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WATER MANAGEMENT ALTERNATIVES FOR STRAWBERRY TRANSPLANT
ESTABLISHMENT AND FREEZE PROTECTION IN FLORIDA
By
IXCHEL M. HERNANDEZ-OCHOA
A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE
UNIVERSITY OF FLORIDA
2013
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© 2013 Ixchel Manuela Hernandez Ochoa
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To my family and friends who stay with me through the whole
process giving me their support, and to my advisor for his lessons
and patience.
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ACKNOWLEDGMENTS
Thanks to my family, especially to my mom, Maria Luisa, my
sister Nina, and my
Uncle Rene, for their support and for being with me through the
whole process, to my
friends for being there supporting me, helping me and making me
laugh. Also thanks to
my committee members Dr. Xin Zhao and Dr. Craig Stanley for
their advice and
especially to my advisor Dr. Santos, for all his lessons and
patience.
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TABLE OF CONTENTS page
ACKNOWLEDGMENTS
..................................................................................................
4
LIST OF TABLES
............................................................................................................
6
ABSTRACT
.....................................................................................................................
7
CHAPTER
1 INTRODUCTION
.......................................................................................................
9
2 LITERATURE REVIEW
...........................................................................................
13
Cultivar Description
.................................................................................................
15 Water Use for Strawberry Production in Florida
...................................................... 16 Chilling
and Freezing Injury
....................................................................................
17 Establishment and Freeze Protection Methods
...................................................... 19
Sprinkler Irrigation
............................................................................................
19 Row Covers
......................................................................................................
21 Crop Protectants
..............................................................................................
23
3 COMPARISON OF FOLIAR AND ROOT-DIPPED CROP PROTECTANTS FOR
STRAWBERRY TRANSPLANT ESTABLISHMENT
............................................... 27
Overview
.................................................................................................................
27 Materials and
Methods............................................................................................
28 Results and
Discussion...........................................................................................
30
Foliar Crop Protectant Study
............................................................................
30 Root-Dipped Crop Protectants Study
...............................................................
32
4 COMPARISON OF FREEZE PROTECTION METHODS FOR STRAWBERRY
PRODUCTION
........................................................................................................
39
Overview
.................................................................................................................
39 Materials and
Methods............................................................................................
40 Results and
Discussion...........................................................................................
41
2011-2012 Season
...........................................................................................
41 2012-2013 Season
...........................................................................................
43
5 CONCLUSIONS
......................................................................................................
55
LIST OF REFERENCES
...............................................................................................
58
BIOGRAPHICAL SKETCH
............................................................................................
66
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LIST OF TABLES
Table page 3-1 Climate conditions from October 12 to October to
October 26 2012, in Balm,
Fl.
.......................................................................................................................
36
3-2 Effects of foliar crop protectants on plant diameter, and
total early fruit number and weight of bare-root strawberry
transplants, 2012-13. Balm, FL. ..... 37
3-3 Effects of root-dipped crop protectants in plant diameter
and total early fruit number and weight of strawberry transplants,
2012-13, Balm, FL...................... 38
4-1 Environmental conditions from October 2011 to March 2012.
Balm, Florida. ..... 47
4-2 Effects of freeze protection methods on plant number and
plant growth, Balm, FL, 2011-12.
.......................................................................................................
48
4-3 Effects of freeze protection methods on the minimum seasonal
air temperatures in each treatment, water use, and early and total
markefruit weight and number, Balm, FL, 2011-12.
.............................................................
49
4-4 Effect of freeze protection methods on the minimum seasonal
temperature for each treatment, and the first six harvests after a
freezing event on strawberry markefruit weight and number, Balm,
FL, 2011-12. ............................................ 50
4-5 Environmental conditions from October 2012 to March 2013.
Balm, Florida. ..... 51
4-6 Effects of freeze protection methods on plant number and
plant growth, Balm, FL, 2012-13.
.......................................................................................................
52
4-7 Effects of freeze protection methods on the minimum seasonal
air temperatures in each treatment, water use, and early and total
markefruit weight and number, Balm, FL, 2012-13.
.............................................................
53
4-8 Effect of freeze protection methods on the minimum seasonal
temperature for each treatment, and the first six harvest after a
freezing event on strawberry marketable fruit weight and number,
Balm, FL, 2012-13. ................................... 54
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Abstract of Thesis Presented to the Graduate School of the
University of Florida in Partial Fulfillment of the Requirements
for the Degree of Master of Science
WATER MANAGEMENT ALTERNATIVES FOR STRAWBERRY TRANSPLANT
ESTABLISHMENT AND FREEZE PROTECTION IN FLORIDA
By
Ixchel M. Hernandez-Ochoa
December 2013
Chair: Bielinski Santos Major: Horticultural Sciences
Florida is the second largest strawberry (Fragaria × ananassa)
producer in the
U.S. Production fields are concentrated in Plant City and Dover
in west-central Florida.
Water resources in this area are shared between agriculture and
urbanization. During
strawberry establishment and freeze protection, the standard
practice is using sprinkler
irrigation (i.e. 17 L m-1), which is inefficient due to the use
of large volumes of water.
Several alternatives to reduce water usage during these phases
were identified (i.e.
reduced-volume sprinklers, row covers, and crop protectants).
The overall goal of this
study is to evaluate and compare the effects of different
transplant establishment and
freeze protection methods on water savings, growth and yield of
strawberry. Foliar and
root-dipped crop protectants were evaluated in two separate
trials. In addition, the effect
of freeze protection alternatives (i.e. reduced-volume
sprinklers, light and heavy row
covers, and a crop protectant) were also assessed.
In the crop protectant trials for transplant establishment,
early marketable fruit
weight and number was the same when using crop protectant
applications as 10 days
of sprinkler irrigation (DSI) in both trials. When using 7 DSI
alone resulted in decrease
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on early marketable yield in both trials. Sprinkler-irrigated
treatments showed lower
plant diameter than crop protectant treatments. For the freeze
protection study, there
was no difference in early and total marketable yield among
treatments when minimum
temperature was (-3oC), temperature inside row covers was
between 2 and 8oC higher
the temperature outside. No difference among treatments in plant
diameter was
observed.
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CHAPTER 1 INTRODUCTION
The cultivated strawberry (Fragaria × ananassa Duch.) is a
perennial woody
plant that belongs to the Rosaseae family. This
commercially-grown plant is a hybrid
derived from the cross between F. chiloensis and F. virginiana.
The primary structure is
the crown from which leaves, runners, roots, axillary crowns,
and inflorescences grow
(Darnell, 2003). The berry develops from the flower receptacle:
a fleshy pith surrounded
by a ring of vascular bundles with branches ending in the
aquenes, which is the real fruit
(Darrow, 1966).
The U.S. occupies the first place among strawberry producers
around the world
with 23,060 harvested ha and nearly 1.3 million t of fruit
during 2011 (USDA, 2012;
FAO, 2011). About 83% of the strawberry production in the U.S.
is concentrated in two
states: California and Florida with 15,378 and 4,000 ha,
respectively (USDA, 2012).
During the winter season, Florida supplies the majority of the
fresh market for the
country (Boriss et al., 2012). Almost 95% of the production
fields are located in the
west-central area of Hillsborough and Manatee Counties (Mossler,
2010).
Strawberries are planted from late September to mid-October, and
the harvest
period occurs from early December through early April (Hochmuth
et al., 1993). During
strawberry establishment and freeze protection, the standard
practice is the use of
overhead sprinklers delivering water at about 17 L m-1.
Bare-root transplants brought
from Canadian nurseries are set into fumigated, black
polyethylene mulched beds in
environments with high temperatures and evapotranspiration.
Sprinkler irrigation is used
from 8 to 12 h day-1 during the first 10 days to provide a
microclimate that reduces
temperature around crowns and promotes new root growth (Hochmuth
et al., 2006;
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Santos et al., 2012). This activity consumes about 4940 m3 ha-1
of water, which is about
one-third of the total water use during the season (Albregts and
Howard, 1985).
During the season, strawberries are subjected from four to six
freezing nights.
Sprinkler irrigation uses about 514 m3 ha-1 per freezing night
(Santos et al., 2011a).
During the unusual winter of 2010, around 11 freezing nights
occurred and large
volumes of water were pumped to protect the crop. As a result of
these freezing nights,
the aquifer level dropped 18 m allegedly causing 750 residential
dried wells and more
than 140 sinkholes in the area. This phenomenon has occurred
three times over the last
ten years. After the last period of freezing nights in 2010 and
2011, the Southwest
Florida Water Management District began working on trying to
implement a water
management plan to protect the aquifers. The Plant City-Dover
area was declared a
water use caution area, causing special rules for water use
during freeze protection to
be developed (SWFWMD, 2011).
The use of sprinkler irrigation for transplant establishment and
freeze protection
are highly inefficient due to the use of large volumes of water,
most of which ends
running off to the drainage canals, leaching nutrients from the
root zone or lowering
aquifer level. During freeze protection, sprinkler irrigation
may result in injury to green
and ripe fruit due to the high impact of water droplets (Bish et
al., 1997; Domoto, 2006;
Hochmuth et al., 1986; Jackson and Parsons, 1994; Perry, 1998;
Poling et al., 1991).
Reduced-volume sprinklers, row covers, and crop protectants
could be alternatives to
sprinkler irrigation. However, more research is needed to assess
the effect of these
techniques on strawberry growth and yield. In general,
sprinklers are set for the worst
scenarios and deliver more water than is needed for crop
protection and growth (Fisher
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and Shortt, 2009; Perry, 1998). Reduced-volume sprinklers might
be a suitable option to
decrease volumes of water used during transplant establishment
and freeze protection.
