Using the Fruitlet Growth Model in Your Orchard

Developed by Dr. Duane Greene, University of Massachusetts, the Fruitlet Growth Model helps to predict if fruit will abscise or persist through June drop 7-9 days after thinning application. This is much sooner than if one were to predict which fruitlets will persist from visual observation alone. This model is based on the assumption that the fruitlets that will remain will grow faster than the abscising fruit. Based on the growth rate of 20 of the fastest growing fruit, the fruitlets that will persist will have a growth rate greater than 50% of the growth rate of the fastest growing fruitlets.

An Excel spreadsheet of the model, developed by Philip Schwallier from Michigan State University, can be downloaded at msue.anr.msu.edu/topic/apples/horticulture.

Steps

  1. Select 5 trees per block.
  2. Count the number of flower clusters per tree.
  3. Mark and number 15 clusters per tree and number each fruitlet of each cluster with a permanent marker (Figure 1).
  4. Determine the number of apples desired/ tree by either two methods.
    1. Base it on the trunk cross-sectional area of the tree (Table 1).
    2. Base it on the desired yield and desired fruit size. Base the desired yield on the history of the orchard block and its yield potential. See Table 2 and 3 and formula below.

      Fruit/tree = Desired yield (bu/acre) x desired tray size ÷ trees/acre

      Example
      Desired yield = 1000 bu/acre
      Desired tray size = 88
      Trees/ acre = 1210

      Fruit/tree = 1000 bu/acre x 88 ÷ 1210 = 73
  5. Determine the % target crop load based on number of apples desired/ tree and the flower cluster count

    % target crop load = number of apples desired/ tree ÷ flower clusters/tree x 5 fruitlets/cluster
  6. Measure fruitlet diameter, with calipers, 3-4 days after application and 4-5 days after the first measurement (Figure 2). Enter in these measurements in the excel spreadsheet found here msue.anr.msu.edu/topic/apples/horticulture. 100-130 growing degrees days, base 4.4°C, should accumulate between the time of application and the second measurement.
  7. If the % crop load is within 5-8% of the % target cropload do not apply a second spray. If it is 5-8% above the target cropload, apply a second spray and repeat step 6 and 7.

Figure 1. 15 Clusters/ tree are marked and numbered with flagging tape. Each individual fruitlet is marked with a number or a number of dots.

Figure 1. 15 Clusters/ tree are marked and numbered with flagging tape. Each individual fruitlet is marked with a number or a number of dots.

Figure 2. Fruitlet diameter is measured with calipers.

Figure 2. Fruitlet diameter is measured with calipers.

Table 1. Crop load recommendations (fruit/tree*) for apple trees based on the trunk diameter (Adopted from Robinson, 2008)
Trunk diameter
(cm)
Trunk
Cross-sectional Area (cm2)
Weak Growing Biennial Varieties (Honeycrisp) Strong Growing Biennial Varieties
(Fuji, Golden Delicious, Crispin (Mutsu), Northern Spy)
Annual Varieties (Gala, Empire, McIntosh, Idared)
0.6
0.3
1
2
2
1.3
1.3
5
6
8
1.9
2.9
11
14
17
2.5
5.1
20
25
30
3.2
7.9
32
40
48
3.8
11.4
46
57
68
4.4
15.5
62
78
93
5.1
20.3
81
101
122
5.7
25.7
103
128
150
6.4
31.7
127
158
190
7.0
38.3
153
192
230
7.6
45.6
182
228
274
8.3
53.5
214
268
321
8.9
62.1
248
310
372
9.5
71.3
285
356
428
10.2
81.1
324
405
486
10.8
91.5
366
158
549
11.4
102.6
410
513
616
12.1
114.3
457
572
686
12.7
126.7
507
633
760

* For Honeycrisp, the recommended crop load = 4 fruits/cm2 trunk cross-sectional area (TCA). For other biennial bearing varieties, the recommended crop load = 5 fruits/cm2 TCA. For annual varieties the recommended crop load = 6 fruits/cm2 TCA.

Table 2. Effect of Apple Fruit Size on Tray and Bin Requirements
Minimum fruit size Tray size Number of apples per bin2
wt (g)1 mm inch
102
57
196
3,920
114
60
2⅜
175
3,500
123
64
163
3,260
133
67
2⅝
150
3,000
145
70
138
2,760
160
73
2⅞
125
2,500
177
76
3
113
2,260
200
79
3⅛
100
2,000
227
83
88
1,760
250
85
3⅜
80
1,600
278
89
72
1,440
313
92
3⅝
64
1,280
357
95
56
1,120
417
98
3⅞
48
960

1 These values are approximate as fruit of the same diameter can differ in weight.

2 20-bushel bin capacity.

Table 3. Effect of Fruit Size in Relation to Crop Load
(Number of apples required per tree for given tree density (trees/acre), desired yield (bu/ac) and fruit size)
Minimum size trees/acre
272
trees/acre
500
trees/acre
1,000
trees/acre
1,500
mm inch bu/ac
400
bu/ac
600
bu/ac
500
bu/ac
750
bu/ac
750
bu/ac
1,250
bu/ac
1,000
bu/ac
1,500
57
320
480
218
327
163
272
145
218
60
2⅜
286
429
194
292
146
243
130
194
64
266
400
181
272
136
226
121
181
67
2⅝
245
368
167
250
125
208
111
167
70
225
338
153
230
115
192
102
153
73
2⅞
204
306
139
208
104
174
93
139
76
3
185
277
126
188
94
157
83
126
79
3⅛
163
245
111
167
83
139
74
111
83
144
216
98
147
73
122
65
98
85
3⅜
131
196
89
133
67
111
60
89
89
118
176
80
120
60
100
53
80
92
3⅝
105
157
71
107
53
89
47
71
95
92
137
62
93
47
78
41
62
98
3⅞
78
118
53
80
40
67
36
53

References

  • Robinson, T., 2008. Crop Load Management of New High-Density Apple Orchards. NY Fruit Quarterly 16(2): 3-7
  • Schwallier, P. and Irish-Brown, A. 2014. Predicting Fruitset Model.

For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca