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                              TECHNICAL PAPER #16
                          UNDERSTANDING CITRUS FRUIT
                              Dr. Murray Gaskins
                              Technical Reviewers
                              Dr. C. W. Campbell
                              William J. Wiltbank
                                 Published By
                       1600 Wilson Boulevard, Suite 500
                         Arlington, Virginia 22209 USA
                    Tel:  703/276-1800 .   Fax:  703/243-1865
                      Understanding Citrus Fruit Growing
                              ISBN: 0-86619-216-6
                  [C]1984, Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in Technical
Assistance to provide an introduction to specific state-of-the-art
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their situations.
They are not intended to provide construction or implementation
details.  People are urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on a purely
voluntary basis.  Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.  VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling typesetting
and layout, and Margaret Crouch as project manager.
Dr. Murray Gaskins, the author of this paper, has been a VITA
Volunteer for 10 years.  He is an expert in crop physiology,
tropical crops, and plant growth substances.   Dr. Gaskins is a
plant physiologist with the U.S. Department of Agriculture in
Gainesville, Florida, and has performed consultancies dealing
with tropical crops for the USDA in Puerto Rico and various South
American countries.  The reviewers of the paper also have experience
with tropical fruits.  Dr. C.W. Campbell is a professor with
the Tropical Research and Education Center of the University of
Florida at Homestead.  William J. Wiltbank is a professor with the
Fruit Crops Department of the Institute of Food and Agricultural
Sciences of the University of Florida at Gainesville.
VITA is a private, nonprofit organization that supports people
working on technical problems in developing countries.   VITA offers
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to their
situations.  VITA maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster of
volunteer technical consultants; manages long-term field projects;
and publishes a variety of technical manuals and papers.
                       by VITA Volunteer Murray Gaskins
Citrus fruits can be useful both as home-produced sources of very
important nutrients, and as a valuable cash crop.   Well-established
trees grow satisfactorily in favorable environments even
when given little care.  However, in most environments young
trees die if neglected, and mature trees grow and produce well
only when cultivated carefully.   With proper management the trees
grow and produce fruit indefinitely.   Therefore, they should be
treated as a lifetime investment deserving constant care.
Citrus production on a small scale can be profitable for the
small landowner, who may sell fruit in small quantities directly
to neighboring consumers or in wholesale quantities to vendors
who will resell to consumers.   Initial investments made in
plants, cultivating tools and labor will not be returned for
several years, and the new fruit grower should be certain of the
commitment to the enterprise before it is begun.
Citrus fruits originated in Asia and were dispersed early in
small history.  Cultivated forms are mentioned in records from
many ancient cultures.  The sweet orange is believed to have developed
in southern China, but has been cultivated for centuries
in many locations throughout the tropics and subtropics.
Horticultural production of citrus, perhaps combined with other
fruit and vegetable crops, can be started on a land area of 1/2
hectare or less.  Since the well-managed orchard becomes increasingly
valuable as time passes, it is important to plan carefully
at the beginning for possible expansion and improvement.  In many
areas security of the planting is important.   Theft of fruit
often is a very serious problem, and may be impossible to prevent
if trees are planted in isolated places.
Source:  J. Soule and F. P. Lawrence, How to Grow Your Own Citrus
         Trees, Circular 339 (Gainesville, Florida:  Florida Cooperative
         Extension Service, University of Florida, 1973),
         p. 1.
Effective management of a small citrus planting, one hectare or
less in size, may require a weekly average labor input of 5-15
hours, depending on skill of the people involved, availability of
labor-saving equipment, terrain, and local pest problems.  Often a
small family can provide all the labor required.   It is important
to recognize, however, that the labor requirements depend on the
season, and are concentrated in the summer rainy season.
Usually small horticultural efforts are best seen as family
enterprises to which all can contribute and from which all can
benefit.  It is often not necessary to hire outside labor.  Nevertheless,
the cash investment is substantial before fruit trees
become self-supporting.  Major costs aside from the land include
cost of the trees and costs of fertilizer and pest control equipment
and supplies.  It is sometimes possible to grow vegetables
or other crops by interplanting among the small trees, and generate
income that will offset the cost of establishing the fruit
tree planting.  Actual costs and returns will vary significantly
among locations, and can best be estimated by analyzing data
collected locally.  It is possible in some countries to obtain
help in planning small farming businesses from the Ministry of
Agriculture and from firms which sell fertilizers and other
agricultural supplies.  All available sources of information and
assistance such as these should be located and used well in
advance of actually beginning the work of planting fruit trees.
Citrus plants grow reasonably well in a variety of soils and
weather conditions.  Their limited ability to withstand subfreezing
temperatures is well known.  Very wet soils, dense shade,
high winds and extreme drought also are particularly detrimental.
However, the widespread occurrence of healthy citrus trees
throughout much of the tropics and subtropics shows clearly that
they are more flexible in adaptation than are many other fruit
bearing species.
Citrus trees are best adapted to subtropical climates with clearly
defined cool-dry and warm-wet seasons.   The centers of commercial
cultivation are found in such regions.   However, tropical
areas can be satisfactory if proper types of fruit trees are
selected.  Most oranges and mandarins are suited only to altitudes
well above sea level.  Lemons, limes, grapefruit, and pomelos, on
the other hand, can be grown satisfactorily in low-elevation
moist tropical areas.
Tolerance of temperature extremes is affected by weather conditions
preceding exposure.  Dormant plants, "hardened" from exposure
to cool, dry, winter weather conditions become capable of
withstanding low temperatures which would kill or severely injure
them if they were actively growing. Dormant orange trees (Citrus
sinensis) often withstand freezing temperatures of about -4[degrees]C
without injury.  Areas where winter temperatures fall below this
point should be considered marginal for growing oranges.  Lime
trees (C. aurantium) are considerably less hardy than oranges and
may be injured by temperatures little below the freezing point.
The lemon (C. limon) is intermediate in cold hardiness.   Some of
the tangerines withstand low temperatures better than oranges.
In all cases, plants are better able to withstand temperature
extremes, whether high or low, when they have been conditioned or
hardened by weather that temporarily suppresses growth.
Citrus trees withstand high temperatures quite well if they are
not deprived of water.  However, high temperatures, particularly
if accompanied by dry winds, increase water requirements.  In
locations where summers are very hot and dry, cultivation is
practical only if the trees can be irrigated.
Citrus trees grow well in a variety of soils.   They are not
adapted to extremely wet soils, and should not be planted in
areas where water accumulates on the surface.   Heavy clay soils
are not ideal, but may be acceptable if well drained.   Many of
the heavier soils of the tropics shrink and crack as they lose
water in the dry season, and the cracks may injure tree roots.
Such injury can be avoided if sufficient water is available to
keep the soil moist throughout the dry season.
Sandy soils are satisfactory, provided trees are able to obtain
sufficient water.  Because small trees often suffer from lack of
water in sandy soils, they may require irrigation.   Mature trees
develop deep root systems and, if properly fertilized, usually
grow well in sandy soils without irrigation except where rainfall
is very low or irregular.
Sweet oranges (e.g., Valencia, Pineapple, Navel), lemons, and