Crop protectants are products designed to reduce environmental
stress. During
transplant establishment, these products can be used either as
foliar or root-dipped
applications. Foliar products are based on natural-occurring
materials like calcium
carbonate, kaolin clay or aluminum silicate. When the product is
applied on plant
canopies, it creates a coat that reflects radiation, reducing
transpiration, and water
stress on transplants. Moreover, root-dipped crop protectants
concentrate water around
the plant crown to provide enough moisture and increase water
availability for the new
roots (Hodges et al., 2006). Crop protectants for freeze
protection are usually composed
of polymeric terpenes that help decreasing the freezing point on
plant tissues, thus
preventing or reducing freezing damage on plants (Perry,
1998).
Row covers are flexible, transparent or semi-transparent
blankets made up of
polyethylene, polypropylene, polyester, and other materials.
They are commonly used
to enhance crop growth and yield by increasing temperature
around the plants during
cold periods and also acting as barriers for pests (Hochmuth et
al., 1987; Nair and
Ngouajio, 2010; Stall et al., 1985). Row covers help in
decreasing the influence of wind,
evaporative and radiational cooling, and convection by keeping
high temperatures for
longer periods of time. The overall goal of this study was to
determine the effectiveness
of water-saving strategies during strawberry transplant
establishment and freeze
protection.
The specific objectives were:
Determine the effects of using crop protectants on strawberry
transplant establishment growth and early marketable yield.
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Compare the effects of reduced-volume sprinkler and
non-irrigation alternatives for freeze protection on water savings,
strawberry growth, and early and total marketable yield.
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CHAPTER 2 LITERATURE REVIEW
Strawberry is one of the most high-value fruit crops in the U.S.
with 1.3 million t
harvested and $2.4 billion in gross sales during the 2011 season
(USDA, 2012). The
main market window for Florida strawberry is during winter
months when California
production is out of the market. Most of the strawberry crop
produced in the state is
utilized for fresh consumption. Strawberries produced nowadays
are a cross between F.
chiloensis and F. virginiana. Both of these species have their
heritage in America. F.
chiloensis was first identified in the coastal areas of Chile
and it was brought and
domesticated in France during the 18th century. F. virginiana is
also an eastern north
American species that was introduced to France during the 1600’s
(Darrow, 1966). After
their cross in Europe, the hybrid was re-introduced to America
during the 19th century.
Strawberries are grown in moderate climates with temperatures
around 20oC and low
humidity conditions (Boriss et al., 2012; Rowley et al., 2010).
In Florida, strawberries are
commonly-grown in soil under hill plasticulture system
(Cantliffe et al., 2007; Chandler
et al., 2000). During early October, bare-root transplants are
set into fumigated
polyethylene-mulched beds and harvested from early December to
late March
(Hochmuth et al., 1993).
There is a broad range of genetic variation among strawberry
species. The
commercially-grown hybrid plant is an octoploid, 2n = 8 (Darrow,
1966). This
heterogozity makes it hard to breed (Nyman and Wallin, 1992).
The plant basic
structure is the crown, which is a compressed stem from where
shoots and roots
develop (Darnell, 2003). The trifoliated leaves grow surrounding
the crown and elongate
until they are fully expanded. The axillary leaf buds may become
axillary crowns or
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runners, depending on the stage of the plant and the
environmental conditions (Darnell,
2003). Each axillary crown has its own inflorescence (Poling,
2012). Runners are
daughter plants, which are vegetative organs that grow with
identical characteristics to
the mother plant. Runners are the most common method for crop
propagation (Bish et
al., 1997). The root system in strawberries is adventitious. In
sandy soils, they can
penetrate 30 cm deep, but most of the roots are concentrated in
the first 15 cm of soil
(Dana, 1980; Darrow 1966).
Flower induction in strawberry is influenced by the combination
of two main
factors: photoperiod and temperature. There are three main types
of cultivated
strawberries, which are divided depending on the flowering
pattern in day-neutral, short-
day or June-bearing, and ever-bearing cultivars. Flowering in
day-neutral cultivars is
determined mostly by changes in temperature, while flowering in
ever-bearing and
June-bearing cultivars is influenced mainly by changes in
photoperiod (Darnell, 2003;
Taylor, 2000). Ever-bearing cultivars produce flowers during
long-day conditions, while
June-bearing cultivars flower during fall when days are less
than 14 h of light. Low
temperatures also increase flower bud initiation (Darnell,
2003). After flower induction,
apical meristems start to differentiate into flower buds. Flower
buds are arranged into
inflorescences with a primary flower, which will develop into
the largest fruit, and the
secondary and tertiary flowers into smaller fruit (Darnell,
2003; Taylor, 2002). The
strawberry flower is perfect with 10 sepals, around five petals,
20 stamens, 60 to 600
pistils depending on the flower. Flower pollination is mostly by
wind and insects
(Darnell, 2003). The real seeds are pollinated pistils that
remain attached to the
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receptacle surface during its expansion, resulting in an
accessory fruit or aggregate
(Darnell, 2003; Poling, 2012).
Cultivar Description
‘Strawberry Festival’ is a short-day cultivar released by the
University of Florida
(UF) in 2000. It was the predominant variety in Florida
west-central area during the
2009-10 season with approximately 60% of the total planted area
(Chandler et al., 2009;
Whitaker et al., 2011). This variety was a result of the cross
between ‘Rosa Linda’ and
‘Oso Grande’ (Chandler et al., 2000). ‘Rosa Linda’ is a variety
released by UF in 1996,
this variety was chosen due to its high early yield and fruit
shape, and ‘Oso Grande’,
which is an University of California variety was selected for
its fruit quality (Chandler et
al., 2000). ‘Strawberry Festival’ fruit is medium size with
conic shape and deep red
color, firmness, and resistant skin to external damage that
makes it excellent for
shipping. The upright growth of leaves makes it easy to harvest
(Chandler et al., 2000;
Whitaker et al., 2012).
‘Florida Radiance’ was released by UF in 2009 as a result of the
cross between
‘Winter Dawn’ and ‘FL 99-35’. ‘Winter Dawn’ was a variety
selected for its high early
yield and large fruit shape. ‘FL 99-35’ was chosen for its
desirable characteristics such
as firm and attractive fruit (Chandler et al., 2009). This
variety ranked in second place
with 15% of the total planted acreage in 2010-11 season
(Whitaker et al., 2012). Its fruit
is medium conic in shape and produce elongated fruit during
early yield. Fruit color
depends on the stage of ripening from glossy bright to dark red
(Chandler et al., 2009).
Plant growth habit is more open with elongated pedicels compared
to ‘Strawberry
Festival’ (Chandler et al., 2009).
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According to Santos et al., (2009), ‘Strawberry Festival’ had
the highest early and
total fruit number during both seasons among other cultivars
such as ‘Winter Dawn’,
‘Florida Elyana’, ‘Florida Radiance’, ‘Ruby Gem’, and
‘Treasure’. However, during early
season and in late February, yields were reduced due to smaller
fruit size (Chandler et
al., 2009). ‘Florida Radiance’ complements this fruit reduction
during that period with
high yield and consistent fruit shape throughout the season
(Chandler et al., 2009;
Santos et al., 2009; Whitaker et al., 2012). ‘Strawberry
Festival’ is intermediate
susceptible to anthracnose fruit rot caused by Colletotrichum
acutatum, and less
susceptible than ‘Sweet Charlie’ to botrytis fruit rot caused by
Botrytis cinerea (Chandler
et al., 2000, 2004). ‘Florida Radiance’ is moderately resistant
to both of these diseases,
which are the two most important fruit diseases during
cultivation (Mackenzie et al.,
2006).
Water Use for Strawberry Production in Florida
Water availability and quality play important roles in
urbanization development.
Primary water withdrawal uses in Florida are for public and
agricultural consumption,
accounting for 80% of the total use. Fresh water withdrawals in
Florida during 2010
were approximately 24.6 million m3 day-1. About 65% of total
water withdrawals come
from surface water sources, which is the most important water
source (United States
Geological Service, 2010). It is estimated that population
growth in Florida from 2005 to
2025 will be up one third, which means an increase in water
withdrawals demand of
≈40% (Florida Department of Environmental Protection, 2010).
Sustainability for water
management is important from an environmental point of view. The
mean to reduce the
amount of water used for agriculture is to identify agricultural
practices that use major
amounts of water and focus on them to offer practical and
sustainable alternatives.
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In Plant City and Dover, Hillsborough County, water resources
are also shared
between agricultural and urban uses (Hochmuth et al., 2006;
Santos et al., 2011a,
2012). Special rules for water use in agriculture were
implemented since 2010, when
unusual winters took place and high amounts of water were
applied to protect the crop
from freezing damage. This activity allegedly caused a drop in
the Floridian aquifer level
of 18 m. It is unclear whether as a result of these events,
about 140 sinkholes and more
than 750 dried wells occurred in the area (Aurit et al., 2013;
Southwest Florida
Management District, 2011). However, public pressure has
suggested there is a link
between both events. Consequently, there is a need to reduce the
amount of water
used for strawberry production, particularly during transplant
establishment and freeze
protection. The common practice in both activities is the use of
sprinkler irrigation, which
uses about two-thirds of the total amount of water utilized
during the season (Hochmuth
et al., 2006). The quantity of water used with sprinkler
irrigation during transplant
establishment is approximately 4940 m3 ha-1, and during freeze
protection, around 556
m3/ha of water per night are used to protect the crop (Hochmuth
et al., 1993 and 2006;
Santos et al., 2011a and 2012).
