Promalin (6-BA + GA4+7) increases cropping of apples
after a freeze by promoting parthenocarpic fruit set
Steve McArtney1 (Southeast Apple Specialist, NCSU) and Jim Wargo (Valent USA Corp.)
Results
Frost or freeze events around
bloom can significantly reduce fruit
set and crop value in apple orchards.
Reproductive
tissues
are
most
sensitive to freeze damage at the
full bloom stage, Air temperatures of
28 °F or 24 °F kill 10 % and 90 % of
the flowers, respectively (Table 1).
In 2007 the apple crop in the
Southeastern USA was severely
reduced by a freeze and in 2012 the
apple crop from Michigan to the
Northwest, and the Southeast, was
greatly reduced by multiple freeze
events.
Air temperatures during the
freeze events. Temperatures
were below freezing for approx.
4 h on the mornings of April 12
and April 13.
The
low
temperature reached 24 °F (-4.5
°C) on April 12 and 28.4 °F (-2.0
°C) on April 13. Air temperatures
across the study site were
remarkably similar (Fig. 2).
Gibberellins
can
stimulate
parthenocarpic fruit development in
apples (Bangerth and Schroder,
1994; Watanabe et al. 2008).
The
objective
of
this
experiment was to evaluate the
potential for Promalin (6-BA +
GA4+7) applications to increase
cropping after a freeze event by
promoting
parthenocarpic fruit
development.
Figure 1. Freeze events during
bloom severely reduced the
apple crop in the Southeastern
USA in 2007 and 2012
Table 1.
Critical spring temperatures for 10% and 90% kill of apple
flower buds at various developmental stages. (Data are
adapted from Washington State and Michigan sources).
Developmental
Stage
Silver tip
Green tip
½ Inch green
Tight cluster
First pink
Full pink
First bloom
Full bloom
Post bloom
Critical Temperature (°F)
10% Kill
90% Kill
15
2
18
10
23
15
27
21
28
24
28
25
28
28
28
25
25
25
Methods
The study was established in a
commercial orchard of ‘Rome’ apples
in Henderson Co. NC early in the
week beginning April 9, 2012 after
long-range
weather
forecast
predicted the occurrence of freeze
events later that week.
A data
logger were placed at either end of
the group of test trees and air
temperatures were logged at 30 min.
intervals. Frosts were recorded on April 12 (low of 24 °F [-4.5 °C]),
and April 13 (low of 28 °F [-2.0 °C]), when the trees were in full
bloom (Fig 2), and on April 24 (low of 30 °F). GA 4+7 (Promalin;
Valent USA Corp.) was applied at 1 pint/acre or 2 pints acre approx.
4 h after each frost event. The treatments were applied to four-tree
plots with an airblast sprayer in a spray volume of 100 gallons per
acre (1 pint per acre = 25 ppm; 2 pints per acre = 50 ppm). The
two spray treatments and an untreated control were arranged in a
randomized complete block design experiment with six blocks. Fruit
set was recorded on two sample limbs on each of the middle two
trees in each block.
20
Logger 1
Air temperature (°C)
Background
15
Logger 2
10
5
0
-5
April 12, 2012
-10
19:12
0:00
4:48
9:36
14:24 19:12
April 13, 2012
0:00
4:48
9:36
14:24
Fruit
set.
Fruit
set
was
Time
extremely low in the control
Figure 2. Air temperatures (°C)
trees, with only 2.6 fruit setting
during freeze events on April 12 and
for every 100 flower clusters.
April 13, in Henderson Co., NC.
Both of the Promalin treatments
resulted in a significant increase
in fruit set compared to the control (Table 2). There was no
effect of the rate of Promalin on fruit set. Crop load on the
approx. 30 % of a full crop. All of the fruit on treated trees is
parthenocarpic and appear to be growing normally.
Table 2. Effects of Promalin application following two
freeze events in 2012 on fruit set of ‘Rome’ apples.
Treatment
Fruit set
(fruit/100 clusters)
Untreated control
2.6 a
Promalin (1 pint/acre; 25 ppm)
17.7 b
Promalin (2 pints/acre; 50 ppm)
14.9 b
Significance (Pr > F)
0.0052
Conclusions
Application of Promalin 4 h after a freeze event that was
expected to cause death of 90 % of the flowers resulted in a
significant increase in fruit set due to the development of
parthenocarpic fruit.
This may be a useful strategy for apple
growers in years or regions where the frost risk during bloom is
high. Previous studies have shown that parthenocarpic apple
fruit are more likely to develop disorders in cold storage. Fruit
quality assessments are planned for the current study, including
measurement of fruit shape, fruit size, and postharvest storage
disorder incidence.
Literature cited
Bangerth, F. and M. Schroder. 1994. Plant Growth Regulation 15, 293-302.
Watanabe, M. et al. 2008. J. Japan. Soc. Hort. Sci. 77, 350-357.