ucf1x2.gif (437x437)

limes may be most suitable for cultivation because they are well
known in most places.  Locally named varieties of these are found
in many markets and are readily recognized and accepted by consumers.
It is generally best to study carefully the local varieties,
and to use similar ones in new plantings.   Unusual varieties
which may be worth planting for home use, may be difficult
to sell.
The performance of a new planting can be predicted with some
assurance if propagated from local trees which perform well.
Often an experiment station or nursery can be found which can
supply locally adapted varieties ready for planting.   When this
is not the case, trees can be propagated as described below.
Citrus fruit trees can be grown to maturity from seeds but such
plants have several disadvantages.   They are extremely slow to
commence fruit production, they have an undesirable growth habit,
and in some instances they produce fruits highly variable in
quality and other characteristics.   These and other disadvantages
are avoided by use of "vegetatively" propagated plants.  Such
plants are produced by "budding" or "grafting" vegetative material
(the scion) from a desirable variety to a suitable rootstock.
The latter is a plant grown from a seed and is usually one
to two years old when used for budding.
          Figure 2.  Collecting Branches Suitable for Budding

ucf2x5.gif (600x600)

          A.   Wood of the second flush from the end of the branch is
              suitable for budding.
          B.   Budwood should be rounded with plump buds (right), not
              angular (left); leaves are trimmed to 1 centimeter
              stubs as budwood is cut from the tree.
Source:  J. Soule and F.P. Lawrence, How To Grow Your Own Citrus
         Trees, Circular 339 (Gainesville, Florida:  Florida Cooperative
         Extension Service, University of Florida, 1973),
         p. 5.
Another easy method of propagating citrus trees is by shield
budding, in which the stock incision is made in the form of the
letter "T." This operation is shown in Figure 3.