Chilling and Freezing Injury
When plants are exposed to low temperatures, they are subject to
two types of
injury: chilling injury and freezing injury. Chilling injury
happens in relative sensible
crops, such as tropical and subtropical fruits. Damage occurs
when plants are exposed
to temperatures between 10 and 15oC, but above 0oC, which is the
freezing point for
water (Wang, 1990). Decrease in temperature has a direct effect
on lowering cell
metabolism. Under prolonged conditions tissue damage, such as
discoloration, surface
damage, and interruption in plant growth and ripening process
may occur (Wang, 1990;
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Wang and Wallace, 2004). When plants are subjected to
temperatures below 0oC,
freezing injury occurs causing disruption of cell walls due to
ice crystal formation
(Lindow, 1983). Populations of ice-nucleation active bacteria
act as centers to initiate
ice formation (Lindow, 1983). A decrease on the population of
ice-nucleation bacteria
causes a decrease on the freezing point of water, this process
is called super cooling.
Solute concentration and rate of temperature drop also have an
influence on ice crystal
formation (Wang, 1990). The amount of solutes dissolved in water
decreases freezing
point. Water in apoplastic spaces has less solute concentration
and freezes first, then
water moves from the intracellular space to apoplastic space by
differences in water
potential, causing dehydration. Under natural conditions,
temperature drops at low rates
and then ice crystals are bigger causing cell membrane breaking.
However, when ice
formation occurs fast, ice crystals are small and the freezing
damage is significantly low.
Plant acclimatation to cold temperatures is called hardening,
which is an increase
in solute concentration in plant tissue or a decrease in
ice-nucleation bacteria or both
processes happening at the same time (wang, 1990). Another
mechanism developed by
the exposure to low temperatures is antifreeze protein
formation. Their function is to
reduce the speed of ice formation by attaching to the crystals
surface (Wang, 1992).
Gene expression also plays a role by promoting biochemical
changes, such as abscisic
acid production, which might be a main factor in developing
plant response to
acclimation (Campalans et al., 1999; Tomashow, 1994). In order
to prevent or reduce
freeze damage many techniques have been developed. Most of them
are based on
either add or hold the temperature in the surrounding air to
keep plant tissues at higher
temperature than freezing point (Perry, 1998), Other techniques
such as
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cryoprotectants aim to prevent ice formation by reducing
population of active ice-
nucleation bacteria on plant tissues (Hew and Yang, 1992).
Critical temperatures for
freezing injury in strawberry will depend on the plant organs.
The most susceptible to
freezing injury are the open blossoms at -1.1oC, followed by
fruit at -2.2oC. Early stages
are less susceptible to low temperatures, tight bud and popcorn
stage suffer damage at
-3 and -5.5oC, respectively (Perry and Poling, 1986).
Establishment and Freeze Protection Methods
Sprinkler Irrigation
Water has been used for a long time to protect strawberries
during transplant
establishment and freeze protection. The physical phenomenon
behind this method is
based on how water absorbs and releases energy, when it changes
from one physical
state to another. A calorie is described as the quantity of heat
needed to raise in 1°C the
temperature of 1 g of water at 1 atm. During transplant
establishment, water is applied
to provide an appropriate microclimate to cool crowns and
promote root and shoot
growth. Cooling occurs when water changes from liquid to vapor.
In this process water
absorbs approximately 540 calories from the surrounding air and
it is called evaporative
cooling. This occurs due to the difference between air and
saturated vapor pressures.
As long as the air vapor pressure, which is a measure of the
water vapor in the air, is
less than the saturated vapor pressure, the process continues
occurring. In addition,
when the temperature of applied water is lower than the
temperature outside, water
absorbs heat due to the temperature gradient and also helps to
decrease the
temperature (Albregts and Howard, 1985).
A similar principle is applied when water is used for freeze
protection. Under
freezing conditions, applied water turns from liquid to ice
releasing energy. For each 1 g
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of water turning to ice, approximately 80 calories are released,
supplying heat to keep
plant tissues near or above 0°C. This process is called latent
heat of fusion. At the same
time, evaporative cooling occurs, which absorbs approximately
540 calories. This is
nearly 7 times from what it is being released by latent heat of
fusion. Therefore, it
becomes necessary to apply 7 times the volume of water needed in
order to keep a
balance between the energy released by latent heat of fusion,
and the energy absorbed
by evaporative cooling. Otherwise, the effect will be the
opposite thus damaging the
crop (Perry, 1998).
Environmental factors such as wind speed, dew point, and
relative humidity
affect appropriate application and distribution of water. Under
conditions where wind
speeds are above 8 km h-1, the water volume needed to get
effective protection is three
to four times more than in calm conditions, because latent heat
will be removed and
uniformity of application will be compromised (Perry, 1998;
Powell and Himelrick, 2000).
Dew point is the temperature at which air becomes saturated with
water vapor and
condenses forming dew on surfaces. It depends on the relative
humidity, since the drier
the air, the lower the dew point (Perry, 2001). When dew point
is low, sprinkler irrigation
needs to be turned on before the temperature in the air
decreases 1°C. When sprinkler
irrigation is applied under dry conditions, evaporative cooling
will be favored instead of
latent heat of fusion, causing an opposite effect of temperature
decrease (Fisher and
Shortt, 2009).
Most of the strawberry production fields are equipped with
sprinklers delivering
17 L m-1, which is equivalent to 155 m3 ha-1 of water per h,
being enough to maintain
and protect the crop during the season. This type of sprinkler
is used for both strawberry
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transplant establishment and freeze protection. The amount of
water used during freeze
protection is comparable to the amount of water used for
transplant establishment
(Albregts and Howard, 1985; Santos et al., 2011a). Sprinkler
irrigation is inefficient due
to the high volumes of water used during the process, most of
which drains off and
lowers the aquifer levels. Another effect of using sprinklers is
injury on green and ripe
fruit due to the high impact of water droplets (Bish et al.,
1997; Domoto, 2006;
Hochmuth et al., 1986; Jackson and Parsons, 1994; Perry, 1998;
Poling et al., 1991).
During transplant establishment and freeze protection, the
amount of water needed can
be reduced with intermittent intervals and reduced-volume
sprinklers (e.g. 13 L m-1)
since these sprinklers are set for the worst scenarios (Albregts
and Howard, 1985;
Golden et al., 2003). Sprinkler irrigation at the field are set
at 17 L m-1 to provide
effective protection to strawberries, and are commonly spaced at
14.6 × 14.6 m, giving
100% overlapping (Locascio et al., 1967). However, when
sprinkler nozzle size is
reduced and distance between sprinklers is larger, freeze
protection might be
compromised (Fisher and Shortt, 2009; Locascio et al.,
1967).
Row Covers
Protected culture is the use of temporary or permanent
structures to provide a
modified environment to enhance plant growth (Wells and Loy,
1993; Jensen and
Malter, 1995). Row covers have been used extensively, especially
to prolong growing
seasons. During winter, row covers are placed over the crop for
two or four weeks to
provide a warmer environment that enhances crop growth
(Dickerson, 2009; Wells and
Loy, 1993). During mild winters, row covers have the potential
to be used for freeze
protection (Hochmuth et al., 1987). In the 1980’s, row covers
started to be used on
commercial farms as an alternative for Florida growers to
protect their crops during
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freezing nights (Hochmuth et al., 1986). Row covers are
flexible, transparent or semi-
transparent blankets made up of polyethylene, polypropylene,
polyester and other
materials. Thickness and weight vary depending on materials and
purpose. For freeze
protection, commonly-used row covers are from 20 to 50 g m2, and
they are placed only
during the freezing nights (Hochmuth et al., 1986; Perry, 1998;
Wells and Loys, 1993).
The mechanism of using row covers is to enclose the mass of air
around the plant
canopy, thus keeping radiational heat already absorbed by the
plant and soil during the
day. Row covers significantly reduce the effect of wind,
evaporative and radiational
cooling, and convection (Hochmuth et al., 1986; Perry, 1998).
Heat loss happens with
the interaction of convection and conduction among the soil,
plant, air mass and the row
cover. Convection is heat transfer by movement of fluids, in
this case the air mass
inside and outside the row cover. Conduction is transference of
heat through a solid
material from molecule to molecule due to temperature gradients
(Snyder and Melo-
Abreu, 2005). With these two processes happening, the
temperature inside row covers
decreases gradually but not as fast as the air temperature
outside.
Using galvanized or fiberglass hoops is a common practice to
anchor the row
covers. Another method is placing the row cover with the crop
supporting it (Wells,
1996). When row covers are placed without hoops, leaves touching
row covers might be
damaged (Dickerson, 2009; Perry, 1998). Using hoops might
prevent damage on leaves
and at the same time the air mass around the plant canopy will
increase, offering more
crop protection. Width ranges between 3 to 15 m, which means
that between 2 and 12
beds could be covered at the same time (Wells, 1996). Degrees of
protection
accomplished with row covers vary from 2 to 7°C, depending on
the material and
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23
thickness (Dickerson, 2009; Hochmuth et al., 1993; Poling et
al., 1991; Santos et al.,
2011a). An advantage of using row covers is that these materials
would not increase
damage on fruit as may happen when using sprinkler irrigation
(Perry, 1998). One of the
major limitations for this system is the high labor cost, which
is about $370 per ha
(Dickerson, 2009; Wells and Loy, 1993).
Crop Protectants
Crop protectants provide protection reducing environmental
stress on plants.
