:
Historic, Archive Document
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on os
Under the supervision of the
UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 33
CITRUS CULTURE IN PORTO RICO
BY
HENRY C. HENRICKSEN
Agriculturist
| Issued February, 1930
For sale by the Superintendent of Documents, Washington, D. Cc. - - - - - Price 10 cents
. L
MeN
PORTO RICO AGRICULTURAL EXPERIMENT STATION, MAYAGUEZ
[Under the supervision of the Office of Experiment Stations, United States Department
of Agriculture]
ee
, Chief, Office of Experiment Stations.
WALTER H. Evans, Chief, Division of Insular Stations,
Office of Experiment Stations.
STATION STAFF
. MAy, Director.
- McCLELLAND, Horticulturist.
. HENRICKSEN, Agriculturist.
. VAN VOLKENBERG, Parasitologist. -
. Tucker, Plant Pathologist.
. Davis, Plant Breeder.
. CARRERO, Assistant Chemist.
RROYO, Minor Scientific Helper.
nS Jr. Riedy Clerk.
Pstats
ersronmmsy
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PORTO RICO AGRICULTURAL EXPERIMENT STATION
MAYAGUEZ, P. R.
Under the supervision of the
UNITED STATES DEPARTMENT OF AGRICULTURE
BULLETIN No. 33
Washington, D. C. : February, 1930
CITRUS CULTURE IN PORTO RICO
By HENRY C. HENRICKSEN, Agriculturist
CONTENTS
Page | Propagation—Continued. Page
eee Lar rears G00 Ps so Se 8 ret a ls Ps 1 Determining the volume weight of the
Coninercial plantings: 22 tsk ee se a 2 PPIRE SS? Abels xe “eS Sa ES a 17
Annual shipments of fruit____._-_._--_-_- 3 Botiehdarag ) 3B yg oy rg Se ih gh ny wee 18
Nati! requirements a oe 4 | Preparation ‘a HE Ted =. 22 ee Rae oe 19
STi Et Bie ane BS EVE PD yk ee eS FP AV ING Drea KS os 2? C2 Ret Rk Oe ie 19
Loeatioi of thé muarsery. 2038 = a) Pinning ane, enliivation ss. 5 AF ee a 21
Tree stocks and their identification__________ Gh vers Mamba Fed a Nea tome Oe P38. 58 SE WAS 25
PEGDAP ALON 21653 ayo it BE Fea ot 14 | Insect and disease control____________________ 27
SElEChIOl Or SECC se ae te © eer ere eee 14 | Harvesting and handling ___________________ 31
Sepdiings 6 eee’ © # acecl 2 bes er be ae Oe oe 15 | Sid SEAT 28 5 Oy Sr eR Pe es EF tS OE 32
BHC Se OCH iOies 352 tee Bee tr kd 16
The Porto Rico Agricultural Experiment Station has kept in close
touch with theJoeal-eitrus-industry from the time of the planting of
the first commercial groves. It has helped to solve many of the
problems having a bearing on the industry and has had to delay the
solution of others because of a lack of adequate facilities for doing
research work. ‘Thousands of replantings must be made following
the loss of citrus trees in the hurricane of 1928, and many new
groves are being planted by persons who have had no special training
in citrus culture. It is important that all growers should know how
they can avoid making mistakes which would defeat the paEpore for
which the plantings are made. Representative groves in the differ-
ent districts were recently studied for the purpose of securing a
foundation for a contempiated campaign of systematic resear rch,
Some of the data obtained are published in this bulletin and should
be of value to fruit growers in general and particularly to those who
are rehabilitating old groves or planting new ones.
EARLY PLANTINGS
Seedling citrus trees are found growing in Porto Rico, as in other
islands of the West Indies. It is impossible to trace the various intro-
ductions, for the seeds usually were brought in as a matter of course,
and no record was made concerning their origin. It is reasonable to
suppose, however, that a great many different lots of seed were
brought in from the various places where desirable fruit was found
§7036°—30 1 4
2 BULLETIN 33, PORTO RICO EXPERIMENT STATION
growing, especially from. around the Mediterranean Sea. This
would naturally be the case with oranges, and to a lesser extent with
shaddocks, grapefruits, lemons, and limes. The shaddock and per-
haps the grapefruit also were probably planted as a curiosity only.
The shaddock is seldom found growing wild in Porto Rico and the
erapefruit only scatteringly in the different districts, the trees grow-
ing singly or in small groups. The so-called native grapefruit tree
is characterized by tall, upright growth, in which respect it resembles
the Triumph variety. Like it also, the native grapefruit tree is im-
mune to citrus scab. The fruit is small to medium in size, and of
rather insipid but slightly bitter taste.
The orange is growing wild or semicultivated more extensively in
Porto Rico than elsewhere in the West Indies, partly because the
tree has been found useful as a coffee shade, and partly because the
fruit can be marketed profitably. In the New York market the fruit
is known as the “ Porto Rican wild orange,’ which is a misnomer,
for it is cultivated as thoroughly as is coffee. The fruit might better
be called the Porte Rico mountain orange. It varies in size, shape,
and quality, but is generally characterized by thin rind, melting pulp,
and extreme juiciness. The fruit varies from light to dark orange
in color, and from seedless to very seedy. Especially selected trees
which have been used for propagation from time to time have been
found to transmit character of the fruit as well as growth habit of
the tree. Unfortunately, a very large number of seedling trees were
destroyed by the hurricane of 1928.
COMMERCIAL PLANTINGS
Local commercial citrus growing dates from about the year 1901.
The first trees which were planted in commercial groves were intro-
duced from Florida, and simultaneously with that introduction
nurseries were started on the island. Rough lemon and sour orange
from locally obtained seed were the first stocks to be planted in
Porto Rico. Later the cultivated grapefruit was also used for stock,
and during the past 15 years seedlings of the so-called native grape-
fruit have been used to some extent. The first groves were of oranges
and grapefruit, with the former greatly predominating. Perhaps
all the varieties of oranges grown in Florida at the time were intro-
duced into Porto Rico either in the form of bud wood or budded
trees. The buds were not selected as they are nowadays; in fact,
they must have been cut frequently from nonbearing trees because
many of the trees in Florida had not attained the bearing stage fol-
lowing the freeze of 1895. The result of this promiscuous cutting
of bud wood became apparent as soon as the trees began to bear.
Some varieties were not true to name, and most varieties bore fruit
which in both quality and quantity was below expectation. Some
of the varieties, moreover, were unlike the same kinds growing in
Florida. The pineapple orange, for example, failed to produce its
characteristic flavor, and the blood orange showed no color, whereas
the navels were coarse and dry as they usually are in Florida. These
variations and the slow growth of the orange tree as compared with
tnat of the grapefruit tree, together with the then-prevailing low
CITRUS CULTURE IN PORTO RICO 3
price of oranges, caused many growers to make new plantings or to
rebud the trees already planted; as a result, very few orange groves
are to be found in Porto Rico to-day.
The introduced grapefruit varieties, like the orange varieties,
comprised practically all that were grown at the time in Florida.
Types resembling the Duncan seem to have predominated, and the
Duncan and the seedless varieties apparently have been preferred
in later propagation work. To-day only these two varieties are
recognized. The seeded type is locally known as the Duncan re-
gardless of visible differences, and the seedless, or nearly seedless
type, is called “ Marsh Seedless.” The variation found in most ot
the groves is considerable, and would seem to point toward a profit-
able field for selection.
The first groves were started by continental Americans, only a
few of whom had had experience in citrus growing. Their experi-
ence had been gained in Florida, and as a result an attempt was
made to apply Florida methods to Porto Rico. Many of the meth-
ods, such as low budding, deep planting, and growing the trees
without wind protection, proved to be great handicaps to a success-
ful industry from the very beginning. Efforts to combat gum
disease, scale insects, ants, leaf-eating beetles, and root-eating grubs
were costly and discouraging. As experience was gained these diffi-
culties were overcome, but many of the surviving trees have re-
quired constant attention ever since. Because of these early mis-
takes the annual loss of trees in Porto Rico has been abnormally
high. During the past 10 years many of the most productive trees
have died from root rot and from gumming induced by low plant-
ing. The life of some of these trees has been greatly prolonged by
inarching and by the judicious use of tree surgery, but most of them
were destroyed by the hurricane of 1928. ‘Thousands of healthy
trees were blown over. ‘Those which were promptly set up and
properly cared for usually recovered, but many trees in the mountain
districts were lost as the result of neglect.
ANNUAL SHIPMENTS OF FRUIT
Production per tree is difficult to estimate, and the average yield
per tree can not be stated because some trees yield 10 to 20 boxes of
fruit, whereas others yield nothing. Of the latter kind, some may
not have reached bearing age, and others may be nonbearers or
practically so. The export figures for oranges can give no clue
to the yield per tree because oranges from the mountain districts
are shipped only when prices warrant, and the island consumption
is always uncertain. The census report of 1920 showed that there
was then a total of 1,300,000 orange trees on the island. A compari-
son of this number of trees with the number of boxes of oranges
exported in 1918 reveals a yield of less than half a box of fruit per
tree, and similar comparisons show that in other years the yield
was one-quarter of a box of fruit or less per tree. Notwithstanding
these facts, the seedling trees are usually very prolific.
