub-optimal temps during thinning = sub-optimal
thinning
2016 Lessons:
 Carbon deficit does not mean that thinner will work
 Carbon deficit only shows if the tree is susceptible to
thinning
 Sub-optimal temps during thinning = sub-optimal
thinning
 When you’re hot - you’re hot!
2016 Lessons:
 Carbon deficit does not mean that thinner will work
 Carbon deficit only shows if the tree is susceptible to
thinning
 Sub-optimal temps during thinning = sub-optimal
thinning
 When you’re hot - you’re hot!
 When you’re not –you’re not!
2016 Lessons:
 Carbon deficit does not mean that thinner will work
 Carbon deficit only shows if the tree is susceptible to
thinning
 Sub-optimal temps during thinning = sub-optimal
thinning
 When you’re hot - you’re hot!
 When you’re not –you’re not!

Jerry Reed, Country artist & Thinning Guru
Effective
Thinner
Mode of Action, Rate,
Timing, Spray Volume,
Water pH / Hardness, etc.
UV Degradation, Spray
Deposition, Drying time, Leaf
Cuticle thickness, Wash-Off,
Re-Wetting, Frost, etc.
Variety, Initial Set, Tree
Age, Cropping History,
Tree Vigor, Pruning, etc.
Weather
Carbon
Balance
Susceptible
Fruitlet
No Control: The Weather
 Weather affects the susceptibility of the tree to
chemical thinners

Carbohydrate supply and demand
 Thinning season:
 Strong demand (Rd) – fruits, shoots, leaves, cambium, roots
 Carbon reserves at seasonal low
 Supply is current photosynthesis (Pn)
Spur leaves
 New shoots are competitors until 7-8 fully expanded leaves

Thinning & Weather
 Chemical thinning will always carry risk, due to
inability to predict or control weather.
 Weather monitoring should be the primary focus
during thinning window.

Adjust timing, rates, and chemicals to account for weather.
 “2 X 4”
 The 2 days before thinner spray and the 4 days after are the
most critical.
Pn and Rd: Sunlight & Temperature
 High light: increased supply: harder to thin
 Low light: reduced supply: easier to thin
 Low temps: low demand: harder to thin
 High temps: high demand: easy to thin
 Worst: low light and warm temps…
 Or is cool and sunny worse?
Thinning & Weather
 Weather can affect the spray directly
 Spray deposit drying time
 Leaf absorption
 UV degradation
 Wash-off / re-wetting
 Thinners must interact with the tree’s metabolic
processes to cause thinning response


Too cold: tree not metabolically active = little thinning
Too hot: tree highly active = STRONG thinning
Weather Matters
 Sunlight and temperature:
 Strength of fruit set
 Tree response to thinner
 Weathering of thinner residue
 Slow drying conditions will max absorption
 The 2 days preceding, and esp the 4 days following
the thinner matter most
Summary:
 Resist scarcity thinking
 Get enough data to see the real story
 Believe your data
 Growing fruit are setting fruit
 You can have a carbon balance deficit and not get
adequate thinning if it is too cold for thinner action
 Carbon Balance Model explains 1 side of the triangle

And ONLY when the forecast is correct
Effective
Thinner
Mode of Action, Rate,
Timing, Spray Volume,
Water pH / Hardness, etc.
UV Degradation, Spray
Deposition, Drying time, Leaf
Cuticle thickness, Wash-Off,
Re-Wetting, Frost, etc.
Variety, Initial Set, Tree
Age, Cropping History,
Tree Vigor, Pruning, etc.
Weather
Carbon
Balance
Susceptible
Fruitlet
Recommendations:
 Key in on initial set: what % of full crop?
 Choose thinners and set strength of dose
 Monitor daily light and temps for “2X4”
 8 mm-on: apply thinner ahead of a favorable forecast
 Adjust dose and timing for fruit growth and forecast
 Be prepared to re-thin at 20 mm
 Use return bloom sprays on AB varieties