During transplant establishment, naturally-occurring materials,
such as kaolin clay,
calcium carbonate, and aluminum silicate are applied on plant
canopies. They act as
reflective barriers to decrease ultraviolet and infrared
radiation, therefore reducing heat
stress on plants (Glenn and Puterka, 2005; Glenn et al., 2002
and 2003). Crop
protectants are widely used to reduce sunburn and decrease pest
incidence on fruit
such as apple (Malus domesticus), pear (Pyrus spp.), tomato
(Solanum lycopersicum),
and pomegranate (Punica granatum) (Cantore et al., 2009; Glenn
and Puterka, 2005;
Glenn et al., 2002; Melgarejo et al., 2004). Previous studies
reported that the application
of kaolin clay, reduced evapotranspiration rates and temperature
on leaf and fruit by 2
to 5oC in apple and pomegranate (Glenn et al., 2002; Melgarejo
et al., 2004; Wand et
al., 2006). White coats also help to reduce pest incidence such
as thrips (Frankliniella
spp.), and increased bud development and fruit set in blueberry
possibly due to reduced
heat stress (Vaccinium spp.) (Spiers et al., 2005). Crop
protectants for transplant
establishment could be an alternative to using sprinkler
irrigation to decrease
temperatures and improve transplant establishment. A study
conducted in strawberries,
resulted in 98% plant survival and no negative effects in plant
growth or early yield
(Santos et al., 2012).
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24
Root-dipped crop protectants are materials based on
water-absorbent crystal
polymers that may absorb from 100 to 1000 times of their weight
in water. The most
commonly used polymers in agriculture are cross-linked
polyacrylamides and cross-
linked acrylamide-acrylate copolymers. These materials last
between 5 to 7 years in the
soil and break down into ammonium, carbon dioxide, and water
(Ekebafe et al., 2011).
They are used especially in dry, arid areas for soil remediation
and landscaping where
water is a limited resource to establish new seeds (Wallace and
Wallace, 1989). The
polymer concentrates moisture around the root system, improving
water use and
fertilizer efficiency (El-Hady and Wanas, 2006; Wang and
Boogher, 1987). The amount
of water available for plant uptake varies between 40% and 95%
depending on the
cross linking and material (Wallace and Wallace, 1989). Many
studies conducted in arid
areas to establish tree species reported increases on plant
survival, and shoot and root
biomass (El-Hady and Wanas, 2006; Orikiriza et al., 2009; Pery
et al., 1995). The same
response was observed when the polymer was used to establish
fruit and vegetable
crops such as citrus and muskmelon (Cucumis melo) (Arbona et
al., 2005; Hodges et
al., 2006).
Bioproducts are especially used in organic agriculture where the
use of chemical
products is restricted. Regalia® is a plant extract from
Reynoutria sachalinensis
commonly used in organic agriculture to control some bacterial
and fungal diseases in
strawberry, tomato, and blueberry (Hai, 2012; McGovern et al.,
2012). The mechanism
of action is given by systemic acquired resistance (SAR) in
plants, which is a natural
defense response resulting from the exposure of plants to a
pathogen. The product
application promotes an increase in phenolic compounds and
pathogenesis related
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25
proteins with antifungal properties, providing resistance to a
broad spectrum of
microorganisms (Glazunova et al., 2009). The biofungicide
application in cucumber
(Cucumis sativus) caused an increase in phytoalexins after the
exposure to powdery
mildew caused by Sphaerotheca fuliginea (Daayf et al., 1997).
Another study conducted
in 2009, reported enhanced photosynthetic activity in bean seeds
(Vicia faba)
(Glazunova et al., 2009). Karavaev et al. (2008) reported
increase in number of stems
and total mass of the grains after the biofungicide application
in barley (Hordeum
vulgare) cultivars. Product effectiveness is determined by the
rate of application and
disease pressure, being more effective as a preventive than as a
curative treatment
(Konstantinidou et al., 2006).
There are many chemical products for freeze protection that are
intended to
prevent or reduce freezing damage on plant tissue. The mode of
action of these
products varies depending on the composition. Some of them are
antitranspirants that
create a physical barrier, reducing heat loss from the plant
tissue (Burns, 1970).
Desikote Max® (40% di-1-p-menthene) is based on polymeric
terpenes derived from
resins. Different studies have been conducted using similar
products with inconclusive
results. Research conducted with young citrus plants using 12
antitranspirants resulted
in no protection in treated plants with minimum temperatures of
-4 and -5oC (Burns,
1970). Another study conducted in tomato, pepper, and peach
plants using a
cryoprotectant and an antitranspirant did not provide freeze
protection when
temperature decreased to -1 and -3.5oC (Aoun et al., 1993; Perry
et al., 1992). However,
a study conducted with grasslands roa (Festuca arundinacea)
resulted in 22% higher
seed yields compared with the unprotected seeds when temperature
went down to -2
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26
and -1oC (Hare, 1995). Winter application on cranberries
(Vaccinium macrocarpon)
resulted in higher fruit number and yield compared with the
non-treated plots (Sandler,
1998). Gardea et al. (1993) reported positive results when using
an antitranspirant to
protect grape (Vitis vinifera) plants at -2oC. However, at lower
temperatures limited
protection was found.
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27
CHAPTER 3 COMPARISON OF FOLIAR AND ROOT-DIPPED CROP PROTECTANTS
FOR
STRAWBERRY TRANSPLANT ESTABLISHMENT
Overview
Sprinkler irrigation is used regularly used to establish
strawberry bare-root
transplants. When water is applied on plant canopies, it reduces
heat stress by lowering
temperatures around crowns and promoting new growth. This
growing phase consumes
about 4500 m3 ha-1 of water, which is equivalent to one-third of
the volume of water
used during the season (Albregts and Howard, 1985). Most of the
applied water ends
running off into drainage canals, leaching nutrients from root
zones, and lowering
aquifer levels (Bish et al., 1997; Domoto, 2006; Hochmuth et
al., 1986). Water use for
strawberry production is a concern due to competition between
agriculture and
urbanization for its use. Sustainable alternatives to reduce the
amount of water used
during strawberry production are important.
Crop protectants for transplant establishment could be suitable
alternatives to
reduce water volumes during transplant establishment. These
products are used as
foliar or root-dipped applications. Foliar applications are
usually based on naturally-
occurring materials that create a reflective coat reducing heat
stress on transplants
(Glenn and Puterka, 2005; Glenn et al., 2002, 2003).
Water-absorbent polymers and
biofungicides are utilized for root-dipped applications.
Root-dipped crop protectants
concentrate moisture around crowns and help to prevent bacterial
or fungal diseases,
as well as promoting development of beneficial soil organisms
(Ekebafe et al., 2011;
Hai, 2012; McGovern et al., 2012; Wallace and Wallace, 1989).
The objective of the
studies were to determine the effects of using crop protectants
on strawberry transplant
establishment growth and early marketable yield.
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28
Materials and Methods
Two separate studies were conducted during the 2012-13 season at
the Gulf
Coast Research and Education Center (GCREC) of the University of
Florida in Balm,
FL. The soil at the experimental site was a Myakka fine sand
siliceous hyperthermic
Oxyaquic Alorthod with 1.5% organic matter and pH of 6.6. Prior
to the experiment the
soil was tilled twice at approximately 20-cm deep to ensure
proper soil structure. In late
August, planting beds were formed using a standard bedder
measuring 69-cm wide at
the base, 61-cm wide at the top, 20-cm high, and 33 cm apart
between bed centers.
Simultaneously with bedding, the soil was fumigated with
1,3-dichloropropene plus
chloropicrin (40:60 v v-1) at a rate of 336 kg ha-1. One drip
tape line (0.03 L m-1 per min,
30 cm between emitters; T-Tape Systems International, San Diego,
CA) was buried 5
cm below the surface. Beds were covered with high density black
polyethylene mulch
(0.025-mm thick; Intergro Co., Clearwater, FL). Transplants were
set in double rows
with 30 cm separation between rows, and transplants were spaced
38 cm from each
other.
Plant nutrients, such as N, K, Mg, Fe, Zn, B, and Mn were
applied following the
current recommendations for the crop in the state (Santos et
al., 2011b). Daily fertilizer
application started at two weeks after transplanting (WAT)
through the drip lines using a
hydraulic injector (Dosatron, Clearwater, FL). Irrigation volume
was the equivalent to the
average reference evapotranspiration for the area from October
to March (Simonne and
Dukes, 2009), and it was split equally into two daily irrigation
cycles starting at 8 am and
1 pm, respectively. Recommendations for insect and disease
control were followed
depending on pest pressure (Santos et al., 2011b).
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29
Bare-root transplants with three to five leaves were brought
from nurseries in
Canada (Lareault Nursery, Lavaltrie, Quebec, Canada).
‘Strawberry Festival’
transplants for the foliar crop protectant trial were planted on
October 9, 2012. ‘Florida
Radiance’ was used for the root-dipped crop protectant trial,
and they were planted in
the field on October 16, 2012. Each plot consisted of 4.6-m long
rows with 20 plants and
1.5-m alleys between each plot. The experimental area was set
with 17 L m-1 sprinkler
heads spaced at 14.6 m. Immediately after transplanting,
sprinkler irrigation was turned
on at 8 am each morning for 8 h day-1. For the foliar
application study, treatments were:
a) 10 DSI (control), b) 7 DSI, c) kaolin clay (Surround WP,
Tessenderlo Kerley, Phoenix,
AZ) at 28 kg ha-1, d) aluminum silicate (Screen Duo WP, Certis
USA, Columbia, MD) at
11.2 kg ha-1, and e) calcium carbonate (Purshade, Tessenderlo
Kerley, Phoenix, AZ) at
28 L ha-1. Foliar crop protectants were dissolved in 560 L of
water and applied on plant
canopies the next day after 7 DSI. For the root-dipped
application study, a water-
absorbent polymer (Supersorb F, acrylamide potassium acrylate
copolymer cross-
linked; Engage Agro, ON, Canada) and a biofungicide (Regalia®,
5% extract of
Reynoutria sachalinensis; Marrone Bio Innovations, Davis, CA)
were compared after 7
and 10 DSI. Root-dipped crop protectants were applied at the
moment of transplanting
at the rate of 10 and 3.5 g L-1 of water, respectively.