Table 1 gives the annual shipments of citrus fruits from Porto
Rico for the period 1910-1928, inclusive.
4 BULLETIN 33, PORTO RICO EXPERIMENT STATION
Taste 1—Annual shipments of citrus fruit from Porto Rico for the fiscal
period 1910-1928 *
Canned Canned
» Grape- , Grape-
Year Oranges : grape- Year Oranges : grape-
fruit Gaiik fruit fine
Bozes Boxes Bozes Bozes Bozes Bores
UGE = pases ee ee 296, 058 AS PAA Me oie 1920. Oe a eee 336, 625 419‘ 6297) | eee
ROMY. 2 eae he ee os 349, 442 O65:1'S9 4] kee ee 1922 ae eee eseeee 162, 395 667, 637.4352 eee
DOLD ii eae BS es 277, 422 LTS SOS TC ese hse ee 1922 sch reters Peeh see de 388, 882 360; 530 bork
pee ee ee tS 353, 690 DOE DAT | ees scene YY ae sap Sa ca Ue a? 732, 973 460, 951 ALCS eae
TOTAL eteawie ds Wyse iets, 348, 927 PUD DOS |i Aes ee es NG D4 site Seles See a ee 192, 363 666, 657 102, 143
(TC EERE sees ae ees 200, 311 DIOS OG |e a= Teese AG 25 ASRe ae xf eee S 336, 761 579, 736 123, 461
ISG Wie he, Fk 404. 451 1 906-645) | Pk A PORp SNA eee 463,709 | 809,038 | 201,888
Sipe Pi ake ae oe 503, 318 ASD eS ss ee Bet 7 fees Ee SS eee 311, 294 692, 052 344, 660
Sisto Se etese 603,226 | 549,125 |_________- 1908 ee erey eee 546,463 | 891,940 | 334, 207
iL? | [2 cae Bae ieee eee 373, 679 Ati 3697 ete | pea
' According to United States customs figures.
A grove-to-grove census, taken shortly after the hurricane of Sep-
tember, 1928, showed that there were then on the island approxi-
mately 6,000 acres planted with 430,000 grapefruit trees of all ages.
Table 1 shows that 891,940 boxes of grapefruit in the fresh state
were exported in 1928 and that 334207 boxes of the canned fruit
were shipped. If 3,793 boxes of fruit are allowed for local consump-
tion, the island will be found to have produced a total of 1,230,000
boxes of grapefruit that year. This production from 300,000 full-
bearing trees would give an average yield of shghtly more than 4
boxes per tree. The table also shows that while production has in-
creased steadily fluctuation has been considerable from yéar to year.
NATURAL REQUIREMENTS
SOILS
That the citrus tree is not very exacting as to its soil requirements
is well illustrated by the fact that the groves are found on soils
covering a wide range from beach sand to heavy clay. A systematic
study of existing groves reveals differences in growth of tree due to
soils which were not readily discernible at the time of planting. This
is true especially of the region between Bayamon and Arecibo, which
is bordered by the ocean on one side and by the railroad and in
places by the pubiic highway on the other. The soils of this region
vary irom sand that is almost devoid of colloidal matter to clay
containing less than 25 per cent sand. The growth of the trees fur-
nishes information regarding the chief limitation of each soil type.
The ‘white sand is poor because it does not retain enough moisture
and plant nutrients to supply the needs of the trees under existing
conditions. ‘The darker sand, containing 2 to 3 per cent colloidal
matter, of which about 1 per cent is organic, is excellent, provided
the subsoil is pervicus and the water level is not too variable. The
red sandy soils containing upwards of 10 per cent colloidal matter
are suitable for tree growth, provided they do not cement. Cementa-
tion is caused by the high amount of iron and the colloidal state in
which it is present in these soils, and it prevents the trees from devel-
oping properly. The clay soils in this region vary from 20 to 60 per
cent clay and are similar to those found in other regions.
.
=
CITRUS CULTURE IN PORTO RICO o
The soil at Plantaje, both surface and subsoil, in which the first
commercial grove on the island was planted, consists of shell sand.
With the formation of land at this point, silt was deposited in the
lower places, whereas the slight ridges remained practically pure
sand. A heavy hammock growth developed, especially in the silt-
covered areas, and after it was removed considerable humus re-
mained. The difference between the poorest and the best soils in
these groves is readily discernible by the color of soil. Analyses
show a difference of 2 to 3 per cent in colloid content, practically
all of which is organic matter. In the better grade of this soil tree
growth is excellent, and even in the poorest grade the trees develop
much better than they do under different conditions.
The water table in these groves reaches within 3 to 4 feet of the
surface in the rainy season, and supphes abundant moisture most of
the time to the citrus trees even on the poorer soil. On the other
hand, the high water table is the main limiting factor in root forma-
tion. The depth of the root system is naturally limited by the
water level, and although a distance of 3 feet is ample for root
development it 1s not sufficient to allow for the changes in water
level that take place periodically. For example, the water may not
reach beyond the 3-foot limit in a certain section for several years,
but during a period of heavy rain it may come to within 214 feet of
the surface. Unless this excess water is drained off, the immersed
roots will decay and the trees may die. At times, especially during
prolonged droughts when the water table subsides 12 inches or more
below normal, the trees may suffer from a severe shortage of mois-
ture. Evidently the pore space in the subsoil is too great to allow for
much capillary rise of water. A lack of moisture can be remedied
cheaply by irrigation; but since the water has an alkalinity equal
to 0.16 gram CaCO, per liter and a pH of 8 to 8.4, it may not be
suitable for the purpose. In the surface soil with a humus content
of about 2 per cent, the pH seldom reaches above 7.4. Whether or
not a pH of 8.4 through the entire soil mass wovld be detrimental is
not yet known.
The sandy soils in all the other groves in the above-mentioned
region consist of silica sand mixed with clay silt and organic matter.
The poorest grade of this sand is exemplified by the narrow ridge
west of Dorado, which broadens out into a plain of several hundred
acres between Vega Baja and Lake Tortuguero. This sand is several
feet deep and is so pervious that water percolates through it readily.
Tt is practically devoid of clay and humus, and consequently all
fertilizers applied leach out immediately following a rain. To main-
tain plant growth, such sand must be given frequent fertilization
and irrigation. Even were this treatment practicable it could not
be satisfactorily used because citrus trees do not thrive well in a
soil that is entirely devoid of humus and soil organisms. Obviously
this extreme type of soil is unsuitable for cultivation, and large
areas of it can not profitably be made suitable. Often, however,
small areas in citrus groves may be improved without great expense.
Clay, when available, may be applied to good advantage at the
rate of 50 tons per acre. The clay should preferably be dry and
pa Nenized, or at least granulated, so that it will readily mix with
the sand.
od
6 BULLETIN 33, PORTO RICO. EXPERIMENT STATION
The addition of stable manure to the sandy soil at the rate of 1
ton per acre will supply the soil with organisms, and then cover
crops can be grown, although fertilizers may be necessary for the
first few crops. Ground limestone may also profitably be applied
to such soils at the rate of 2 to 5 tons per acre. A rock crusher
should be available for use on plantations where limestone is plenti-
ful because most of the soils in which citrus trees are planted are
greatly benefited by an application of lime. The poor sandy-clay
soils may be improved by an application of both ground limestone
and organic matter. Citrus trees should not be planted on areas
where the subsoil is high in iron and much cemented. |
The suitability of sandy soils containing small amounts of clay
and organic matter depends upon the permeability and water-holding
capacity of the subsoil, as well as upon the level of the water table
and its fluctuations. Provided that the water table does not reach
to within 10 to 12 feet of the surface at any time and the subsoil has
great water-holding capacity and is sufficiently pervious for root
formation, the citrus tree usually will not suffer much from lack of
moisture in most of the citrus districts in Porto Rico. In such soils
trees with a taproot are more likely to succeed than those without.
In fact, the presence or absence of a taproot is probably often the
cause of differences in tree growth in many such soils. Wherever
the level of the water table varies within wide limits the root system
will adjust itself to the highest level maintained for a considerable
length of time; consequently, the trees may suffer from drought
when the level falls. The cultivation practice suggesting itself un-
der such conditions consists in the growing of cover crops during
the rainy season and the maintenance of a dust mulch during the
dry season.
In sandy soils the leaching of fertilizers may be a problem of
some importance. Usually, however, it is of minor importance, as
is illustrated by the following example: A soil which in situ weighs
40 kilograms per cubic foot and has a water-holding capacity of 15
per cent will retain moisture equal to 2.5 inches of rain per foot.