ucf3x6.gif (600x600)

          Figure 3.  Steps in Shield Budding
          A.   The stock is cleaned and trimmed-up to provide working
          B.   The incision is made on the stock and the bark flaps
              pried up.
          C.   The shield or bud is cut from the budwood.  (Note that
              the underside is flat and the sliver of wood is left in
          D.   The shield is inserted and wrapped securely.
Source:  J. Soule and F.P. Lawrence, How to Grow Your Own Citrus
         Trees, Circular 339 (Gainesville, Florida:  Florida Cooperative
         Extension Service, University of Florida, 1973),
         p. 6.
              Figure 4.  Methods of Forcing a Bud to Sprout

ucf4x7.gif (600x600)

          A.   Lopping--The stock is cut part-way through and the top
              laid over.
          B.   Topping--The stock top is removed in one operation
              (left) or two (right).
          C.   Bending--the stock top is bent over and tied down.
              (Note that the scion must be inserted much higher than
              is usual practice in Florida.
Source:  J. Soule and F. P. Lawrence, How to Grow Your Own Citrus
         Trees, Circular 339 (Gainesville, Florida:  Florida Cooperative
         Extension Service, University of Florida, 1973),
         p. 8.
                  Figure 5.  Developing the Framework

ucf5x8.gif (600x600)