Sprinkler irrigation was turned on
immediately after transplanting for 7 days. Both experiments
were set in a randomized
complete block design with four replications.
To assess the effect of treatments on strawberry growth and
development, five-
randomly selected plants were chosen to measure canopy plant
diameter at 4, 8, and
12 WAT. Plants in borders were not used for this measurement.
The same plants were
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30
used for the three observations. Canopy plant diameter was
measured perpendicular to
the direction of the rows. Plots were harvested twice a week on
Mondays and
Thursdays, marketable fruit was defined as a fruit over 10 g in
weight, physiologically
mature with more than 80% dark red skin, and free of defects or
disease injury. Early
marketable fruit weight and number were collected for the first
10 harvests to identify
the effects of the crop protectant applications on the early
yield. Climate data from
2012-13 season were collected from Florida Automated Weather
Network (FAWN).
Maximum and minimum temperatures between 15 to 25 cm above
canopy level were
monitored for each treatment with temperature loggers (HOBO data
loggers, Onset
Corp., Bourne, MA). Data were analyzed using the general linear
model (P≤0.05) and
treatment values were separated using Fisher’s protected least
significant difference
tests (Statistix Analytical Software, version 9, Tallahassee,
FL).
Results and Discussion
Climatic conditions corresponding to the establishment phase are
presented in
Table 3-3. Average minimum and maximum temperatures from 12 to
26 October, 2012
were 18.8 and 31oC, respectively. No rain was reported during
this time. From
November to January, 2012, minimum temperatures ranged between
0.6 to 18.9oC, and
maximum temperatures were between 13.9 and 29.4oC. Rainfall
volumes were 2.5, 62,
and 7.6 mm, respectively, during the same period.
Foliar Crop Protectant Study
There was no difference in plant diameter among treatments at 4
WAT. Sprinkler
irrigation combined with foliar applications resulted in the
same plant diameter as using
sprinkler irrigation alone, values ranged between 15 and 17 cm
(Table 3-2). However,
plant diameters at 8 and 12 WAT were affected by the combination
of foliar application
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31
and days of sprinkler irrigation. The highest values were
observed in treatments using
foliar application and 7 DSI alone with 37 cm. Using 10 DSI
resulted in 33 cm, which
was the lowest value (Table 3-2). The same tendency was observed
at 12 WAT, with no
differences in plant diameter for foliar application treatments
and 7 DSI alone, averaging
39 cm. Using 10 DSI showed the lowest plant diameter with 36 cm
(Table 3-2).
Early marketable fruit weight and number were also affected by
foliar applications
and days of sprinkler irrigation. The highest early marketable
fruit weight and number
were observed in plots with 10 DSI and 7 DSI plus foliar
applications, averaging 14.8 t
ha-1 and 558,220 fruit ha-1. Using 7 DSI alone affected
negatively early marketable fruit
weight and number reducing yields to 11.6 t ha-1 and 464,187
fruit ha-1, which were the
lowest (Table 3-2). Based on field temperature sensors, crop
protectants decreased
plant canopy temperature between 1 and 4oC (Table 3-2). Total
water used during
transplant establishment in control plots was 5600 m3 ha-1,
whereas 3900 m3 ha-1 where
applied when crop protectants were utilized. Therefore, foliar
applications had an effect
on strawberry transplant establishment in reducing air
temperature around the crowns
and improving plant growth and early yield.
Transplant response to foliar applications might be related to
reduction in
environmental stress due to high temperatures. The benefits of
foliar applications of
kaolin clay, calcium carbonate, and aluminum silicate has been
reported in several
studies. Most of the research shows the effects of crop
protectant applications to reduce
fruit sunburn. Glenn et al. (2002 and 2003) conducted several
studies to report the
effects of kaolin clay application on apple trees, which
resulted in a decline on fruit
temperature, possibly due to increased stomatal conductance
associated to reduced
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32
leaf temperature and reflection of ultraviolet light (Glenn et
al., 2002 and 2003; Glenn
and Puterka, 2005). Jifon and Syvertsen (2003) reported
reduction in leaf temperature
and air vapor pressure in grapefruit (Citrus paradisi) trees.
Cantore et al. (2009)
observed a reduction in fruit temperature by 4oC and decreased
in stomatal
conductance and transpiration. Santos et al. (2012) conducted a
study using kaolin clay
during transplant establishment in strawberry where either 7 or
8 DSI followed by the
foliar crop protectant application resulted in 98% plant
survival and same early yield as
using 10 DSI. Even when a decrease in photosynthetic activity
has been reported while
using these products, application results more beneficial due to
an increase in yield
when fruit sunburn is reduced up to 95% (Cantore et al., 2009;
Glenn and Puterka,
2005).
Root-Dipped Crop Protectants Study
There was a treatment effect in plant diameter at 4 WAT. The
largest plant
diameter was observed in treatments with 7 DSI plus Supersorb F,
averaging 19 cm,
followed by 7 DSI plus Regalia® application and 10 DSI, which
did not differ between
each other (Table 3-3). Using 7 DSI alone decreased plant
diameter by 30% (Table 3-
3). Plant growth at 8 WAT was the same for all treatments
ranging from 33 to 37 cm. At
the end of the experiment, plants with root-dipped application
showed the widest plant
diameter averaging 41 cm, compared to 36 cm when sprinkler
irrigation was used alone
(Table 3-3).
Early marketable yield was affected by the combination of
root-dipped crop
protectants and sprinkler irrigation during transplant
establishment. The highest early
marketable fruit weight was reported in plots treated with
either 7 DSI plus the water-
absorbent polymer and the biofungicide or plots that received 10
DSI, averaging 10.6 t
-
33
ha-1. Using 7 DSI alone decreased early marketable fruit weight
to 8.9 t ha-1, which was
the lowest fruit weight value (Table 3-3). Similar effects were
observed on early
marketable fruit number. There was no difference among
root-dipped treatments and 7
DSI alone with 454,850 fruit ha-1. The lowest early marketable
fruit number was
observed in plots with 10 DSI. In conclusion, water-absorbent
polymer and the
biofungicide application resulted in the same plant growth and
early yield as using 10
DSI. Applying 7 DSI alone produced the lowest yield. This
reduction was due to
decrease in fruit size. Total water used during transplant
establishment in control plots
was 5600 m3 ha-1, whereas 3900 m3 ha-1 when the crop protectants
were applied.
When using Supersorb F, effects on plant growth and yield could
be due to
higher water retention around crowns, increasing its
availability for plant uptake. This is
a likely advantage for growers since sandy soils are known for
their low water holding
capacity. Moreover, response in plant biomass using root-dipped
crop protectants has
been described by several authors. Viero et al. (2002) used a
water-absorbent polymer
to establish eucalyptus (Eucalyptus grandis) seeds, resulting in
optimum plant survival
when using 2 L of water with 12 g of the product incorporated at
planting. Furthermore,
Orikiriza et al. (2009) reported an increase in dry shoot and
root biomass in eight of nine
tree species planted in pots with 0.2 and 0.4% (w w-1) of a
water-absorbent polymer.
Hodges et al. (2006) described an increase in fresh and dry
weights and leaf area index
in muskmelon at 3 weeks using a water-absorbent polymer for
establishment. The same
response was reported by Woodhouse and Johnson (1991) when
applications up to
0.5% (w w-1) in potted barley increased up to six-fold seedlings
dry weight. Additionally,
superabsorbent polymers are commonly used for soil remediation
in arid areas where
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34
water availability is major issue to establish new seedlings
(Ekebafe et al., 2011;
Wallace and Wallace, 1989; Viero et al., 2002).
Plant diameter and early yield was improved when using Regalia®,
probably to
the systemic action of the biofungicide, which promoted
photosynthetic activity and SAR
activation on the new transplants. Mode of action in seed growth
is not well known.
However, several studies suggest that it stimulates
photosynthetic activity, which is
possibly related to an increase in electron acceptors in
photosystem II. Moreover this
product is an SAR activator that promotes production of
phytoalexins and other phenolic
compounds with antifungal properties. Product application
resulted in enhancement of
photosynthetic activity in barley, causing an increase of grain
dry biomass and number
of productive stems (Karavaev et al., 2008). Moreover, research
conducted in bean
leaves (Vicia faba) reported stimulation of photosynthetic
activity (Glazunova et al.,
2009). Furthermore, ornamental plants such as Impatients
valleriana also showed
increase in dry shoot biomass when using the crop protectant
(Cochran et al., 2011).
Response in cotton seedlings was also observed with increased
emergence rate and
early growth (Su, 2011). As an SAR activator, biofungicide
application in cucumber
resulted in increased phytoalexins production and reduced
incidence of powdery mildew
(Daayf et al., 1997). Another study showed that biofungicide
application resulted in
reduction of disease incidence probably due to increase in
phenolic compounds related
to SAR activation (Wurms et al., 1999). Konstantinidou et al.
(2006) reported that
product application had a direct effect in conidial germination,
resulting in disease
reduction by 40% to 65% when used in tomato. Product
effectiveness is determined by
-
35
the rate of application and disease pressure, being more
effective as preventive than as
curative treatment (Konstantinidou et al., 2006).