If such a soil is fertilized after having been saturated by a heavy
rain, the water-soluble salts will go into solution quickly, and if 2.5
to 8 inches of rain fall shortly after the soil is fertilized a great por-
tion of the soluble salts will be leached below the foot of soil. The
actual amount of salts so leached will of course depend upon the
amount of colloidal matter present in the soil. Another rainfall of
9.5 inches will wash an additional portion of soluble salts from the
topsoil and also some of the salts from the subsoil. When the root
system of the tree is deep and the subsoil contains considerable clay,
fertilizers are not so hkely to be washed below reach of the roots
except during periods of unusually heavy rains.
The differences in productivity of clay soils are difficult to describe
and difficult to determine by analytical means. ‘The terms “ heavy ”
or “ light,” when applied to clays, do not refer to actual weight of the
soil but rather to soil plasticity, which is not governed by the con-
tent of clay or by any other constitutent, but rather by the state of
the entire mass of colloidal matter. A clay subsoil, for instance, is
usually termed “heavy” because the colloidal clay and iron com-
CITRUS CULTURE IN PORTO RICO 7
pounds present are compacted. When brought to the surface such
subsoil may become hard on drying and plastic on wetting or it may
become crumbly. Crumb structure does not usually develop in
such soils until after they have become thoroughly mixed with 2 per
cent or more of humus. It may erroneously be inferred from this
that the plowing under of 15 tons of dry grass per acre will result in
the formation of a 6-inch layer of topsoil. Theoretically this is cor-
rect, but in reality only a small portion of the organic matter re-
mains in the soil. In fermenting, the humus becomes thoroughly
incorporated with the soil, and a crumb structure results. Several
annual applications of dry grass each of 15 tons would be required
to produce the desired effect and at the same time leave a deposit
of 2 per cent of humus in the soil. |
A 6-inch layer of soil is not sufficient for root formation, especially
in groves where the topsoil is disturbed periodically by cultivation.
Usually seedling trees the roots of which are not disturbed by cul-
tivation are shallow rooted where the soil layer is thin and the sub-
soil hard. Hundreds of such trees when blown over by the hurri-
cane of September, 1928, were found to have a very shallow root
system. In cultivated groves having a shallow surface soil the
subsoil must of necessity be -pervious to the tree roots, or growth
will be very slow. A great many of the groves were found to con-
tain small and large areas having a shallow surface soil when the
trees were planted. Holes for planting were dug in the subsoil and
sometimes dynamite was used to break up the surrounding soil area,
but the tree growth usually was very unsatisfactory nevertheless.
Before planting is done it is often difficult to determine whether
the subsoil is permeable enough for the roots to penetrate, but after
the trees are planted and have attained the age of about 10 years
the suitability of a soil can readily be judged by the root develop-
ment. Unfortunately an impervious subsoil can not readily be
improved, and usually it is more profitable to discontinue cultivating
a grove on such soils than to attempt to improve them. Improve-
ment, when practicable, may be had by. the practice. of deep
subsoiling. : : .
The water tablein clay soils is not usually high enough to inter-
fere with root development, but often drainage is slow, and the soil
remains saturated for prolonged periods. An excess of water will
cause the roots to decay because the area occupied by them is then
poorly aerated. In the deep, well-drained clay soils of the Manati
Valley the trees do not suffer from an oversupply of water even
when the rains are heavy and prolonged. These soils, because of
their crumb structure, are in some respects similar to sandy soils.
The water drains quickly from them and evaporates from the surface
to such an extent as to make drought a problem unless the dust-
mulch method of cultivation is practiced. The crumb structure in
these soils is readily destroyed by cultivating when the moisture
content is too high. This is true of all soils, but some are much more
difficult to handle than others.. One type of soil in the Cidra Valley
retains its crumb structure much better than do the soils of the
Manati Valley, and these again better than most of the soils in the
Bayamon section. ,
8 BULLETIN 33, PORTO RICO EXPERIMENT STATION
LOCATION OF THE NURSERY
In selecting a location for the nursery the water supply should
receive first consideration. An abundant supply should be available,
and overhead irrigation is very desirable. Preparation of the soil
should begin about a year before planting is done. ‘The soil should
be deeply ‘plowed and harrowed and then planted with a leguminous
crop. This should be plowed under when it has fully developed and
left for about three months to disintegrate. During this period the
soil should be harrowed frequently and any growth of Para grass
(Panicum barbinode), or malojillo as it is locally called, should be
removed.
FIGURE 1.—Grapefruit budded on rough lemon stock and planted on extremely hard
soil
TREE STOCKS AND THEIR IDENTIFICATION
In all citrus-growing countries opinion differs regarding the kind
of root stock to ‘be used. This is natural in view of the fact that the
problem is complicated. In studying the stocks in the older groves
it was found that they consisted of lemon (fig. 1), sour orange (fig.
2), cultivated grapefruit, and some so-called native or wild grape-
fruit. They are growing in all the different types of soil and in
some of the groves in adjc oining rows.
Since the different stocks could not be identified except by means
of the sprouts (fig. 3) which arise from below the bud union and
are seldom present, it became necessary to find a* chemical method
permitting of readily differentiating one kind from another. The
method found is simple enough for ‘planters to use. The following
directions will serve for field use: A piece of root from a tree carry-
CITRUS CULTURE IN PORTO RICO 9
ing sprouts emanating from the stock should be used as the standard
with which to compare the unknown. A piece of this root should
be washed and wiped, and the bark should be scraped from it. A
FIGURE 2.—Grapefruit budded on sour orange stock. Trees are the same age as is
that shown in Figure 1 and are planted adjacent to it
Ficurp 3.—Leaf forms of citrus: A, Lemon without apron: B, grapefruit with apron
overlapping leaf blade: C, sour orange with apron not overlapping leaf blade; D,
sweet orange with small apron; E, lime, very small leaf with very small apron
1-gram sample should be macerated in a mortar with a few cubic
centimeters of water, after which about 20 to 30 cubic centimeters
of water should be added. One cubic centimeter of a 5 per cent
87036°—30——2
10 BULLETIN 33, PORTO RICO EXPERIMENT STATION
ferric chloride solution should next be added. The mixture should
then be poured into a measuring cylinder and thoroughly stirred,
and water added to make 50 cubic centimeters. The color produced
by the ferric chloride when mixed with the root bark of sour orange
is very black. With cultivated grapefruit it is much lighter, and
with native grapefruit it is still lighter, whereas with lemon prac-
tically no color develops.
The roots should preferably be taken from trees that are growing
vigorously, as dormancy may interfere with results. When the color
is not distinct enough, the following laboratory method may be satis-
factorily used: Add a small portion of aluminum cream to the mix-
ture and filter by means of vacuum. The depth of the color of the
different solutions can then be determined in a colorimeter in com-
parison with a standard solution of naringin to which ferric chloride
has been added. Lemon shows practically no color change; native
grapefruit has a color equaling 4 to 6 milligrams naringin in 50 cubic
centimeters of water; cultivated grapefruit has a color equaling 6
to 10 milligrams naringin, and sour orange has a color usually much
deeper than 10 milligrams. 2 | |
The rough or goat lemon, iocally known as limén de cabra, has been
and still is used extensively for stock. Unfortunately the method of
identification does not permit of differentiating between the different
varieties of lemons, but growth characteristics indicate the probable
use in some instances of a variety other than rough lemon. The vis-
ible characters of trees which have been budded on lemon stock are
(1) comparatively vigorous growth regardless of soil type, and (2)
an uneven bud union, with the trunk usually enlarged below the
union. (Fig. 4, A.) Frequently the enlargement is visible on trees
5 to 10 years old; after that time it may be obliterated. Trees having
received little cultivation and small amounts of fertilizer up to 10
years of age may not show enlargement. (Fig. 4, B.) On the other
hand, grapefruit trees on lemon stock may be decidedly constricted
below the bud union, a condition which would indicate the probable
use of some variety less vigorous than the rough lemon.
The sour orange (Citrus aurantium), commonly called naranja in
Porto Rico, was used considerably as budding stock 25 years ago, and
trees budded on it can be found in most of the older groves. Trees
on sour-orange stock usually are (1) smaller than those on lemon or
grapefruit stock, and (2) where grapefruit was used, constricted be-
low the bud union. (Fig. 5, A.) In one grove having a loose, fri-
able loam soil, trees 25 to 28 years old on lemon and sour-orange
stocks were found to be of practically the same size. In another
grove having an extremely light sandy soil, 25-year-old trees on sour-
orange stock were dwarf in comparison with those on lemon stock.
The same was observed in several groves having an extremely heavy,
poorly aerated soil. Constriction below the bud union develops
especially when the trees are well cultivated and fertilized. With
no cultivation and scant fertilization, the growth is slow, and the top
may develop no faster than the stock. When sweet orange is budded
on sour-orange stock, the bud union usually is smooth, and constric-
tion does not develop.
