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100
I NORTH FLORIDA EXPERIMENT STATION
Quincy, Florida
70 -i August 27, 1969
North Florida Station Mimeo Report NFS 70-1
EFFECTIVENESS OF VARIOUS CHEMICALS ON THE THINNING OF PEACHES IN NORTH FLORIDA
J. B. Aitken!/
INTRODUCTION
The most important single step in peach production is thinning the young fruit so that
those remaining will reach optimum size. In order to obtain the proper size of the fruit at
harvest, the fruit are generally thinned to a distance of about 6 inches apart along the
limb. Maintaining this average spacing between fruit is very difficult with the inexperi-
enced labor available in North Florida. One of the most prevalent problems in thinning is
the failure to realize how much the young fruit will increase in size before harvest if
given proper care. Hand thinning is a monotonous job which requires continuous supervision
to be done properly. In recent years it has been very difficult to secure sufficient labor
to handle the thinning task.
In 1969, a study was initiated at the North Florida Experiment Station to determine the
effectiveness of various chemicals on thinning young fruit on peach varieties grown in
North Florida. These chemicals were evaluated on degree of thinning and size of fruit at
harvest.
MATERIALS AND METHODS
Ethrel(2-chloroethylphosphonic acid)(Amchem 68-240), Casoron E-lR(dichlobenil) and
Penar were used as full bloom sprays, while 3-CPA(3-chlorophenoxy-alpha-propionamide) was
used as a post-bloom spray. All treatments were applied to run-off using a portable pumping
unit operating at 100 psi. This study was divided into 3 experiments as follows:
I. Bloom thinning of Armgold peach: Four-year-old Armgold peach trees were used in
this experiment. Four replications of single-tree plots for each treatment were arranged in
a randomized complete block design. The treatments were the following: 100 and 500 ppm
Ethrel, 800 and 1200 ppm Penar and 100 and 200 ppm dichlobenil. An unthinned control was
used for comparison.
II. Bloom thinning of Maygold peach: Nine-year-old Maygold peach trees arranged in a
randomized complete block were used in this experiment. Each of the following treatments
consisted of 2 replications of single-tree plots: 50, 100, 300, and 500 ppm Ethrel; 800
and 1200 ppm Penar and 100 and 200 ppm dichlobenil. An unthinned treatment was used for
comparison.
III. Post-bloom sprays on Maygold peach: Each treatment was replicated 5 times using
single-tree plots of 9-year-old Maygold peach trees. The treatments consisted of 2
concentrations, 300 and 400 ppm 3-CPA, and 6 application times, 14, 18, 22, 26, 30, and 34
days after full bloom. An unthinned control was included in each replication. Fruit
samples were taken at each time of application to obtain fruit weight, fruit diameter, and
ovule length.
/Asst. Professor (Asst. Horticulturist), North Florida Experiment Station, Quincy.
Harvest data on the Armgold trees was taken by removing and counting all of the fruit
from each tree. This same method was used for the bloom thinning experiment on Maygold with
the additional weighing of the fruit as well as counting. On the 3-CPA experiment, random
samples were taken by removing all of the fruit from randomly selected limbs around the tree.
Fruit weight was used as an index of thinning efficiency.
RESULTS AND DISCUSSION
The effectiveness of Ethrel as a bloom thinner for Armgold peaches is clearly shown in
Table 1. Trees sprayed with 500 ppm Ethrel had fewer fruit than did those sprayed with 100
ppm, but even the lower rate provided adequate thinning. Dichlobenil at 200 ppm could be
used as bloom spray followed by supplemental hand thinning. All of the other treatments
were no better than the unthinned control. It should be noted that this year the period of
bloom was extremely long and that some of the blossoms opened after the treatments were
applied. In a year of normal bloom patterns, the results could be even more striking.
Ethrel at 500 ppm applied at full bloom to Maygold peach trees produced the fewest
number of fruit but also the largest sized fruit (Table 2). The average fruit weight from
this treatment was 93.07 grams and fruit diameter was 5.66 cm. Application of 300 ppm of
Ethrel left 27.6% of the fruit on the tree as compared to 16.1% for the 500 ppm rate and the
fruit was slightly smaller both in weight and diameter. Both of the Ethrel treatments above
produced fruit in the 2-2 1/4 inch range. All of the other treatments proved ineffective
for use as bloom thinners on a commercial basis.