In conclusion, foliar applications of kaolin clay, calcium
carbonate, and aluminum
silicate during transplant establishment help to decrease the
temperature around the
crown, promoting growth in strawberry transplants without
reduction in yield. Moreover,
root-dipped application resulted in same plant growth and early
marketable yield than
using 10 DSI, probably due to higher water retention around the
root area, and possible
plant growth improvement due to photosynthetic activity
stimulation and systemic
acquired resistance activation in transplants, promoting root
and shoot growth.
Implementation of these techniques for transplant establishment
means potential water
savings of about 1700 m3 ha-1 of water, which means that 6.7
million m3 of water in
Plant City area can be saved during the season.
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36
Table 3-1. Climate conditions from October 12 to October to
October 26 2012, in Balm, Fl.
Air to1 RelHum
avg. Total rain
SolRad avg. Wind speed
Days Avg. Min. Max.
Avg. Min. Max.
oC
% mm w m2
km h-1 12-Oct-12 22.4 16.9 29.0 76.0 0.0 218.7 7.1 2.1 18.4
13-Oct-12 22.8 17.1 29.0 77.0 0.0 202.5 8.8 2.2 25.3 14-Oct-12
24.0 19.1 30.2 81.0 0.0 188.2 7.3 1.9 25.4 15-Oct-12 24.3 20.6 30.4
86.0 0.0 175.9 5.2 0.3 21.3 16-Oct-12 23.1 17.7 29.6 83.0 0.0 180.6
4.1 0.1 13.1 17-Oct-12 21.6 18.0 27.0 90.0 0.0 90.7 3.1 0.0 9.3
18-Oct-12 23.7 18.3 31.0 86.0 0.0 199.8 4.1 0.0 15.8 19-Oct-12 23.4
16.4 30.0 84.0 0.0 193.0 3.8 0.0 14.4 20-Oct-12 22.3 14.5 28.7 70.0
0.0 211.2 5.6 0.1 17.4 21-Oct-12 19.3 12.4 27.1 68.0 0.0 214.8 6.5
0.8 18.2 22-Oct-12 21.5 16.0 28.7 76.0 0.0 181.6 9.0 2.2 23.4
23-Oct-12 21.5 16.7 27.2 82.0 0.0 119.6 7.8 1.9 20.9 24-Oct-12 23.4
18.2 29.5 80.0 0.0 180.8 9.9 2.2 27.5 25-Oct-12 23.0 20.3 27.9 87.0
0.0 96.9 8.9 2.0 25.8 26-Oct-12 24.0 21.2 29.2 75.0 0.0 155.0 12.8
3.8 31.0
1Air to = air temperature measured at 60 cm; SolRad= solar
radiation measured at 2 m; wind speed measured at 10 m; ReHum=
relative humidity; Avg.= average; Min.= minimum; Max.= maximum.
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37
Table 3-2. Effects of foliar crop protectants on plant diameter,
and total early fruit number and weight of ‘Strawberry Festival’
bare-root strawberry transplants, 2012-13. Balm, FL.
Treatments
Plant diameter Early yield1
4 WAT 8 WAT 12 WAT Fruit
weight Fruit
number Max. to2
cm
t ha-1 no.ha-1
oC
10 DSI (control) 16 33 b 36 b 15.6 a 554,120 a 34.5
7 DSI 16 36 ab 39 ab 11.6 b 464,187 b 34.5
7 DSI + Surround on the 8th day 15 37 a 40 ab 16.0 a 573,756 a
33.2
7 DSI + Screen Duo on the 8th day 17 37 a 39 ab 13.3 ab 523,245
ab 30.9
7 DSI + PurShade on the 8th day 17 36 ab 39 a 15.6 a 585,352 a
30.1
Significance (P≤0.05) NS * * * *
1Early yield= first 10 harvests; DSI = days of sprinkler
irrigation; WAT = weeks after transplanting. 2Max to = maximum
temperature 15 cm above plant canopies was measured at 2 pm. NS and
* = non-significant and significant at P
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38
Table 3-3. Effects of root-dipped crop protectants in plant
diameter and total early fruit number and weight of ‘Florida
Radiance’ strawberry transplants, 2012-13, Balm, FL.
Treatments
Plant diameter Early yield1
4 WAT 8 WAT 12 WAT Fruit weight Fruit number
cm
t ha-1 no. ha-1
10 DSI (control) 16 b 33 36 b 11.4 a 406,142 b
7 DSI 13 c 34 37 b 8.9 b 419,036 ab
Supersorb F dip + 7 DSI 19 a 37 41 a 10.1 ab 464,162 ab
Regalia® dip + 7 DSI 16 b 37 41 a 10.6 a 481,354 a
Significance (P≤0.05) * NS * * *
1Early yield: First 10 harvests; DSI = days of sprinkler
irrigation; WAT = weeks after transplanting. NS and * =
non-significant and significant at P
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39
CHAPTER 4
COMPARISON OF FREEZE PROTECTION METHODS FOR STRAWBERRY
PRODUCTION
Overview
Sprinkler irrigation is the most common method for freeze
protection in fruits and
vegetables. Water is used to provide heat energy to keep the
plant tissue near or above
0oC and prevent freezing injury. This activity is highly
inefficient due to the use of large
volumes of water, which may cause running off to drainage
canals, leaching nutrients
from root zones, lowering aquifer levels, and damage of green
and ripe fruit (Hochmuth
et al., 1986; Jackson and Parsons, 1994; Perry, 1998).
Sustainable ways to reduce
water volume during this stage without compromising crop yield
are needed. Most of the
alternatives for freeze protection are based on either adding or
keeping the heat around
the plant canopy (Perry, 1998).
Reduced-volume sprinklers, row covers, and crop protectants
could be
alternatives to sprinkler irrigation. In general, sprinklers
deliver more water than is
needed for crop growth (Fisher and Shortt, 2009). Reduced-volume
sprinklers might be
a suitable alternative to decrease water volumes used during
freeze protection. Row
covers are flexible blankets that help to decrease the influence
of wind, evaporative and
radiational cooling, and convection by keeping higher
temperature for longer periods of
time (Hochmuth et al., 1986). Crop protectants provide
protection by reducing
environmental stress. When used for freeze protection, they
prevent or reduce freezing
damage on plant tissues. The objective of this study was to
compare the effects of
reduced-volume sprinkler and non-irrigation alternatives for
freeze protection on water
savings, and strawberry growth, early and total marketable
yield.
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40
Materials and Methods
Field preparation was conducted the same as described in the
previous chapter.
‘Strawberry Festival’ bare-root transplants with three to five
leaves from nurseries in
Canada (Lareault Nursery, Lavaltrie, Quebec, Canada) were
planted on 12 and 16
October 2011 and 2012. Each plot consisted of four 9.1-m long
rows with 100 plants
and three replications. A non-treated buffer zone of 7.6-m long
at the end of each plot
was set to avoid water overlapping of treatments. Immediately
after transplanting,
sprinkler irrigation with 17 L m-1 sprinkler heads was turned on
at 8 am each morning for
8 h day-1 during the first 10 days to ensure plant
establishment.
Treatments consisted on: a) 17 L m-1 sprinkler heads (4.31 mm
nozzle; Rain Bird,
Azusa, CA), b) 13 L m-1 sprinkler heads (3.55 mm nozzle; Rain
Bird, Azusa, CA), c) light
row covers on the crop canopy (21 g m2; Gardener’s Suply
Company, Burlington, VT),
d) light row covers on 0.5-m high minitunnel hoops, e) heavy row
covers on the crop
canopy (31 g m2; Agribon row cover, Environmental Green
Products, Phoenix, OR), f)
heavy row covers on 0.5-m high minitunnel hoops, and g) crop
protectant polymer
(Desikote Max®, 40% di-1-p-menthene; Engage Agro, ON,
Canada).
Treatments were set in a randomized complete block design with
three
replications during both seasons. Sprinklers were set at 14.6-m
apart. Sprinklers were
turned on when air temperature at 1.2-m above the surface was
1oC and they were
turned off when the ice was completely melted. Row covers were
placed between 2 and
5 pm on the afternoon of the forecast freezing event, and they
were held using 2 kg
sand bags, and were removed once the freezing event ended. The
crop protectant was
applied at the rate of 5.1 L ha-1 at the same time when row
covers were placed to allow
the formation of the protective film.
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41
Leaf number and plant diameter were measured at 6, 12, and 20
WAT following
the same procedures as the previous chapter. The same parameters
were used for
harvest. Early and total marketable fruit weight and number were
collected from the first
10 and 24 harvests, respectively. Climate data from 2012-13
season were collected
from Florida Automated Weather Network (FAWN), equipment located
at the GCREC.
Temperatures between 15 and 25 cm above canopy level were
monitored for each
treatment with temperature loggers (HOBO data loggers, Onset
Corp., Bourne, MA).
Data were analyzed as described in the previous chapter.
Results and Discussion
2011-2012 Season
There was no significant season by treatments interaction,
however, data were
presented by season. Monthly climate conditions from October
2011 to March 2012 are
presented in Table 4-1. Minimum temperatures ranged from -2.9 to
6.4oC and maximum
temperatures between 28.6 to 32.1oC. About 57% of total rain
during the growing
season was recorded in October. Relative humidity was around 79%
for the whole
period. Solar radiation ranged between 134 to 217 w m2. FAWN
reported 5 freezing and
near freezing nights (≤ 1oC) on January 4th, January 5th,
January 15th, February 12th,
and February 13th, with minimum temperatures of -1.3, -2.9,
-1.9, -0.8, and -0.4oC,
respectively. Calm wind conditions were reported during most
part of the nights.