CITRUS CULTURE IN PORTO RICO 11
FIGURE 4.—Grapefruit budded on rough lemon stock: A, Typical bud union; B, occa-
sional bud union
12 BULLETIN 33, PORTO RICO EXPERIMENT STATION
FIGURE 5.—Grapefruit budded on Sour-orange stock: A, Typical bud union; B, occa-
sional bud union
*
CITRUS CULTURE IN PORTO RICO Ba 1
Apparently grapefruit was not used for stock in the island’s first
nurseries, and perhaps none of the trees of the early introduction
from Florida were budded on grapefruit stock. That it soon be-
came popular is attested by the fact that many of the grapefruit
trees in 20-year-old groves are budded on grapefruit stock. Field
studies did not show that grapefruit has been used as a budding
stock for orange. Mee, A
Some years after grapefruit began to be extensively used as a
stock, a few growers started to use the seedling or so-called native
grapefruit. No differences could be detected between the trees
budded on this and other grapefruit stock in two small groves, 12 to
14 years old, which were observed. The growth of the seedling tree
indicates that it may prove to be a desirable stock. In addition to
having vigor and health, it is immune to scab. Grapefruit when
FIGURE 6.—Grapefruit budded on grapefruit stock. Typical
bud union
budded on grapefruit stock and grown in good soil is characterized
by (1) vigorous growth and large size, indistinguishable from trees
budded on lemon stock, and (2) a smooth bud union and a trunk
which is seldom constricted below the union like the sour orange or
enlarged like the lemon.
As a result of observations made in many groves, it 1s concluded
that the rough lemon may be used as stock for both orange and grape-
fruit. It may be planted in any soil that is suitable for citrus grow-
ing, and is preferable to other stocks in extremely light sandy and
in very impervious soils. The indications are that time of fruiting
may be controlled more readily with trees budded on lemon stock
than on sour-orange stock. Trees which are budded on lemon stock
are said to be more likely to produce coarse fruit than are those
budded on the other stocks. This was not found to‘be the case, but
may be in a more extensive study when more fruit is available. The
a
14 BULLETIN 33, PORTO RICO EXPERIMENT STATION
longevity of trees which are budded on lemon stock is said to be less
than that of those budded on the other stocks, but this fact was not
established. :
Lemon is said to be more severely attacked by foot rot than is
grapefruit, whereas the sour-orange stock is reputed to be practically
immune. This fact could not be substantiated in field studies be-
cause all the older trees were budded too low to permit of learning
whether gumming started in the stock or in the bud. In most of the
groves gumming of various kinds is as prevalent on trees budded on
sour-orange stock as on those budded on lemon stock, although in
one grove those budded on lemon stock were found to be the most
affected. :
The sour orange may be used as stock for sweet orange, but is
less desirable for grapefruit than is grapefruit stock. The sour
orange may be planted in any good grade of loose loam, but should
never be planted in extremely hght or in extremely heavy soils.
The sweet orange as a stock has not received a thorough trial in
Porto Rico, and may be worthy of consideration. The growth of the
seedling tree in all types of soil indicates that it may prove to be a
desirable stock, at least for the orange.
The grapefruit may be used as stock for grapefruit, and un-
doubtedly for orange as well, although it has not been used for the
latter in Porto Rico. It may be planted on any fair grade of soil,
but does not thrive as well as the rough lemon in very hght sand or
in very heavy clay.
PROPAGATION
SELECTION OF SEED
In commercial citrus growing the foundation of the tree is the seed.
The seedling is planted either for the purpose of producing fruit
as such, or for use as stock. Seedling trees may produce desirable
fruit in abundance, as is well illustrated by the thousands of seed-
ling orange trees grown in Porto Rico. The fruit is variable, how-
ever, and therefore not so valuable as that from budded trees. The
difference in value may be too small to warrant the planting of
budded trees in coffee plantations, especially where cultivation other
than weeding is impracticable. In cultivated groves, on the other
hand, where intensive culture is practiced, the cost is too great to
risk the planting of seedlings. The seed should be carefully selected
regardless of whether the seedlings are wanted for fruit or for use as
stock. Seed selection always pays notwithstanding the fact that the
seedlings do not always reproduce the desirable characters of the
parent tree.
If the seedling is to serve as stock, the selection may be con-
fined to tree characters only, in which case the primary require-
ments of the mother tree are vigor and size. Health may well
be considered also and perhaps shape, but prolificacy and fruit
characters are immaterial factors. If, on the other hand, the seed-
ling tree is to bear fruit, the mother tree must be selected for fruit
as well as for tree characters. The tree must be large, vigorous,
well shaped, healthy, prolific, and produce fruit of such color, shape,
and quality as the market demands.
CITRUS CULTURE IN PORTO RICO B5
After the tree has been selected all fruit below medium size should
be discarded. Likewise, after the seed has been removed from the
fruit, the abnormally small ones should be discarded. This may be
accomplished partly by washing the pulp away from the seed, when
the very light ones float off, and partly by hand picking. After
having been cleaned the seed may be dipped in any of the organic
mercury preparations which are sold especially for seed disinfec-
tion. The seed may be planted without having been dried, or it
may be dried in a drafty but shady place and stored in a tight con-
tainer for some time and then planted. It should preferably be
planted immediately when possible, however.
SEEDLINGS
In Porto Rico, where the nurseries are comparatively small, germi-
nation boxes should be used in preference to seed beds. The boxes
may be of any convenient size, but should not be less than 6 inches
FIGURE 7.—Germination box with seedlings 8 weeks old. Note perfect stand due to
disinfection of seed and planting in soil free from organic matter
deep. Holes one-half to 1 inch in diameter should be made about
6 inches apart in the bottom of the boxes to provide for drainage.
The boxes should preferably be placed on racks of convenient height
and be partly shaded. An open shed with boxes placed near the
sides will afford a suitable location, or a frame having a covering
of slats so spaced as to provide half shade may be used. Pieces
of rock should be placed over the drain holes and the boxes filled
with soil to a depth of 6 inches. Any kind of soil that is not too
heavy may be used, although pure sand is unsuitable and much
organic matter is undesirable for the reason that it may harbor
damping-off fungi. A friable subsoil may be used because it is
nearly sterile. Commercial fertilizer consisting of inorganic matter
should be applied and mixed with the upper layer of soil; about
an ounce per square foot of a 4-8-8 mixture will do. The seed bed
16 BULLETIN 33, PORTO RICO EXPERIMENT STATION
will then be comparatively free from damping-off fungi, growth will
be rapid, and the plants stocky. (Fig. 7.) The seed should be
scattered on top of the soil at the rate of about four per square
inch and covered with a thin layer of soil. This in turn should
be topped with a thin layer of fine coconut fiber. The soil should
be kept constantly moist until the plants are ready for transplanting.
If fresh seeds are planted the resulting seediings will be large
enough for removal into other boxes in three months. All seedlings
below medium size should be discarded. The young plants should
be set one-half inch apart in boxes containing 8 inches of prefer-
ably a rich loam soil. They may be left in the boxes for six
months, during which time they will require less attention than
would similar plants in the nursery. |
In the nursery the seedlings may be planted 12 inches apart in
rows 814 to 4 feet apart. When the soil is clayey the plants should
be set at least 18 inches apart in the row to permit of removing a
ball of earth with the tree in transplanting. Preparatory to the
removal of the plants from the propagating boxes a shovel should
be run along the bottom to loosen the earth. After this they can
be taken up with less likelihood of breaking the roots. All under-
sized plants should again be discarded. It is a good practice to
remove only a few trees at a time, dip the roots in a slurry consist-
ing of clay and cow dung, and place them in a covered basket from
which they can readily be removed for transplanting. Holes large
enough to accommodate all the roots without cramping should be
made in a straight line across the field. The plants should then be
set and the soil firmed around the roots. Water should be applied
immediately.
After planting is done the soil should be kept constantly moist,
mellow, and fertile. It is well to use fertilizers two weeks after
planting and then at intervals of two months; applications of 1
ounce per plant of about a 4-6-6 mixture will produce stocky plants
which should be ready to bud in about 12 months from seed.
BUD SELECTION
In bud selection the main points for consideration are the tree
and the fruit. The tree should have a branching habit with a
spread as great as, or greater than, the height. It should be large
in order that the bearing area may be large, and free from imperfec-
tions that may be attributed to inheritance. The fruit produced
should be large and evenly distributed and advantageously placed.
Grapefruit, for example, should hang singly, or nearly so, rather
than in large clusters.
The most essential points to be considered in regard to fruit are
time and degree of maturity, form, size, weight in proportion to
size, color and smoothness of rind, thickness of rind, flavor and con-
sistency of pulp, number of seeds, fiber and number of sections, and
finally uniformity of all these points in a large number of fruits.
Special attention should be paid to uniformity of characters. The
fruit from a certain tree may be generally desirable, but when indi-
vidual specimens differ much in regard to any one character the
indications are that the character is not stable and may not be
CITRUS CULTURE IN PORTO RICO ivé
uniformly transmitted through the bud. The fruit from some par-
ticular branch may prove to be more desirable than that from the
other branches on the same tree, in which case the buds should be
taken from the one branch only. Of course the records for one
year will not be as reliable as data covering a period of at least five
years. There are many tree records now on the island, and a com-
parison of them may yield valuable data leading to the establish-
ment of a foundation for future work. Planters who have not yet
adopted the system of indexing their trees individually should do
so at once. They will soon learn of the benefits to be derived from
such a system and the importance of selecting bud wood from trees
whose records have been kept
for a number of years. Two
numbers should be painted on
each tree, one to indicate the
row and the other the number
of the tree in the row.
