1-3
4-6
1-3
4-6
The earliest and cheapest fruit thinning is done with a pruning shear. . . . Crop load
adjustment is the most reliable way to increase fruit soluble solids content.
IMPROVING FRUIT SIZE:
THINNING AND GIRDLING NECTARINES, PEACHES, AND PLUMS
1and Ted M. DeJong2
University of California
Pomology, University of California-Davis, Davis, CA 95616
Argentina, June 16-20, 1998
INTRODUCTION
It is essential to achieve adequate fruit size in order to satisfy consumers.This improvement is
generally achieved by one method—regulation of crop load.While proper cultural practices
such as correct plant nutrition and adequate irrigation are essential, these do not affect final fruit
size to the same degree as crop load adjustment.
A mature, open-center trained peach or
nectarine tree may have 3000 to more than 8000 flowers—even after pruning.In a “normal”
year, approximately 50% (or more) of these flowers will set fruit.This leaves 1500 to 4000 or
more fruit on a tree that can properly size less than half that amount.For this reason it is
important to figure out proper dormant pruning levels that leave only as much fruitwood as is
necessary.Remember that the earliest and cheapest fruit thinning is done with a pruning shear.
FRUIT GROWTH DYNAMICS
Stone fruits grow according to what is called a double sigmoid growth curve.In general, fruit
grow very fast for several weeks after bloom in a period called stage I.Fruits then begin to slow
down for a varietal specific period of time called the “lag phase,” or stage II.Fruits once again
begin growing rapidly about 4 to 6 weeks before harvest in what is called stage III, or “final
swell.”
extremely important in relationship to final fruit size.Since fruit are growing at a logarithmic
rate during stage I, it is essential to optimize fruit growth during that period; otherwise potential
size can be lost.In other words, any size that is lost early in the development of a fruit can never
be regained.This is very much like the law of compound interest in that small investments over
a long period of time yield much more than large investments over a short period.
FRUIT THINNING
Date of Thinning
For the above reasons, fruit should be thinned as early as reasonably possible.Numerous studies
have been performed that illustrate this concept.One of the more recent studies and one of few
performed on plums (DeJong et al., 1990) demonstrated how late thinning could greatly harm
fruit size (Table 1).In that study, unthinned trees actually had more marketable fruit per acre
Such studies emphasize the
need to thin fruit as early as is economically possible.
Royal Diamond plum thinning study with trees thinned on varying dates (after DeJong
et al., 1990).
(tons/acre)
(tons/acre)
9.0a7.9ab7.5b5.5b6.6c
zFor each line, values followed by the same letter are not significantly different.
Severity of Thinning
This concept states that crop load is inversely proportional to fruit size.In other words, the more
fruit removed from the tree, the larger the remaining fruit will grow.Of course, the more fruit
removed, the less total production the tree will have—note in Table 1 how the unthinned
treatment had 33.6 tons of fruit per acre, while the others were vastly lower.However, the three
earliest thinning treatments had more marketable fruit yield than the other treatments.
Research has
shown that crop load adjustment is the most reliable way to increase fruit soluble solids content,
more so even than delaying harvest by a few days (Crisosto et al., 1997; Day et al., 1994).
what size fruit is required. After that, total crop load must be adjusted annually to produce fruit
that meet that market demand. There is no practical way to do this other than by trial and error,
estimates, experience, and fruit counting.
Hand Thinning
Virtually all thinning done on stone fruits in California is done by hand.Numerous attempts
have been made to develop mechanical or chemical thinning methods.To date, neither has
proven successful for commercial production.Table 2 presents data on a thinning trial for
Loadel canning cling peaches (DeJong et al., 1991).The results show that the early thinning
Also note the
differences in the amount of fruit removed at thinning (2.36 vs. 40.18 kg/tree) between the
extreme treatments.As mentioned above, these fruits which are thinned off represent a great
deal of photosynthate energy that could have been diverted into crop had the trees been thinned
earlier.
other to better identify which fruits will be the largest on a particular fruiting shoot.The first
reason is nearly always a mistake unless the variety is obsolete or the market is very bad (and
under those conditions growing no fruit loses less money than growing much fruit).Those who
consider trying to save money by waiting should take a close look at Table 2.The second reason
makes sense with some very early-season varieties that often have erratic size ranges.However,
it is best to wait no longer than necessary to thin.
Summary of fruit thinning trial of Loadel canning cling peaches thinned on three
different dates in 1991 (after DeJong et al., 1991).
(number/tree)
(g/fruit)
(g/fruit)
(number fruit/tree)
(tons/acre)
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