Minimum temperature directly above crop canopy in covered plots
was between
4 and 7oC higher than the outside air regardless the cover
weight and the use of hoops
(Table 4-4). Moreover, no water for freeze protection was needed
in plots using row
covers and crop protectant, compared with 1135 and 874 m3 ha-1
of applied water when
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42
using 17.5 and 13 L m-1 sprinklers, respectively. Water savings
when using 13 L m-1
sprinklers was 23% compared with the control (Table 4-2).
Plant number and plant diameter were not affected by treatment
application
(Table 4-1). There was no difference in plant diameter at 6 WAT
with 41 cm. Plant
growth remained the same at 16 and 20 WAT averaging 43 cm in
both sampling dates
(Table 4-1). Treatment application affected early marketable
fruit and number. The
highest early marketable fruit weight and number was obtained in
plots using light row
covers with hoops, heavy row covers without hoops, and the crop
protectant plots,
averaging 5.2 t ha-1 and 226,159 fruit ha-1 (Table 4-2). The
lowest yield was reported
when using 17 L m-1 sprinklers to protect the crop with 3.7 t
ha-1 and 220,047 fruit ha-1
(Table 4-3). These values were about 30% lower than those
obtained in plots where row
covers and the crop protectant were applied. Similar tendency
was observed in total
marketable fruit weight and number. Using non-irrigation
alternatives for freeze
protection resulted in the highest total marketable fruit
weights at the end of the
strawberry season with 23 t ha-1 and 905,709 fruit ha-1 (Table
4-3). Lowest yield was
observed in plots using sprinkler irrigation, regardless of
water volume with 17.3 t ha-1
and 889,645 fruit ha-1 (Table 4-2). These values represent
approximately 25% more
yield when using the alternative methods, regardless of the
output volume used for
freeze protection (Table 4-2).
The first six harvests after January 5th and February 13th, 2012
were analyzed to
identify treatment effects after the freezing nights. Crop
protection was affected by
treatment application. For the first event, the highest yield
was recorded in plots using
row covers and the crop protectant, averaging 2.96 t ha-1 and
157,758 fruit ha-1. The
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43
lowest yield was registered when using 17 L min-1 sprinklers for
freeze protection (Table
4-4). Similar results were found for the second freezing event,
non-irrigation treatments
had the highest yield with 12.67 t ha-1 and 580,658 fruit ha-1.
Reduction in yield using
sprinkler irrigation was 28% (Table 4-4).
2012-2013 Season
The same treatments were repeated during this season. Monthly
climate
conditions from October 2012 to March 2013 are presented in
Table 4-5. Average
temperature during this period was 18.8oC with minimum
temperatures ranging from -
0.2 to 6.4oC and maximum temperatures between 28.1 to 33.2oC.
About 40% of total
rain during the growing season was recorded in October, followed
by December with
30% of total rain. Relative humidity was around 78% for the
whole period. Solar
radiation ranged between 126 to 206 w m2. During this season 3
near freezing nights (≤
1oC) were registered on February 4th, February 18th, and March
4th which required
turning on the sprinkler irrigation. Minimum temperature was
1.5oC, 1oC, and 0.2oC.
Calm wind conditions were observed during these nights.
Minimum temperature directly above the crop canopy in covered
plots was
between 2 and 8oC higher than the outside air regardless the
cover weight and the use
of hoops (Table 4-7). Regarding water volumes needed for freeze
protection, no water
was needed in plots with row covers and crop protectant, whereas
approximately 681
m3 ha-1 were used in the control plots (17.5 L m-1 sprinklers)
and 525 m3 ha-1 when
using 13 L m-1 sprinklers to protect the crop. Water savings
using 13 L m-1 was 23%
compared with the control (Table 4-7).
Plant number was the same through the growing season (Table
4-6). Plant
diameter was not affected by treatment application, there was no
difference among
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44
treatments at 6 WAT averaging 36 cm, plant growth at 14 cm and
20 WAT remained the
same averaging 42 cm (Table 4-6). There was no difference among
the treatments on
early marketable fruit weight and number averaging 8.3 t ha-1
and 425,052 fruit ha-1
(Table 4-7). At the end of the season there was no difference in
total marketable fruit
weight and number among treatments with 27.19 t ha-1 and
1,363,876 fruit ha-1 in
average (Table 4-7). All treatments provided the same protection
as the 17 L m-1
sprinklers (control) during these mild winter conditions. There
was no reduction in yield
due to the use of sprinkler irrigation to protect the crop.
Similar results were observed
when analyzing the first six harvests after each near freezing
night. All treatments
provided the same crop protection. There was no difference in
marketable fruit weight
during the three freezing nights with 11.74, 7.31, and 7.4 t
ha-1, respectively. Values for
fruit number were 601,323, 414,307, and 413,202 fruit ha-1,
respectively.
Results from this trial are similar to the ones found by several
authors. Locascio
et al. (1967) achieved protection in strawberries at minimum
temperature of -4.4 when
using 17 L m-1 and 13 L m-1 sprinklers, however when wind speed
was higher than 8 km
h-1 or temperature was lower than -4.4oC, reduced-volume
sprinkler provided limited
protection. Poling et al., (1991) found that light row covers
and heavy row covers
protected strawberries subjected to -4.1oC, no difference was
observed in yields
compared with 17 L m-1 sprinkler irrigation. Hochmuth et al.,
(1993) reported that heavy
row covers from 30 to 50 g cm2 protected strawberry plants at a
minimum temperature
of -4.4oC, the same protection was measured when using sprinkler
irrigation at 17 L m-1.
Santos et al. (2011) found that row covers provided 7oC of
protection when temperature
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45
in the outside air was -6oC, reduction in yield was observed
when sprinkler irrigation
was used to protect the crop.
Crop protectant application provided freeze protection at -3oC
with calm wind
conditions during the recorded freezing nights. While few
authors report similar results
to the ones found in this experiment, results may be explained
by understanding how
the critical temperature varies depending on plant and organ
tissue. Moreover, product
formulation differs among these types of products, and
represents a main factor when
choosing a crop protectant for specific areas. Hare (1995) found
that application of a
cryoprotectant and biodegradable detergent provided protection
to tall fescue seedlings
at a minimum temperature of -2oC, increasing yield by 22% in
plots with the crop
protectant. Gardea et al. (1993) reported a reduction in leaf
disks injury in grapes by
25% when two types of cryoprotectants were used at a minimum
temperature of -2oC.
In contrast to these results, research conducted in pepper and
tomato transplants
found no protection at -1 and -3.5oC when using two types of
cryoprotectant (Aoun et
al., 1993; Perry et al., 1992). Burns (1970) found similar
results in citrus were 12
antitranspirants were tested, no protection was provided at a
minimum temperature of -
4 and -5oC. In conclusion, when using freeze protection
alternatives, data showed that
alternatives performed the same as sprinkler irrigation in the
evaluated conditions.
These alternatives have to be chosen according to the conditions
from each place.
During the tested conditions all alternatives protect the crop.
However, during hard
freezing nights mixing these techniques might work better than
just choosing one
technique. If reduced-volume sprinklers are choose it would mean
about 129 m3 ha-1
per freezing night in water savings, which represents about
516,000 m3 in the Plant City
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46
area. Moreover, if non-water alternatives are adopted it would
mean 561 m3 ha-1 per
freezing night and approximately 2.2 million m3 of water savings
in the Plant City area.
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47
Table 4-1. Environmental conditions from October 2011 to March
2012. Balm, Florida.
Month Air to Total rain RelHum
SolRad avg.
Avg. Min. Max.
oC
mm % w m2
October 21.3 8.8 30.3 109.2 79 169.82
November 19.1 6.9 29.9 15.0 81 152.58
December 17.6 1.9 28.7 7.6 82 134.64
January 15.2 -2.9 28.6 22.9 75 155.34
February 18.7 -0.8 30.5 14.2 79 155.24
March 21.1 1.7 32.1 21.3 75 217.94
1Air to = air temperature measured at 60 cm; SolRad= solar
radiation measured at 2 m; wind speed measured at 10 m; RelHum=
relative humidity; Avg.= average; Min.= minimum; Max.= maximum.
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48
Table 4-2. Effects of freeze protection methods on plant number
and plant growth,
Balm, FL, 2011-12.
Plant number Plant diameter
Treatments
6 WAT 12 WAT 20 WAT
no. ha-1
cm
17 L m-1 sprinklers (control) 41259 40 44 42
13 L m-1 sprinklers 41259 42 44 42
Light row cover on canopy 40972 44 44 41
Light row cover with hoops 41832 42 43 42
Heavy row cover on canopy 42405 41 44 41
Heavy row cover with hoops 41832 40 42 41
Desikote Max® 42978 40 42 43
Significance (P≤0.05) NS NS NS NS
WAT = weeks after transplanting. NS and * = non-significant and
significant (P
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49
Table 4-3. Effects of freeze protection methods on the minimum
seasonal air temperatures in each treatment, water use, and early
and total marketable fruit weight and number, Balm, FL,
2011-12.
Early yield1 Total yield2
Water use
Treatments Fruit no. Fruit
weight Fruit no. Fruit
weight
no. ha-1 t ha-1 no. ha-1 t ha-1 m3 ha-1
17 L m-1 sprinklers (control) 220047 d 3.68 c 889645 c 17.83 b
1135
13 L m-1 sprinklers 259587 bcd 4.32 bc 863858 c 16.74 b 874
Light row cover on canopy 248397 bcd 4.47 bc 1101140 ab 23.31 a
0
Light row cover with hoops 318099 a 5.50 a 1137313 ab 22.82 a
0
Heavy row cover on canopy 295066 abc 4.98 ab 1199699 a 23.86 a
0
Heavy row cover with hoops 243150 cd 4.17 bc 1049291 b 21.76 a
0
Desikote Max® 306218 ab 5.06 ab 1170255 ab 23.02 a 0
Significance (P < 0.05) * * * *
Early yield= first 10 harvests; Total yield= 30 harvests. NS and
* = non-significant and significant (P
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50
Table 4-4. Effect of freeze protection methods on the minimum
seasonal temperature for each treatment, and the first six harvests
after a freezing event on strawberry marketable fruit weight and
number, Balm, FL, 2011-12.