          A.   The stock is cut on a slant just above the scion.
          B.   The young tree is staked and tied with soft twine at
              intervals of 15 to 45 centimeters.
          C.   The tree is headed at a height of 45 centimeters.
          D.   Four to six well-spaced branches are allowed to grow;
              all sprouts below these are removed as they appear.
Source:  J. Soule and F.P. Lawrence, How to Grow Your Own Citrus
         Trees, Circular 339 (Gainesville, Florida:  Florida Cooperative
         Extension Service, University of Florida, 1973),
         p. 9.
Success or failure in cultivation of fruit crops often depends on
use of suitable rootstocks, because these determine the adaptation
of the mature plants to different soils, their tolerance of
certain diseases, and many other important characteristics.  It
is likely that if various rootstock plants are available, expert
local advice about their selection may be available also.  It is
important to make use of local knowledge about matters such as
this because it is difficult and expensive to correct mistakes
after trees have been planted.
The varieties known as sour orange (C. aurantium), rough lemon
(C. jambhiri), and Cleopatra orange (C. reshni) probably have
been most used in the past.  Each has its peculiar advantages and
certain weaknesses.  It is likely that if various rootstock
plants are available, expert local advice about their use will be
available also.  This should be sought and used, since proper selection
of a rootstock requires knowledge of local conditions.
The scion to be propagated should be known to grow and fruit well
locally.  If fruit is to be sold local consumer preferences
should be considered.  Where citrus production is commercially
important, great care is exercised to develop and maintain disease-free
plants from which new trees can be propagated.   Several
virus diseases attack citrus, and when present in the scion
these persist in the new plant.   In the absence of propagating
material known to be free of such diseases, the best procedure is
to select scion material from mature trees which have grown well
and fruited regularly for several years, and which show no deformed,
mottled or small leaves, dead limbs, bark abnormalities,
or other disease symptoms.
Citrus trees are usually grown in the nursery for one year after
budding, and then transplanted to their permanent location.  In
the tropics trees are usually transplanted at the beginning of
the rainy season.  If they are given good care trees can be
transplanted at any time, but trees moved at the beginning of the
growing season, as summer rains begin, have a distinct advantage
over those moved during the dry season or late in the rainy
season after they have begun growth.   Trees are usually planted at
six to eight meter intervals in rows spaced eight to ten meters
apart.  Local customs vary considerably for a variety of reasons.
Frequently annual crops or other tree crops such as coffee are
grown among the immature citrus trees.   This is advantageous for
the farmer, and need not interfere appreciably with growth of the
citrus trees if they are managed carefully.   Intercropping increases
efficiency of land use, and helps to defray costs of
bringing a citrus planting into production.   It is important to
promote growth of the young trees, however, and to avoid their
neglect as other crops are cultivated.
In heavy and poorly drained soils where surface water may accumulate
citrus trees may be planted so that they are elevated 5 to
10 centimeters above the surrounding area.   On hillsides and on
all well drained soils where flooding does not occur, the trees
are planted so that in the field they will be at the same level
as in the nursery row from which they were moved.   Except in arid
climates, the planting sites should not be excavated to form
depressions.  for the purpose of retaining water around the young
trees.  While it is desirable in some instances to maintain a
basin around newly transplanted trees, so that irrigation water
will be used efficiently, this should be done after the tree is
planted as described above, by throwing up around the tree a low
ring of soil to form a circle one to two meters in diameter.
Fertilizer recommendations should be obtained if possible from
nearby experiment stations.  These can serve as guides to indicate
ratios of major nutrient elements, and help to avoid excessive
use of those not needed.  The mineral requirements of citrus
trees are not unusual, and locally recommended fertilization
schedules which support good growth of other evergreen plants can
be used with confidence.
The fertilizer applied to commercially grown citrus trees on the
deep sand soils of Florida in their first year after transplanting,
would be equivalent to about 2 kilograms per tree of a
6-6-6 formula (N-[P.sub.2] [O.sub.5]-[K.sub.2] O).   In the second year this would be
increased to 4-5 kilograms per tree and in the third to 8-12
In all cases these quantities would be divided among four or-five
applications.  These high rates of fertilization are economically
practical in Florida but would not be in many circumstances.  At
the opposite extreme, plantings are sometimes established without
use of any mineral fertilizer.   The most satisfactory approach to
fertilization will be established by local conditions which determine
availability, cost, response of the plants, and market
demand for the fruit.
It is common in commercial citrus production to regulate fertilization
rates of mature trees in accordance with their estimated
or actual fruit producing capability.   This may not be known when
citrus is a new or relatively unproven crop.   In most instances,
useful information can be obtained from local experiment stations.
As noted above, when such information is not available,
recommendations pertaining to other evergreen tree crops may be
used as a general guide.
It is particularly important to determine soil characteristics
that affect the ability of plants to absorb needed mineral elements.
Soils vary widely not only in content of various minerals
but also in acidity, organic matter content, water holding capacity
and other characteristics which affect nutrient absorption.
The availability of certain minerals which may be used only in
minute quantities may profoundly affect plant growth.   A large,
heavily bearing citrus tree may actually need in a year only a
few grams of iron, the amount present in a small nail, but if
this quantity is not absorbed, its growth will be severely impaired.
This often occurs in the case of trees growing in alkaline
(high pH) soils.  Where trees are unable to obtain certain
mineral elements from the soil, it becomes necessary to supply
these by spraying the trees with solutions containing the needed
minerals.  In regions where this is necessary, local experiment
stations usually can give advice about preparation and use of
such sprays.
Nitrogen is the mineral element required in greatest quantity.
The actual amount of elemental nitrogen (N) contained in 100