DETERMINING THE VOLUME WEIGHT
OF THE FRUIT
In bud selection the grower
usually judges the desirability
of the fruit without regard to
weight, measurements, or other
accurate tests, which is unsatis-
factory. The acidity may be
determined by titration and the
total solids by means of a
hydrometer, the methods of
determining which are well
known to most planters. The
weight should preferably be
ascertained by means of scales
that are correct to within the
limit of 1 gram. The exact
size may be ascertained by
means of a container shown in
Figure 8. The container is
filled with water until a few
drops escape from the spout,
when the measuring cylinder _ | sae oe volo eg ie ul gee
is set under the spout. The “‘""":ermining volume weight of fruit
fruit is immersed in the water
and is held down by means of the point of a nail. The number of
centimeters of water escaping from the container may be learned
from the cylinder and indicate the volume corresponding with that
of the fruit. The weight in grams, formerly ascertained, divided by
the volume in centimeters will give the density, the variations of
which are governed mainly by the thickness of the rind. - An orange
having a density much below 1 has a very thick rind, judging from
Porto Rican standards, and a grapefruit having a density below
0.85 has a thick rind. It is well to peel the fruit and weigh the rind.
87036°—30——3
18 BULLETIN 33, PORTO RICO EXPERIMENT STATION
Macerate the pulp and squeeze the juice out through a piece of muslin
and weigh the two separately. A tabulation of the data obtained
in this manner will yield valuable information.
BUDDING
The nursery trees are ready to bud when they are large enough, at
6 inches from the ground, to hold a bud. About 6 inches is the
proper height to insert the bud; by budding lower the tree may
pecome increasingly susceptible to foot rot, and budding higher is
of no advantage.
The operation of budding is familiar to most growers. The main
points to be remembered are that the knife must be sharp, and the
bud must be cut at a slant from the base to the tip of the knife blade
in order that the wood may not split. (Fig. 9.) Adhesive bud-
ding tape which readily sheds water should be used instead of
FIGuRB 9.—Method of shield budding: A, Cutting the bud; B, bud; C, making the in-
cision; D, bud partly inserted; E, bud wrapped
twine in wrapping the buds. The wrapping is usually partly re-
moved a week or 10 days after the operation, and the bud if alive
will be ready for forcing a few days later. Forcing consists in
girdling the stock 2 or 3 inches above the bud, or cutting the stock
partly off and lopping the top over. Some growers prefer to cut
the stock partly off and lop the top and others prefer to sever the
whole stock a few inches above the bud. The whole stock should be
removed only when the seedling is growing vigorously, however. As
soon as the bud starts growth it should be fastened to a stake placed
beside each tree. Frequent tying is needed to insure straight growth.
After two or three months the stock should be cut off smoothly,
slanting from the bud downward. All these details are very im-
portant. Asa result of their having been neglected in the past, many
of the nursery trees on the island are undesirable. Given proper
attention as regards irrigation, cultivation, fertilization, and all other
details, a first-class budded tree may be produced in less than two
years from seed. Abnormally small or defective trees should be
CITRUS CULTURE IN PORTO RICO 19
discarded in the nursery. The trees may be transplanted when they
are 3 feet high. Old, overgrown nursery trees are not as desirable
as young, vigorous ones. About two months before transplanting is
done, watering and cultivation of the nursery should be discontinued
in order to allow the young growth to mature. Frequently the
weather is rainy and growth continues, in which case transplanting
can be done without loss of the immature growth.
PREPARATION OF THE FIELD
The present-day field practice is based upon the experience gained
during the first few years of commercial citrus growing in Porto
Rico. Some of the growers who have come to the island within the
past 15 years are prone to deviate from this practice, and others
who are starting to plant now are not always convinced that it is
right under all conditions. Nevertheless, with some minor changes,
the earlier practice will continue to prevail because, as was shown by
the data collected in field studies, it is built tupon correct premises.
The two principal facts to be borne in mind are that (1) the trees
must be planted on mounds, and (2) wind protection must be pro-
vided before planting is done. Preparation should therefore be
started several months in advance of planting. The field should be
measured off and stakes placed where the trees are to be planted.
Usually the trees are planted at distances of 25 or 30 feet each way.
The latter distance is not too great to accommodate grapefruit trees
after they reach the age of about 20 years, but until that time 25 by 25
feet is sufficient, and budded orange trees do not usually need more
room at any age. Planting 25 by 25 feet will permit of the growing
of 69 or 70 trees to the acre. A mound of soil with a diameter of
4 to 6 feet is usually built up around each stake to a height of 18
inches or more, and the trees are planted on top of these to safeguard
them from foot rot. On heavy soil where the water table is high
the following practice is recommended for trial: Plow as deep as is
practicable and follow the plow with a subsoil breaker. Make the
beds to extend across the slope of the land, and use each line of
stakes as the center of a bed. This may be done most economically
by using a road scraper, proceeding in the same manner as when
building a road. If this practice is followed the size of the mounds
can be greatly lessened.
WINDBREAKS
Protection against wind may be provided by planting annuals or
biennials broadcast in the field, and tall-growing perennials in strips
at suitable intervals so that they will cross the field at a right angle
to the prevailing wind direction. Leguminous plants are desirable
for planting in the field because they supply the soil with nitrogen.
The crotalarias make efficient windbreaks, and especially Crotalaria
juncea because of its tall growth. The pigeon pea or gandul
(Cajanus indicus) is perhaps the second choice among the legumes
although Tephrosia candida seems to be equally desirable. ‘The
gallito (Agati grandiflora) grows well wherever the soil is not too
dry, and is an excellent nitrogen gatherer. (Fig. 10.) The seeds
20 BULLETIN 33, PORTO RICO EXPERIMENT STATION ;
FIGURE 10.—Gallito trees planted on the windward side of a young citrus tree
Figtre 11.—Sugarcane planted for local wind protection
CITRUS CULTURE IN PORTO RICO 21
. may be sown thinly or broadcast, and the trees will develop similarly
to annuals. If they are planted 2 to 3 feet apart these trees will
attain good size and serve as windbreaks as long as they are needed.
The main drawback is the poor wind resistance of the gallito, which
may result in damage to the citrus trees during heavy winds.
Several nonleguminous crops may also be used as windbreaks in
Porto Rico. Sugarcane makes a desirable windbreak in places where
the crop can be disposed of at a profit. (Fig. 11.) The banana is
another economic plant that may be used for wind protection
wherever the soil moisture is ample for the development of later
crops. Corn is not as extensively used as it might be, at least during
the first year. A combination of either corn or sunflowers and climb-
ing peas or beans is desirable. Pigeon peas, planted thinly so as to
produce stout plants, will provide a good support for velvetbeans,
and afford one of the best combinations both for wind protection and
for soilimprovement. The latter factor is very important. The soils
of citrus groves in Porto Rico can all be greatly improved by the
addition of humus, which must be added while the trees are small
enough to allow crops to be grown in the grove.
For permanent windbreaks the bamboo has been used extensively.
It is objectionable because citrus trees do not thrive in proximity to
a bamboo hedge. A bamboo windbreak, therefore, entails the loss
of two or three rows of citrus trees but is a fairly good insurance
against heavy winds. Most of the tall tropical trees are too slow
growing for use as windbreaks, and they are usually less wind re-
sistant than the citrus trees. The so-called Australian pine (Casua-
vina eguesetifolia) 1s promising for the purpose since hedges of it
and isolated trees generally withstood the hurricane of September,
f2p. (Wig. 12. )
PLANTING AND CULTIVATION
The method of planting citrus trees with a ball of earth surround-
ing the roots is especially desirable where nursery trees are not
shipped long distances. Young, vigorous trees taken up with a ball
of earth survive the shock of transplanting without the loss of follage
and with very little loss of roots. If the trees are not removed with
a ball of earth adhering to the roots, all the leaves should be cut off,
the branches should be cut back to within a few inches of the trunk,
and a portion of the longest root and all broken ends should be
pruned off. Immediately after the roots are pruned they should be
dipped in a thin slurry consisting of clay and cow dung and wrapped
in wet sacking. The trees should then be kept in a shady place until
they are wanted for planting. The method of planting on top of a
mound involves the difficulty of conserving the soil moisture. The
mounds should, therefore, be mulched with trash cut in the field, or
brought from without, if necessary. (Fig. 13.) |The soil surround-
ing the tree should be well watered at the time planting is done, and
then often enough to maintain a-high soil-moisture content until root
erowth is well advanced: An. ¢xamination of transplanted trees
usually shows that on young vigorous trees with thin roots the root
scars heal over in a few months and new roots form rapidly enough
to support the leaf growth; and, on the other hand, that on old trees
with dormant and thick roots the root scars do not heal quickly.
