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David H.
Byrne, Texas A&M University and Terry Bacon, Sun World
International, Inc.
 Stone fruit trees such as peaches,
n ectarines, plums, and apricots develop their vegetative and
fruiting buds in the summer and, as winter approaches, the already developed
buds go dormant in response to both shorter day lengths and cooler
temperatures. This dormancy or sleeping stage protects these buds
from oncoming cold weather. Once buds have entered dormancy, they
will be tolerant to temperatures much below freezing and will not
grow in response to mid-winter warm spells. These buds remain dormant
until they have accumulated sufficient chilling units (CU) of cold
weather. When enough chilling accumulates, the buds are ready to grow
in response to warm temperatures. As long as there have been enough
CUs the flower and leaf buds develop normally. If the buds do not
receive sufficient chilling temperatures during winter to completely
release dormancy, trees will develop one or more of the physiological
symptoms associated with insufficient chilling: 1) delayed foliation,
2) reduced fruit set and increased buttoning and, 3) reduced fruit
quality.
INSUFFICIENT CHILLING SYMPTOMS
Delayed Foliation. A
classic symptom of insufficient chilling is delayed foliation. A tree
may have a small tuft of leaves near the tips of the stems and be
devoid of leaves for 12 to 20 inches below the tips. Lower buds will
break eventually but full foliation is significantly delayed, fruit
set is reduced, and the tree is weakened. Furthermore, heavy
suckering from lower parts of the tree causes management problems,
and normal development of next year’s fruit buds can be impaired.
Reduced Fruit Set and Buttoning.
Flowering, in response to insufficient chilling, often follows the
pattern seen with leaf development. Bloom is delayed, extended, and
due to abnormalities in pistil and pollen development, fruit set is
reduced. In many peach cultivars, flowers drop before or around shuck
split, but in others such as ‘Jersey Queen’ and ‘Harvester’, buttons
form. Buttons result from flowers which apparently have set but never
develop into full-size fruit. The fruit remains small and misshapen
as they ripen. If you cut these fruit open, the seed is dead. Because
buttoning is not apparent early in the season, growers can not thin
off the abnormal fruit and the developing buttons serve as a food
source and overwintering site for insects and diseases.
Reduced Fruit Quality. The
effects of insufficient chilling on fruit quality are probably the
least discussed but appear to be very common especially in central
and south Texas. The effects on leaf growth and fruit set are
dramatic but the effects of insufficient chill on fruit quality are
subtle, and can occur when other symptoms do not. Insufficient
chilling will cause many cultivars to have an enlarged tip and
reduced firmness. Furthermore, fruit ground coloration may be greener
than usual, possibly due to the fruit losing firmness before the
ground color can fully change from green to yellow. The extent of these
quality problems depends on the cultivar and the degree of chilling
deficiency.
CHILL ACCUMULATION MODELS
The question of what is dormancy and what constitutes ‘chilling
temperatures’ has yet to be clearly defined. Most people agree that
temperatures below freezing or above 60 degrees F are not effective
for chilling unit accumulation. There are various models used to
calculate chilling, each one defining what a chilling unit is. The
three most common models are: the number of hours below 45 degrees F model,
the number of hours between 32 and 45 degrees F model, and the Utah
model (Table 1.). The first two models are simple and define a
chilling unit as one hour below or between certain temperatures. The
Utah method is more complex because it introduces the concept of
relative chilling effectiveness and negative chilling accumulation
(or chilling negation).
Table 1. Common chill accumulation
models
450 F and Under Model
1 hour <= 450 F =1.0 chill unit
32-450 F Model
1 hour between 32 and 450 F =1.0 chill unit
Utah Model
1 hour below 34 degrees F =0.0 chill unit
1 hour 35-36 degrees F =0.5 chill units
1 hour 37-48 degrees F =1.0 chill units
1 hour 49-54 degrees F =0.5 chill units
1 hour 55-60 degrees F =0.0 chill units
1 hour 61-65 degrees F =-0.5 chill units
1 hour >65 degrees F =-1.0 chill units
All of these models require hourly temperatures to be recorded for
calculation, and the point in time to begin recording chilling
temperatures is not well defined. In addition, since these models were
developed in states where high chill-requirement peach cultivars are
grown, their usefulness under medium and low chill-accumulation
conditions has been limited.
