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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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