22? BULLETIN 33, PORTO RICO EXPERIMENT STATION
Ficgurb 13.—Young tree planted on mound with the soil well
mulched. Note mulch has been removed from one side
CITRUS CULTURE IN PORTO RICO ae
Decay may set in before the scars heal, continue into the trunk, and
eventually cause the death of the tree. The root growth of such trees
is frequently too slow to permit the sap to reach the leaves rapidly
ae to prevent wilting, especially when the air is dry and moving
ast.
The value of wind protection for young trees is mainly that of
maintaining a humid atmosphere. With the checking of excessive
air movement the tree may survive under conditions which would
make growth impossible without wind protection. The checking of
excessive alr movement also promotes the growth of beneficial fungi.
FIGURE 14.—Two-year-old tree which was planted on mound.
The mound has practically disappeared and the crown
roots are exposed
This was noted in the groves first planted on the island. Most of
the trees in these groves were severely attacked by the purple scale,
which was not kept in check by beneficial fungi until after wind pro-
tection was established. Nowadays newly planted trees which are
similarly exposed do not suffer from scale to such an extent, because
the beneficial fungi, especially the red-headed fungus (Sphe@rostilbe
coccophila), seem to be able to maintain themselves under more ad-
verse conditions than formerly. Planting tall-growing plants for
cover crops among the young trees has nevertheless proved to be a
most economical and time-saving practice in the establishment of
citrus plantations. Under this practice cultivation during the first
few years consists in plowing under as soon as it matures each crop of
24. BULLETIN 33, PORTO RICO EXPERIMENT STATION
plants grown for wind protection and planting another as soon
thereafter as possible. In establishing a grove the first point need-
ing consideration is the maintenance of such wind protection as will
afford the most favorable conditions for tree growth and eliminate
the necessity of spraying. The second point is determining the best
method of improving the soil so that e will continue to support
growth during the many years the trees are expected to produce
profitable crops.
The only treatment required by the tree mounds is that of keeping
them free from weeds and sufficiently well supplied with trash to
maintain a high degree of soil moisture. The soil will gradually
level down and after a few years will leave the trees with the
FicuRE 15.—Tree which was planted on mound. Crown roots are well exposed and
trunk is free frcm gum disease
crown roots entirely exposed. (Figs.14 and 15.) These roots should
be carefully protected from injury. No implement of any kind
should be allowed in the proximity of the trees, and weeding should
be done by hand. Plows and harrows may be used 4 to 6 feet
from the tree trunk. Annual deep plowing is advisable, but inter-
mittent plowing is inadvisable. If plowing is deferred for several
years the large roots within reach of the plow are likely to be cut,
and the tree may thus be seriously injured.
The time and necessity for cultivation and the kind of implement
to be used vary with the different local conditions. In one 20-year-
old grove where the medium heavy clay soil is kept moist with water
from surrounding springs, plowing and cultivation have not been
practiced since the trees were young. Para grass (Panicum barbi-
node) grows luxuriantly in the grove and is cut at intervals. Most of
CITRUS CULTURE IN PORTO RICO 25
it is left to serve as a mulch. This practice is not advisable where
soi] moisture is not abundant at all times. The general practice is to
plow or to cultivate, or to do both, although in some heavily fertilized
groves the soil seems to retain its moisture well under a luxuriant
growth of Para grass. Disk plows pulled by tractors are being used
successfully, but the area that can be plowed naturally decreases
with the spread of tree growth. In the older groves plowing is
usually confined to two or three furrows in the spaces between the
rows of trees. Disk or cutaway harrows are used almost entirely
for cultivation.
FERTILIZING
A study of the practice followed locally in fertilizing citrus trees
showed a lack of uniformity in the mixtures used, the quantities ap-
plied, and the number and times of application. The quantities ap-
plied differ greatly in the various groves, but applications are usually
made in excess of 50 pounds per tree annually. In .some instances
60 to 80 pounds are applied and in others 100 pounds annually are
applied to each. large bearing tree. Such large quantities may not
always be necessary for maximum tree growth and fruit production,
but the practice has undoubtedly much to recommend it, inasmuch
as many growers seem to find it profitable. The number of applica-
tions varies from two to three or more a year. Many growers apply
half the fertilizer in November or early in December and the other
half in June. Some who question the desirability of this practice
apply fertilizer three to six times a year. The practice of making
four to six applications seems to have nothing to recommend it, but
three applications are undoubtedly preferable to two. An applica-
tion in December has a tendency to bring on bloom not later than
February. However, since the fertility of the soil may not be suffi-
cient for the development of the fruit, it may be well to make a
second application about March 1 and a third application about
June 1.
For present purposes the question of fertilizing may be summa-
rized as follows: With few exceptions the aim is to ship most of the
crop before October. This can be done provided the bloom appears
in January. Often it does not appear until toward the end of Feb-
ruary, in which case the fruit has only six months or slightly more
in which to develop. Such fruit is not usually very palatable, even
though it may pass the Government maturity test, and can be im-
proved in quality by being left on the tree six to eight weeks longer.
In other words, the bloom should appear in early January in order
that the fruit may have a chance to mature early in September.
Although no infallible method is known at present for inducing
the trees to bloom in early January, the following practice, if con-
sistently employed from the time the trees are young, may produce
the desired result. Apply fertilizer about December 1 and follow
with plowing and cultivation. Ifa bloom is induced about the latter
part of January or the first part of February, apply fertilizers again
the latter part of February and continue cultivation until about June
1. Then apply fertilizers again and cease cultivation until the first
of December. If the trees are far enough apart to permit growing
eg SAARI IAL EAI Oe or ~....
26 BULLETIN 33, PORTO RICO EXPERIMENT STATION
other plants, a cover crop may profitably be planted at the time
eg ena ceases.
/1n regard to the kind and amount of fertilizer to be used, the fol-
dJowing suggestions may be helpful: Suppose a planter has found ©
/ by experience that it pays him to apply, say, 60 pounds per tree
/ annually of a 6-8-10 mixture (3.6 pounds nitrogen, 4.8 pounds phos-
phoric acid, and 6 pounds potash). Rather than divide the mixture
into three equal parts, let him use 20 pounds of a 3-2-2, or 10 pounds
of a 644 mixture in December, 20 ‘pounds of a 2-8-3, or 10 pounds
of a 4-6-6 mixture in March, and 20 pounds of a 1-3-5, or 10 pounds
of a 2-6-10 mixture in June. ~The reason for this suggestion is
obvious. The aim is to have the fruit mature early and the tree
attain as high a stage of dormancy as possible by early December ;
hence the minimum amount of nitrogen in the June application and
cessation of cultivation during the summer. In December the aim
is to break the dormancy of the trees as quickly as possible; hence
the large amount of nitrogen in the application, followed by intensive
cultivation.
The following observation will serve to emphasize some of the
problems in citrus fertilization. It was found during the past year
that small trees in 25-gallon containers continued growth even when
the soil nitrogen content was very low. It was also observed that
wild trees produce considerable growth and fruit in soil contain-
ing only small amounts of plant nutrients. This indicates that
the citrus tree can supply its needs from very weak soil solutions.
Field experience, on the other hand, seems to show that for quantity
production and early maturity liberal amounts of fertilizers are
necessary.
In regard to methods of application, most growers apply the fer-
tilizers broadcast in groves 6 years old or more. Some claim te
get better results by applying it in a narrow band around the tree,
about as far from the trunk as the spread of the branches. Others
who are now irrigating apply the fertilizers entirely under the tree,
in a space radiating from the trunk to the tips of the branches.
The results obtained from these different methods will soon show
which is the superior one.
In regard to the kind ana amount of ingredients in a fertilizer
mixture, variations might be made to meet soil requirements. In _
very light sandy soils, for instance, there is loss by leaching of some
nitrogen and perhaps some potash, but probably not much phosphate.
A great deal of phosphate may be found in the upper few inches
of such soils at any time during the year. An accumulation of
phosphate under such conditions would seem to justify the use of
fertilizers containing decreased amounts of the ingredient after a
few years.
In clay soils-both phosphate and potash may accumulate, but the
phosphate is often present in a form that is not available to the
tree. Whenever leguminous cover crops are used as a source of
nitrogen the content of this ingredient in fertilizers may usually
be diminished. ‘The amount to be applied can be partly determined
by the appearance of the tree. Applied in excess of needs, nitrogen
PTO wee
will cause the rind of the fruit.to be coarse and thick. -
- CITRUS CULTURE IN PORTO RICO 27
INSECT AND DISEASE CONTROL
In addition to gumming and various decays of the roots; the
trunk, and the branches, the chief enemies of the citrus-fruit indus-
try include scab, several species of scales, and the rust mite. The
control measures used for these are not uniform. Some growers
spray with Bordeaux mixture for scab, others spray with lme-
sulphur, and still others follow the Bordeaux mixture with sulphur
in the powdered form. Scale insects can usually be kept in check
by means of beneficial fungi, provided that Bordeaux mixture is not
used. The use of Bordeaux mixture entails the use of scale washes,
and too much spraying is costly as well as harmful to the trees.