kilograms of citrus fruit is less than 500 grams, but trees
growing in poor soil and producing this quantity of fruit might
require 1 kilogram or more of N (about 5 kilograms of ammonium
nitrate), applied as fertilizer each season.   Other mineral elements
are required in smaller quantities, but are of equal importance.
It is wasteful and often entirely useless to apply fertilizers
without knowledge of the local soil properties and plant
performance.  In many parts of the tropics liberal use of phosphate
fertilizers is necessary to maintain satisfactory plant
growth.  Plants actually use only small quantities of phosphorus,
but their ability to extract it from some soils is limited and
for this reason fertilizers containing large amounts of phosphorus-bearing
minerals must be used.
Citrus plants are attacked by a variety of insects and disease
organisms.  Aphids, mites, and scales are usually the most injurious
insect pests.  In some localities fungus diseases such as
scab and melanose injure leaves and fruit of certain varieties.
Fruit flies destroy enormous amounts of fruit.   In the tropics,
fruit-piercing moths and leaf-cutting ants can be very destructive.
Pink disease and greasy spot are important diseases in both
tropical and subtropical areas.  Spray treatments can be used to
control these and other disease and insect problems.   Some of the
spray materials are very dangerous to humans if used improperly,
and it is imperative that these materials be used with great
care,  Guidelines provided by responsible local officials should
be strictly observed.
Citrus plants that are well adapted to local conditions may
produce fruit regularly without protection from pests.   It may
not be practical or necessary in all circumstances to maintain a
pest control spray program.  Usually when production is attempted
on a commercial scale some control measures are well justified by
the improvement in quality and quantity of marketable fruit
produced.  The full productive capability of carefully cultivated
trees will not be achieved if pest control measures are neglected.
Similarly, a good pest control program will not be highly
beneficial if other management elements are neglected.
It is often possible for a grower to establish and maintain a
reputation for fruit of superior quality, for which buyers will
readily pay premium prices.  The additional costs of production
resulting from pest control and other management efforts then may
be offset not only by higher yields but by higher prices as well.
The producer willing to invest the capital and personal effort
required to produce high quality fruit should recognize and
attempt to exploit this possibility.   In many localities, seedling
orange trees produce fruit which finds its way into the
markets at very low prices for an extended period of time.  It is
important for the grower to select varieties which produce mature
fruit while the market is not saturated with fruit of inferior
quality from local seedling trees.
Most of this discussion has assumed establishment of a small
planting largely by an individual or a single family.   Some
experience with a small planting will yield answers to many
questions which cannot be resolved fully in advance.   Among such
questions, that of optimum size is important.   Some cost reduction
is associated with larger scale, but a greater risk of
failure may be present as well.   The transition from a family
effort to one requiring frequent use of hired labor may radically
change capital requirements.   Such issues should be analyzed
carefully in determining appropriate size.
The questions which follow will help identify problems which may
arise, and which should be given careful thought in advance.
     1.   Is the land available of suitable quality for a citrus
     2.   Will erosion control be difficult if fruit trees are
     3.   Are the fertility characteristics of the soil sufficiently
     4.   Can a suitable fertilizer regime be relied upon to
         supply all mineral requirements of the plants?
     5.   Will the trees require irrigation?
     6.   If so, is sufficient water available for this purpose
         at reasonable cost?
     7.   Is the terrain suited for growing trees with irrigation?
     8.   Is sufficient capital available to purchase needed
         equipment and supplies?
     9.   Is the owner able to invest the necessary management
         effort to make the venture a success?
     10.   Have supply sources been found for plants, fertilizers,
          pest control chemicals and other needed supplies?
     11.   Has information been assembled about available varieties,
          and has a decision been made about which to
     12.   If plants must be propagated, has a suitable source of
          rootstock seeds and scion budwood been found?
Boy Scouts of America.  Fruit and Nut Growing.  North Brunswick,
    New Jersey:   Boy Scouts of America, 1974.
Brogdon, J.E. Insects and Mites of Commercial Citrus and their
    Control.   Gainesville, Florida:   Florida Cooperative Extension
    Service, University of Florida, 1971.
Fletcher, W.A.  "Propagation of Citrus Trees."   New Zealand Journal
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Florida Cooperative Extension Service.   University of Florida.
    Budding Nursery Stock.  Gainesville, Florida:   Florida Cooperative
    Extension Service, Undated, pp. 18-29.
Jackson, L.K., and Sauls, V.W.   Tangerine and Tangerine Hybrids.
    Gainesville, Florida:  Florida Cooperative Extension Service,
    University of Florida, 1978.
Johnston, J.C.  Citrus Soil Management.  Visalia, California:  California
    Agricultural Extension Service, University of California,
____________.  Growing Citrus Seedlings.  Visalia, California:
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    Extension Service, University of California, 1959.
Laurence,  F. P.   Planting and Care of Young Citrus.   Gainesville,
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    of Florida, 1975.
Prosser,  D.P., Jr.  Hedging Machine of Citrus Groves.  Bulletin
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    University of Florida, 1953.
Puffer, R.E., and Turrell, F.M.   Frost Protection in Citrus.  Visalia,
    California:   California Agricultural Extension Service,
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Sauls, J.W., and Jackson, L.K. Citrus Propagation.   Fruit Crops
    Fact Sheet.   Gainesville, Florida:   Florida Cooperative Extension
    Service, University of Florida, 1978.
____________.  Lemons, Limes, and Other Acid Citrus.  Fruit Crops
    Fact Sheet.   Gainesville, Florida:   Florida Cooperative Extension
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Smoot, J.J.;  Houcky L.G.;  and Johnson, H.B.   Market Diseases of
    Citrus and Other Subtropical Fruits.  Agricultural Handbook
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    Circular 339.  Gainesville, Florida:   Florida Cooperative
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