Jan. 4th, 2012 Feb. 13th, 2012
Treatments Fruit
number Weight number Min. To1 Fruit number
Weight number Min.To
no. ha-1 t ha-1 oC no. ha-1 t ha-1 oC
17 L m-1 sprinklers (control) 126930 c 2.19 c -2.9 476196 bc
10.20 bc -0.4 13 L m-1 sprinklers 1432627 bc 2.59 bc -2.9 399411 c
8.27 c -0.4
Light row cover on canopy 153861 bc 2.88 ab 0.6 597675 a 13.26 a
4.6 Light row cover with hoops 177928 a 3.28 a 0.6 557570 ab 12.2
ab 5.2 Heavy row cover on canopy 153861 abc 2.91 ab 0.6 625181 a
13.41 a 7.0 Heavy row cover with hoops 142402 bc 2.69 abc 0.6
546104 ab 12.2 ab 7.0
Desikote Max® 160737 ab 3.03 ab -2.9 576764 a 12.47 a -0.4
Significance (P < 0.05) NS * * *
1Min. To = minimum air temperatures in the row covered plots
were taken 15 cm above plant canopies, whereas the temperatures in
the sprinkler-treated plots were the air temperatures without
irrigation. NS and * = non-significant and significant (P
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51
Table 4-5. Environmental conditions from October 2012 to March
2013. Balm, Florida.
Month Air to Total rain RelHum
SolRad avg.
Avg. Min. Max.
oC
mm % W m2
October 22.5 6.4 33.2 83.8 82 170.2
November 16.7 3.6 28.5 2.5 79 144.4
December 17.0 0.6 28.1 63.5 81 126.3
January 17.9 2.9 29.5 7.6 81 130.4
February 17.1 0.1 30.4 25.4 77 167.6
March 15.1 -0.2 29.8 25.4 68 206
1Air to = air temperature measured at 60 cm; SolRad= solar
radiation measured at 2 m; wind speed measured at 10 m; ReHum=
relative humidity; Avg.= average; Min.= minimum; Max.= maximum.
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52
Table 4-6. Effects of freeze protection methods on plant number
and plant growth, Balm, FL, 2012-13.
Plant number Plant diameter
Treatments
4 WAT 8 WAT 12 WAT
no. ha-1
cm
17 L m-1 sprinklers (control) 42978 35 42 43 13 L m-1 sprinklers
42978 37 43 42
Light row cover on canopy 42978 35 42 42 Light row cover with
hoops 42978 34 39 42 Heavy row cover on canopy 42978 34 43 41 Heavy
row cover with hoops 42978 36 43 42
Desikote Max® 42978 37 41 41 Significance (P≤0.05) NS NS NS
NS
WAT = weeks after transplanting. NS and * = non-significant and
significant (P
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53
Table 4-7. Effects of freeze protection methods on the minimum
seasonal air temperatures in each treatment, water use, and early
and total marketable fruit weight and number, Balm, FL,
2012-13.
Early yield Total yield
Water use
Treatments Fruit no.
Fruit weight Fruit no.
Fruit weight
no. ha-1 t ha-1 no. ha-1 t ha-1 m3 ha-1
17 L m-1 sprinklers (control) 437516 8.70 1304812 25.77 681
13 L m-1 sprinklers 554416 8.71 1518843 28.12 525
Light row cover on canopy 357577 7.23 1297936 26.68 0
Light row cover with hoops 399695 8.17 1476724 29.69 0
Heavy row cover on canopy 400555 8.08 1326301 26.27 0
Heavy row cover with hoops 379066 7.83 1329739 27.49 0
Desikote Max® 405712 8.17 1292778 26.30 Significance (P≤0.05) NS
NS NS NS NS
NS and * = non-significant and significant (P
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54
Table 4-8. Effect of freeze protection methods on the minimum
seasonal temperature for each treatment, and the first six harvests
after a freezing event on strawberry marketable fruit weight and
number, Balm, FL, 2012-13.
Feb. 4th, 2013 Feb. 18th, 2013 Mar. 4th, 2013
Treatments Fruit
number Weight number Min. To1
Fruit number
Weight number Min. To
Fruit number
Weight number Min. To
no. ha-1 t ha-1 oC no. ha-1 t ha-1 oC no. ha-1 t ha-1 oC
17 L m-1 sprinklers (control) 552697 10.77 1.5 444392.52 7.94
1.0 481353.6 8.77 0.2
13 L m-1 sprinklers 598254 11.79 1.5 490809 8.98 1.0 513157 9.51
0.2
Light row cover on canopy 600832 11.96 6.5 379926 6.77 2.7
377347 7.01 3.4
Light row cover with hoops 669597 12.74 6.7 406572 7.05 1.3
384223 7.00 2.7
Heavy row cover on canopy 591377 11.30 9.9 393678 6.40 7.5
381645 6.30 6.4
Heavy row cover with hoops 635215 12.54 7.3 402274 7.04 3.2
379926 6.51 3.6
Desikote Max® 561293 11.07 1.5 382504 6.95 1.0 374768 6.90
0.2
Significance (P≤0.05) NS NS NS NS NS NS
NS and * = non-significant and significant (P
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55
CHAPTER 5 CONCLUSIONS
Based on the results from the foliar crop protectant trial,
applications during
transplant establishment help to decrease the temperature around
the crown, promoting
growth in strawberry transplants. No reduction in yield was
observed, regardless of the
material. Furthermore, root-dipped application treatments
resulted in the highest plant
diameter and the same yield as using 10 DSI, this result was
probably due to higher
water retention around the root area. Biofungicide application
also helped to improve
transplant establishment possibly due to stimulation of
photosynthetic activity and
systemic acquired resistance activation in transplants,
promoting root and shoot growth.
All crop protectants combined with 7 DSI resulted in the same
early yield as using 10
DSI. When using 7 DSI alone, plant growth was not affected at
the beginning of the
season. However, later effects were observed on early yield,
where reduction in fruit
weight and number was found.
It is estimated that about 9.4 L h-1 of diesel fuel are needed
to pump water to
irrigate 1 ha. If sprinkler irrigation is used for 8 h day-1,
cost of pumping will be about
$240 for three days. Pumping costs are comparable with costs
when using crop
protectant applications. Growers will be able to choose among
the alternatives which
one is the most practical and less costly depending on
availability of material for their
area. Cost of foliar crop protectants varies, most of them are
between $25 and $90 per
ha, plus cost of application, which is around $85 per ha
including labor and tractor fuel.
Foliar applications can be done with the same machinery used for
pesticide application.
On the other hand, cost of root-dipped materials is between $50
and $200 per ha.
Implementation of these techniques will have a direct impact on
water savings with
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56
approximately 1700 m3 ha-1 of water, which means that 6.7
million m3 of water in Plant
City area can be potentially saved during the season.
When using freeze protection alternatives, data showed that
alternatives
performed the same as sprinkler irrigation in the evaluated
conditions. Reduced-volume
sprinklers provided freeze protection to strawberries. Row
covers protected the crop
regardless of material thickness during both seasons. However
when analyzing the first
six harvests after a freezing event during 2011-12 season,
significant differences where
found. Moreover, the highest yield was found in plots where row
covers and crop
protectants were used. During 2011-12 season higher early and
total yields were
reported in plots where row covers were used. Use of hoops when
putting row covers
had no major effect in degrees of protection and yields.
However, in northern areas with
severe freezing events, use of hoops might have an influence
increasing air mass and
keeping temperature for longer inside the row cover. When using
the crop protectant,
protection was accomplished at minimum temperature of -3oC,
where wind speeds
below 3 km h-1 prevailed during most of the night. However, when
using this type of
product, growers have to be aware about degrees of protection
because at lower
temperature, crop protection might be compromised.
During the strawberry season, about six to eight freezing nights
may occur. Cost
of these techniques varies. About 9.4 L h-1 of diesel fuel are
needed to pump water to
irrigate 1 ha, which means $80 ha-1 cost of application per
night, translated to $640 ha-1
in eight freezing nights. When using the reduced-volume
sprinklers, the same plumbing
can be used but it is required to change the sprinkler nozzle
size, which will cost around
$250 (approximately $5 per nozzle). Cost of row covers depends
on thickness of the
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57
material and ranges from $2500 and $3300 per ha, plus labor cost
in installing and
removing the covers, which is around $370 per ha. Duration of
the material is between 2
and 4 years depending on use and carefulness. One advantage of
using row covers is
that water damage in fruits was reduced, increasing marketable
yield. Approximate cost
when using the crop protectant polymer is around $80 per ha per
night plus cost of
application which is around $85 per ha including labor and
tractor fuel. Future research
is needed to determine what the effect of applying once every
freezing event would be
since two consecutive freezing nights is most common. These
alternatives have to be
chosen according to the conditions from each place. In mild
conditions all alternatives
protect the crop. However, during hard conditions with lower
temperatures, mixing these
techniques could work better than just choosing one technique.
If reduced-volume
sprinklers are chosen it would mean about 129 m3 ha-1 per
freezing night in water
savings, which represents about 516000 m3 in the Plant City
area. Moreover, if non-
water alternativ