A comparison of the efficiency of the different control measures
used in the different groves for the past several years fails to show
that one is superior to the other. It is generaily agreed that Bor-
deaux mixture is the most efficient remedy for scab, but some growers
claim that practically as high a percentage of scab-free fruit can be
had from the consistent use of lime-sulphur as from the use of
Bordeaux mixture. This may be largely due to the fact that spray-
ing can not be done as often and as efficiently as necessary. Dusting,
being much the more rapid of the two methods, may overcome this
difficulty, but it has not yet been in use long enough to warrant its
recommendation. |
The use of Bordeaux mixture is objected to principally because
it kills the scale parasitic fungi. Scale insects can be kept in check
by spraying the trees with oil emulsion; yet it is not unusual to
find groves in which the trees as well as the fruit are very scaly.
This may be due to ineffective spraying, or to poorly applied sprays,
but usually it is due to neglecting to spray at the proper time.
Such neglect is often inevitable, because even with a thorough
equipment only a certain number of trees can be sprayed in two to
three weeks, and during some seasons spraying must be repeated
at two to three week intervals.
Scab attacks the leaves and the fruit of the lemon, the sour orange,
and some varieties of the grapefruit tree. The Triumph and the
native grapefruit have so far been immune to scab, and the sweet
orange is seldom attacked by it. Grapefruit leaves are seldom so
severely attacked as to necessitate spraying. However, it may become
necessary at times to spray the nursery stock before budding is done
unless, of course, the Triumph type is used for stock. When scab is
kept out of the nursery it may likewise with great care be kept out
of the isolated grove in which the scab-free trees are planted. When
scab does attack the young trees spraying should be deferred until
the fruit needs it. In fact, it is better to destroy the first year’s crop,
which consists of a small amount of coarse fruit, than to spray for
the purpose of keeping it free from scab. When spraying is re- ~
sorted to for the second and succeeding crops the following points
should be kept in mind: Spray (1) before the bloom appears in
order to cover all the foliage, (2) at the time the petals are falling,
and (3) often enough to keep every part of the tree covered until
the fruit is three-fourths to 1 inch in diameter. These directions
apply whether Bordeaux mixture or sulphur is used and whether
they are applied in the form of sprays or dust. After attaining a
28 BULLETIN 33, PORTO RICO EXPERIMENT STATION
_ diameter of about 1 inch grapefruit is not attacked by scab. In view
of the fact that many trees have a habit of blooming over long
periods, spraying for scab must be done almost continuously where
attempt is made to keep all the fruit clean.
The formula for Bordeaux mixture is usually 3-38-50. Some
growers use commercial calcium caseinate as a spreader, whereas
others use a paraffin oil-soap emulsion, the quantity of which varies
according to the hardness of the water.
After sprayings with Bordeaux mixture for scab have been dis-
continued, sprayings with oil emulsions for scale should be begun
and followed with sprayings for rust-mite control. Failure to spray
for rust mite has resulted in the shipment of very little bright grape-
fruit from Porto Rico. That which is called bright usually con-
tains hundreds of rust-mite punctures. Sulphur is the best remedy
for rust mite and does the least harm to the foliage and the fruit
when applied dry. A dusting machine should be found in every
large grove for covering the trees with sulphur after sprayings with
oil have ceased. Only the pure sulphur should be used.
Lime-sulphur (1 gallon, having a density of 32° Baumé, to 30
or 40 gallons of water) should be applied for scab control when the
trees are the most dormant. When they are blooming the strength
should be reduced (1 gallon to 60 or 75 gallons of water), and after
the fruit has set and the foliage has matured the mixture should be
1 gallon to 40 or 50 gallons of water. Lime-sulphur, although a
fungicide, does not entirely destroy beneficial fungi but does prob-
ably injure them. It is also used as an insecticide against the various
scales, and when used frequently enough is effective against the rust
mite. In the powdered form sulphur can be applied more cheaply
and more frequently.
- The most common root diseases observed in field studies appar-
ently are the results of cutting the large roots. Injured roots are
the rule rather than the exception in cultivated groves, and it is
readily apparent that if infection took place as the result of every
injury the root system would be continually diseased. One form of
root disease which attacks trees growing in certain areas and spreads
from one tree to another is not caused by root injury. This disease
has been studied by the pathologist of the station for the past two
years, but the causal organism has not as yet been found. Fortu-
nately the disease is not very prevalent.
Foot rot appears on the crown roots and lower part of the trunk
and causes heavy losses in most of the groves. (Fig. 16.) Obser-
vations indicate that the disease can be prevented by planting
the trees on mounds so as to expose the crown roots to the air.
Such roots are not attacked, but may readily become infected after
_ they are injured; hence the practice of planting on mounds involves
a strict adherence to the rule of keeping all cultivation away from
the tree. In some of the old groves diseased trees have been saved
by removing the soil from immediately around the trunk to expose
the crown roots. This practice, however, is successful in sandy soils
only; the water accumulating in the excavation during rainy weather
would be fatal to trees in clay soil. On land having a gentle slope
the excavations could probably be advantageously connected with |
drainage ditches. |
-
iti Got hana ania
CITRUS CULTURE IN PORTO RICO 29
Rebudding which was done years ago has been the cause of much
trouble. The large trunks were not always properly cut and the
scars properly covered; as a result the trees started to rot and could
be saved only by means of tree surgery. This consists in removing
all dead wood and filling the cavity with some material such =
Portland cement. Cement is not always satisfactory, because it
usually shrinks away from the sides of the cavity. Once water gets
between the cement and the tissue the decay proceeds more rapidly
than ever. Asphalt has given better results than cement, but is satis-
factory only when used by skilled workmen. The cavities may also
be left open provided they are well cleaned out, waterproofed, and
drained. ike
Some growers plant three or four seedlings around an old dis-
eased tree and inarch them into the trunk. By means of this treat-
Figure 16.—Tree which was planted on level ground.
Crown roots are Dot visible and the trunk is infected with
gum disease
ment and the giving of proper attention to wounds, the life of the
tree may be prolonged. Wounds should always be treated with
Bordeaux paste, or some wood preservative that will not injure the
live tissue. Of late years gas tar has been used by many growers.
It is not always suitable, but the grade produced and sold in San
Juan does not seem to injure the live tissue of the tree. In choosing
a wound dressing it is well to keep in mind the fact that such mate-
rials as Bordeaux paste and other similar antiseptics do not make
the wound water and air tight. They must be followed by an appli-
‘cation of tar, asphalt, or some other preparation that will form a
perfect permanent seal.
Wither tip or die-back of the small branches causes considerable
loss in some of the groves. Like the various gum diseases of the
trunk, wither tip is associated with fungi. Losses from it are small
30 BULLETIN 33, PORTO RICO EXPERIMENT STATION
unless the trees are suffering also from a lack of moisture or nutri-
tion. The remedy is, of course, to keep the tree growing, which is
not always possible because of climatic conditions. ‘The tree, there-
fore, must be constantly watched and the dead tips and branches
pruned off at the termination of a dormant period. New growth
may start from the live tissue, and the dead parts may drop, but
very frequently the decay continues unless the dead tissue is removed
by pruning. Wounds following the removal of branches one-fourth
inch in diameter or over should be dressed.
Two decays cause appreciable damage to citrus fruit in Porto
Rico after it is picked. One decay is due to the blue mold (Penicil-
lium italicum) and sometimes to the green mold (P. digitatum).
These fungi do not usually enter the rind of the fruit unless it has
been bruised or mechanically injured and do not seriously attack
grapefruit because the rind does not bruise readily. They are a
very serious pest of oranges having a thin rind, especially oranges
. grown in the mountain districts. The avoidance of bruises is not
always possible under local conditions, and dipping bruised fruit in a
borax or sodium bicarbonate solution is not very advantageous.
Even cooling to a temperature of 40° F. or below does not prevent
bruised fruit from decaying. The method suggesting itself for
stabilizing the mountain-orange industry is that of handling with-
out bruising, whenever that is possible, and precooling and shipping
under refrigeration. The oranges of the mountain districts are of
fine quality and can be grown cheaply and should therefore be
entered on the market in competition with oranges from elsewhere.
The other decay is the so-called stem-end rot caused by the fungus
Diplodia natalensis, which results in great loss to grapefruit growers
in Porto Rico. Like the Penicillium fungi, the Diplodia fungus
does not usually enter the rind of fruit unless it has been bruised.
Since it is usually found about the stem the name stem-end rot has
been given to it. The fungus may enter through the stem end before
the fruit is picked, in which case it almost invariably drops soon after.