Mean Temperature Model.
Another approach, the mean temperature model, uses mean winter (December
and/or January) monthly temperatures to estimate accumulated chilling
units. Researchers in Georgia and Florida independently developed a
relationship between the mean monthly temperature of their coldest
month(s) and total chill unit accumulation. Combining data from both
studies (Figure 1.), the Stone Fruit Breeding Program at Texas
A&M University developed a method to estimate chill accumulation
which has demonstrated to be accurate for estimating chill
accumulation in Texas from the Lower Rio Grande Valley up to the Red
River, and should work well throughout the southeastern U.S.
Chilling accumulation, determined with this model, has been tested
and compared to peach tree behavior at Stephenville, Fredricksburg,
College Station, Yoakum, and Weslaco, TX. The coldest month or months
are used for the calculation (Table 2.). In low chill regions
(regions where average January temperature is 59-63 degrees F) where
January represents the dormancy season, January mean temperature is
most accurate for estimation. In high chill regions (regions where
average January temperature is below 48 degrees F) a mean
December-January temperature is recommended. In medium chill regions
(regions where average January temperature is 48-58 degrees F)
January mean temperature has been best for calculating chill
accumulation except in years when mean temperatures between December
and January differed by more than 6 degrees F. In this case, the
December-January mean was more accurate.
Table 2. Mean temperature chill
accumulation model calculations.
For January mean: Total chill accumulation= 3547-54(January
mean temperature)
For December-January mean: Total chill accumulation=
4280-68.8(December+January mean temperature/2)
The accuracy and the simplicity of calculating chill accumulation
with mean temperatures will aid fruit researchers and growers in
several ways. Mean temperature data is routinely kept by cities and
state climatologists and is usually easily accessible for tracking
chill accumulation for a specific area over long periods of time.
This will make it easy for a researcher, extension agent, or grower
to more accurately match cultivars to a given locale. Figure 3
illustrates chill accumulation at College Station TX over a 12 year
period from 1986-1997. This demonstrates that if a grower had planted
a 750 C.U. requirement peach cultivar prior to 1986, it would
probably have experienced some degree of insufficient chilling
symptoms in at least 4 of those 12 years (33.3%).
Also, this method will make it possible for the grower to know,
before fruit thinning time, if chill accumulation was sufficient for
a given cultivar. If insufficient chilling is suspected for a
cultivar, the grower can implement management and marketing
strategies to reduce the impact on costs and labor. Furthermore,
chemical sprays such as Dormex that help break dormancy are being
researched. These chemicals can be used in late January or early
February if insufficient chilling is suspected. On the other hand,
the expense of a dormancy-breaking chemical can be avoided if the
grower knows that trees have received sufficient chill accumulation.
Hopefully the mean temperature chill model will prove to be a tool
that can help us select cultivars better and manage our present
cultivars to minimize detrimental effects of mild winter.
Further Reading:
Byrne, D. H., and T. A. Bacon. 1992. Chilling
estimation: its importance and estimation. The Texas Horticulturist
18(8):5, 8-9.
Gurdian, R. J. and R. H. Biggs. 1964. Effect of low
temperature on terminating bud dormancy of Okinawa, Flordawon,
Flordahome, and Nemaguard peaches. Proc. Fla. State Hort. Sci.
77:370-379.
Richardson, E. A., S. D. Seeley, and D. R. Walker.
1974. A model for estimating the completion of rest for Redhaven and
Elberta peach trees. HortScience 82:302-306.
Sharpe, R. H. 1970. Sub-tropical peaches and
nectarines. Fla.State Hort. Soc. 82: 302- 306.
Sherman, W. B. and B. L. Topp. 1989. Peaches do it
with chill units. Fruit South. 10(3):15-16.
Swartz, H. J. and S. E. Gray. 1982. Annual chill
unit accumulation in the U.S. Fruit Var. J. 36(3):80-83.
Weinberger, J. H. 1950. Chilling requirements of
peach varieties. Proc. Amer. Soc. Hort. Sci. 56:122-128.
Weinberger, J. H. 1956. Prolonged dormancy trouble
in peaches in the southeast in relation to winter temperatures. Proc.
Amer. Soc. Hort. Sci. 67: 107-112.
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