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Chemical thinning is an established and essential
practice performed by fruit growers each spring. In recent years,
the apple market has caused chemical thinning to become the single
most important cultural practice that many growers undertake. The
purpose of this leaflet is to summarize thinning recommendations and
to discuss the conditions that may influence your chemical-thinning
program.
Fruit thinning is done to increase fruit size and
enhance repeat bloom. The fewer fruit that are allowed to develop on
a tree, the larger those fruit will be. In general, reduction in the
number of fruit to 4 to 6 fruit per cm of limb circumference is
required for good size. Good return bloom usually can be assured if
fruit density is reduced to 8 to 10 fruit per cm within four weeks
of bloom. Therefore, more severe thinning is required to obtain good
fruit size than to get adequate return bloom.
Growers must consider thinning all mature, bearing
trees. Trees that carried a light crop last year will have a heavy
bloom and will require extensive thinning this spring. Furthermore,
trees with a light bloom this year will set a higher percentage of
flowers than those with heavy bloom, thus they may require some
thinning.
Carbaryl
Carbaryl or
Sevin is the workhorse of thinning agents. It is the mildest and
safest thinner that we have. It generally is used at 0.5 to 2 lbs
50WP or 0.25 to 1 qt XLR per 100 gal, assuming dilute application.
The XLR formulation of Sevin is preferred over the 50WP, because of
the particle size of the 50WP formulation. Particles of 50WP Sevin
are approximately the same size as pollen grains, thus increasing
the likelihood that these particles will be carried to the hive by
bees. Sevin XLR has a much smaller particle size, reducing, but not
eliminating, the likelihood of bee damage. Sevin XLR contains a
surfactant, so it may be absorbed more readily than other Sevin
formulations. It also may improve the penetration of other chemicals
with which it is applied, such as NAA.
NAA
Naphthalene
acetic acid is the most potent thinner that we have. It is used at
rates between 2.5 and 20 ppm. Because it is a more active compound
than Sevin, there is a greater chance of overthinning, especially
when high temperatures (85o or higher) follow application. NAA
should be used, but it should be treated with respect. Fruit size
may be reduced or pygmy formation may increase with late or
high-concentration applications or when it is very hot.
NAD
Naphthalene
acetamide is less active than NAA. It frequently is used in
situations where flagging of foliage is a problem. It generally is
applied at 25 to 50 ppm. NAD never is used on Delicious, since small
seedless fruit (pygmies) will result and persist to harvest. It
often is used in a petal-fall spray.
Timing:
Apples can be thinned satisfactorily any time from
bloom to 3 to 4 weeks after bloom. The exact length of the thinning
period will be determined by the weather. If warm temperatures occur
during the 3 weeks prior to treatment, results of thinning may be
disappointing. However, if cool conditions prevail before treatment,
thinning may be accentuated and the thinning window may be extended.
Over the last number of years, the preferred time,
assuming that the weather cooperated, was when fruit were 8 to 10 mm
in diameter. As mentioned above, the market has caused thinning to
become the most important cultural practice for many orchardists.
Thinning must work! Inadequate thinning will result in significant
losses of crop value, much more than will be experienced by light to
moderate over thinning. You must create every opportunity possible
to thin your fruit chemically.
Because of this need, we recommend that you begin
thinning at petal fall or shortly thereafter. At this time, we
suggest the use of Sevin alone, or where more severe thinning is
needed, a combination of Sevin and NAA (5-10 ppm).
Assess the response and initial fruit set when
fruit are 7 to 12 mm in diameter (about 7 to 10 days after petal
fall), and treat again if necessary with Sevin alone or Sevin plus
NAA.
When fruit are over 15 mm in diameter (after
another 7 to 10 days), assess the response again and determine
whether or not any stress periods have occurred, and treat if
necessary with Sevin only.
This approach will improve your results possibly in
two ways. Multiple applications should provide more thinning
response, and multiple applications will spread the applications
through a number of possible weather windows for optimal response.
The effectiveness of chemical thinners is
influenced by the weather in several ways.
Penetration
All chemical thinners, to be
effective, must diffuse across the cuticle, which covers all aerial
portions of the plant, including leaves, flowers, and young fruit.
Waxes on the cuticle provide a major barrier to penetration. When
the weather is cold and cloudy, little wax is secreted on the leaf
surface, thus permitting greater penetration of the chemical.
Conversely, when the weather is sunny and dry, wax is secreted and
deposited at an accelerated rate, thus restricting penetration of
thinning chemicals. Therefore, chemical thinners, generally, are
less effective after warm, dry periods and more effective after
cool, moist periods.
Stress
Regardless of
the mode of action of each thinning chemical, all require some type
of stress to be imposed by weather for an ideal thinning response.
Competition among flowers, fruit, leaves, and growing points occur
for water, nutrients, metabolites, photosynthate, and growth
regulators. Chemical thinners increase this stress on fruit, causing
weak fruit and those with a small number of seeds to abscise. It is
our opinion that in years where chemical thinners have worked
poorly, weather-imposed stress has occurred before thinner
application, or stress has not occurred until after the time that
fruit abscision can be altered. Satisfactory thinning results may be
obtained if thinning is followed by 3 days of sunny weather with
temperatures in the mid to upper 70's. Ideal results, however,
require 3 days of sunny weather in the 80's. Cloudy, rainy weather
following chemical thinning has been shown to result in enhanced
thinning in the Mid Atlantic region; however, this response has not
been confirmed in the Northeast.
Pollination weather
If weather during pollination was good and considerable bee
activity occurred, then a heavy initial set of fruit with many seeds
is likely. Thinning will be required and may be difficult. If the
weather was less than ideal for bee activity and developing fruit
have few seeds, set may be reduced and thinning may occur more
easily.
Frost damage
Frost which may kill only a
few flowers low in the tree also may injure spur leaves throughout
the tree. Absorption of thinning chemicals into these injured fruit
and leaves is likely to be greater than into noninjured fruit and
leaves, and therefore the thinning response may be greater. Further,
impaired photosynthesis and stress caused by reduced carbohydrate
supply also may accentuate thinning.
Winter injury
Winter
injury to either the tree top or the roots can accentuate thinning
activity.
Vole damage
Trees that have been damaged
by voles will be weakened and will thin more easily. Root damage
caused by pine voles is not always apparent, and effects may not be
seen until it is time to thin chemically.
Heavy crop the previous year
Trees
bearing a heavy crop store less reserve food than trees bearing a
light crop. These less vigorous trees will be more responsive to
thinning sprays the following year.
Low-vigor trees
Trees that have had
insufficient nitrogen or have a deficiency of another essential
element will thin more easily.
Young trees
Care must be taken to
chemically thin young trees very carefully. They thin very easily,
and at most, reduced rates of chemical thinners are all that is
needed.
Thick, dense trees
Interior spurs, those
located on shaded (or shaded prior to pruning) portions of the tree,
and spurs on lower limbs usually are weak and low in carbohydrate
reserves. These spurs thin easily. If trees were pruned to expose a
number of previously shaded spurs, realize that these spurs will be
thinned easily.
Chemical thinning is a difficult process, often
thought to be more art than science. Regardless, it is one that
orchardists must master to be successful. Poor thinning results in
loss of more crop value than all other problems together.
Adapted fromDuane W.
Greene and Wesley R. Autio, Department of Plant & Soil
Sciences,University of Massachusetts Extension, 2001
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