Usually very little fruit drops because of Diplodia infection in Porto
Rico. After the fruit is picked the fungus enters through the stem
scar or through the stem cavity after the button loosens. The latter
is the usual mode of entrance because the space under the calyx al-
ways harbors dirt and fungus spores. Two remedies have been tried
to control stem-end rot in Porto Rico. One consists in disinfecting
and sealing with shellac the cut end of the stem at the time the fruit
is picked, but it did not prove to be efficient for the reason that respi-
ration of the fruit is very high in the usual air temperature in Porto
Rico. When respiration is vigorous the gas exchange is very much
greater through the stem end than through any part of the rind;
consequently sealing does not endure and infection readily takes place
once the seal is broken. The other remedy tried consists in removing
the button and sealing the stem cavity, but that has not been found
successful locally, at least not with the early fruit. In removing the
button from fruit that is not thoroughly mature the rind is torn ona
great many specimens, and infection follows.
Diplodia will be much less of a problem in the future since precool-
ing and a more efficient refrigeration are now available on the
steamers. eh
CITRUS CULTURE IN PORTO RICO 31
HARVESTING AND HANDLING
_ Phe work of harvesting and handling the fruit in Porto Rico
is better systematized than any other part of the industry. The fruit
is picked by means of clippers, and generally it is carefully handled.
It is transported from the field to the packing house in field crates, of
which several suitable kinds are to be found on the market. Most of
the packing houses are equipped with standard machinery, and use
standard methods throughout. Packing-house problems awaiting
solution include coloring, cleaning and grading, and decay.
Coloring the fruit artificially is necessary early in the season be-
cause citrus fruit does not change color from green to yellow as soon
as it reaches the stage of maturity at which it is edible. Coloring is
induced by keeping the freshly picked fruit in an air-tight compart-
ment for varying lengths of time, at varying temperatures, and with
varying amounts of ethylene. Variations in the above-mentioned
factors are due to variations in the coloring quality of the fruit.
These have been studied by the writer for many years, and some of
them may profitably be discussed at this time.
One set of variations is due to differences in dormancy of the rind
tissue. Starvation of the tree or prolonged drought causes the rind
of immature fruit to turn yellow, after which irrigation and fertili-
zation with nitrogen may cause it to turn green again. Fruit which
has turned partly yellow because of tree dormancy is readily colored
by means of ethylene even when very immature; whereas fruit which
has again turned green because of renewed growth is very difficult to
color by any method. This helps to prove that the trees should not
be fertilized and cultivated later than about June 1 if they are to
produce marketable fruit in September. Of course‘adverse weather
conditions may upset all calculations, and some trees may remain in
a vigorous condition long after others have become dormant. The
desirable state of dormancy or colorability of the rind can be deter-
mined by measuring the catalase content. .This has not as yet been
correlated with the amount of gas needed nor with other coloring-
room conditions.
Rind blemishes and covering of various kinds cause differences in
colorability. For example, the rind of fruit which is attacked by
scab can not readily be colored artificially, probably because of a
change in the surface tissue which practically inhibits respiration.
The effect of the purple scale (Lepzdosaphes beckit) is similar to
that of scab. Fruit which is infested with scale may color perfectly
when left to mature on the tree. However, when the fruit is picked
green and colored artificially the spots underneath and immediately
surrounding the live scale will be found to remain green and the
green spots to contain more catalase than the surrounding tissue.
The rufus scale (Selanaspidus articulatus) produces the very
opposite effect. The spots covered by it are light yellow long before
the fruit reaches maturity, and the colorability of the intermediate
rind is not affected. Fruit that has been sprayed with oil emulsions
shortly before it is picked is not readily colored by ethylene, prob-
ably because of the oily covering which inhibits respiration. The
same effect is produced by any covering which leaves a thin film
32 BULLETIN 33, PORTO RICO EXPERIMENT STATION
on the rind. Coloring is greatly retarded by washing the fruit in
soapy water, or by running it over the polishing brushes, during
which process some of the oil cells in the rind may be ruptured.
Before it is packed the fruit is soaked in water usually containing
some detergent, after which the extraneous matter is removed by
brushing machines. The surface of the rind is often covered with
various scales and scale-destroying fungi and also with dust and
spray residues. ‘This covering when dense is very hard to remove.
Of the many different soaps, emulsions, and soap powders tried at
the station for the purpose, a soft soap of the nature of shaving
cream when mixed with water to the consistency of lubricating oil
was found to give the best result.
After the fruit 1s washed and polished it is sized by mechanical
sizers and graded by hand according to color and blemishes. For
the latter kind of work standards of comparison have been generally
agreed upon by the growers, but the agreement is not always ad-
hered to. 3
Attack by decays such as blue and green molds and stem-end rot,
which follow injury to the fruit, may of course be prevented by
careful handling of the fruit so as to avoid bruising, by keeping it
cooled to a temperature of 40° F., and transferring it directly from
the refrigerator to an insulated, refrigerated hold of the steamer.
Infection takes place in fruit the rind of which has been torn,
especially when the fruit is placed in a hot, moist atmosphere such
as prevails in the coloring room or in the hold of a nonrefrigerated
steamer. The paper in which the fruit is wrapped inhibits evapora-
tion and moisture conditions then favor fungus growth regardless
of where the fruit is stored. The fruit should be thoroughly dried
to eliminate as much moisture as possible from around the calyx.
Fruit which is well dried and kept in dry air is not nearly so likely
to decay as is that which is wrapped and packed. However, since
wrapping and packing are necessary and since infection is induced
by high respiration, the only remaining remedy is refrigeration. By
means of refrigeration both respiration and the vitality of the fungus
are greatly lessened.
’ Infection is likely to take place at any time between picking and
refrigeration, and this period should therefore be shortened as much
as possible. This matter seems to be understood by growers, and a
modern precooling plant has been erected on the steamship wharf
in San Juan. After the precooling system is put into practice brown
spotting or cold-storage pitting of the rind may become a problem.
However, it will not be serious unless the fruit is held at 40° F. or
below for some time and can be partly prevented by careful han-
ding. Spots always develop on parts of the rind which has been
slightly scratched or squeezed.
SUMMARY
In a study made of the present practice of citrus growing in Porto
Rico the origin of some of the methods now in use are traced and
those giving promise of best results are discussed. Operations for
CITRUS CULTURE IN PORTO RICO 33
which no well-defined methods have as yet been perfected in Porto
Rico are considered on the basis of practice successfully used
elsewhere.
Commercial planting of citrus in Porto Rico dates from about the
year 1901. ‘The first plantings consisted mostly of orange trees, but .
these were generally rebudded to grapefruit a few years later. The
more recent plantings consist almost entirely of grapefruit.
The area devoted to grapefruit approximated 6,000 acres in 1928,
when the estimated yield was 1,235,000 boxes of that fruit. The
quantity of oranges produced fluctuates from year to year. In 1924
less than a quarter of a million boxes of oranges were exported,
whereas the previous year the exportation was nearly three-quarters
of a million boxes. Most of this fruit was produced by trees growing
in the mountain districts, where they are used as shade for coffee
trees.
The commercial citrus groves are located mainly on soils ranging
from light beach sand to heavy clay. The sandy type of soil is
suitable for citrus culture, provided it contains a fair amount of
humus and the water table does not at any time rise to within the
soil area occupied by the larger portion of the root system.
In a preliminary study of tree stocks a chemical method was de-
vised permitting of readily differentiating one kind of stock from
another by means*of the color produced when ferric chloride was
added to an aqueous extract of the root bark.
The selection of seeds for the production of stock as well as of seed-
ling trees is emphasized, and nursery methods suitable to local con-
ditions are described. Bud selection, based upon recognized, de-
sirable trees and fruit characters, is discussed.
Field methods, based upon practical experience in Porto Rico,
are outlined. These include the provision of windbreaks for the
citrus tree and the planting of the latter on raised mounds. Planting
such permanent windbreaks as the bamboo or other similar tall,
quick-growing plants is recommended. The use of smaller plants,
erown among the citrus trees as windbreaks, also is advocated, espe-
cially for young plantings. The purpose of the latter is primarily
that of maintaining as high an air humidity among the trees as
possible. By this means scale insects, which are very destructive to
citrus trees, are kept in check by scale-destroying fung1.
The planting of citrus trees on raised mounds is described. By
the use of this method the crown roots become exposed after the
mound subsides, which effectively prevents foot rot, a disease doing
considerable damage to citrus trees planted too deep.
Fertilizing, based upon local field experience, is discussed. It is
pointed out that most local planters apply 50 pounds or more of
mixed fertilizer to each bearing tree annually, and that some apply
as much as 100 pounds to large bearing trees. The formulas of the
various fertilizer mixtures used are too variable to serve as a basis
for standardization of fertilizer formulas. The number of annual
applications is variable, but the practice of applying the fertilizer
three times a year, the first part of December, March, and June, is
desirable for bearing trees.
34 BULLETIN 33, PORTO RICO EXPERIMENT STATION
Mention is made of the destructive insects and diseases commonly
encountered in citrus groves and after the fruit has been picked,
and a brief description is given of the control measures commonly
employed for them.
Harvesting and handling of citrus fruits is fairly well standard-
ized, but some of the packing-house operations. need further per-
fecting. Coloring of fruit artificially in the early season and the
various factors affecting coloring have been studied and are discussed.
U. S. GOVERNMENT PRINTING OFFICE: $830
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