The data shown in Table 3 indicates that 400 ppm of 3-CPA applied 26 days after full
bloom resulted in the development of the largest fruit. The effectiveness of 3-CPA is
related to seed length at time of application. The treatment mentioned above had a seed
length of 9.44 mm as shown in Table 4. In reviewing the data in Table 3, it appears that
400 ppm is better than 300 ppm in thinning efficiency due to the increase in fruit size at
each application date.
SUMMARY
This report is based on 1 year of research on chemical thinning of peaches. Ethrel
applied as a full bloom spray at 500 ppm produced the largest fruit and left the fewest
number of fruit on the trees. Ethrel at 100 ppm on Armgold and 300 ppm on Maygold provided
adequate thinning, but not as good as the 500 ppm rate. The use of 400 ppm of 3-CPA applied
when the seed length is about 9-10 mm would give the most efficient thinning on Maygold
peach. Research with these chemicals and several new ones on the market will be continued
next year.
120 CC
JBA
8/27/69
Table 1. Effect of several chemicals applied at full
bloom on thinning Armgold peaches.
Chemical Number of fruit per tree
Treatment Total % of Control
Control 476.0 100.0
Ethrel, 100 ppm 217.0 45.6
Ethrel, 500 ppm 177.0 37.2
Penar, 800 ppm 491.0 103.2
Penar, 1200 ppm 506.0 106.2
Dichlobenil, 100 ppm 468.0 98.3
Dichlobenil, 200 ppm 292.5 61.4
Table 2. Effect of several chemicals applied at full
weight of Maygold peaches.
bloom on number and average
Chemical Number of fruit per tree Average 2/ Fruit3/
Treatment Total/ %7 of Control Fruit Weight, g- Size, cm-
Control 2477.0 c 100.0 44.72 a 4.10
Ethrel, 50 ppm 2281.0 c 92.1 48.12 ab 4.20
Ethrel, 100 ppm 1786.5 bc 72.1 58.79 abc 4.48
Ethrel, 300 ppm 682.5 ab 27.6 84.44 bc 5.38
Ethrel, 500 ppm 397.5 a 16.1 93.07 c 5.66
Penar, 800 ppm 1559.0 abc 62,9 54.26 ab 4.33
Penar, 1200 ppm 2908.5 c 117.4 34.73 a 3.68
Dichlobenil, 100 ppm 2481.0 c 100.2 39.04 a 3.82
Dichlobenil, 200 ppm 2612.0 c 105.4 39.86 a 3.85
1/Means not having a letter in common are significantly different at the 5% level.
2/Means not having a letter in common are significantly different at the 1% level.
!/Fruit size determined from standard curve based on fruit weight.
Table 3. Effect of concentration and time of application
of 3-CPA on the average fruit weight of Maygold
peaches.
Days after Average Fruit
Conc. Full bloom Fruit weight, gI Size, cm/
300 ppm 14 41.04 a 3.88
18 37.50 a 3.78
22 44.40 abc 4.00
26 52.30 abc 4.28
30 41.77 ab 3.91
34 42.13 ab 3.92
400 ppm 14 67.74 abcd 4.80
18 65.01 abcd 4.70
22 74.82 cd 5.05
26 94.52 d 5.72
30 73.64 bcd 5.00
34 43.13 abc 3.96
Control 47.22 abc 4.12
-/Means not having
different at the
a letter in common are significantly
1% level.
2/
2Fruit size determined from standard curve based on fruit
weight.
Table 4. Fruit measurements on each application date
of 3-CPA on Maygold peaches.
Days after Fruit Fruit Ovule
full bloom Diameter, mm Weight, g Length, mm
14 5.60 0.18 3.85
18 7.34 0.32 5.12
22 9.80 0.97 6.26
26 14.83 2.44 9.44
30 18.75 4.94 11.08
34 22.11 7.41 12.71
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