The Florida avocado industry

Material Information

The Florida avocado industry
Series Title:
Bulletin University of Florida. Agricultural Experiment Station
Ruehle, George D
Place of Publication:
Gainesville Fla
University of Florida Agricultural Experiment Station
Publication Date:
Physical Description:
100 p. : ill. ; 23 cm.


Subjects / Keywords:
Avocado -- Florida ( lcsh )
Avocado -- Diseases and pests -- Florida ( lcsh )
City of Miami ( local )
City of Homestead ( local )
Avocados ( jstor )
Fruits ( jstor )
Plant roots ( jstor )
government publication (state, provincial, terriorial, dependent) ( marcgt )
bibliography ( marcgt )
non-fiction ( marcgt )


Bibliography: p. 97-100.
General Note:
Cover title.
General Note:
"A contribution from the Sub-Tropical Experiment Station"--T.p.
General Note:
"A complete revision of Experiment Station Bulletin 272" --T.p.
Statement of Responsibility:
by George D. Ruehle.

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
000927026 ( ALEPH )
18287633 ( OCLC )
AEN7729 ( NOTIS )


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[ By George

Bulletin 602


D. Ruehle

J. R. BECKENBACH, Director
(A contribution from the Sub-Tropical Experiment Station)



INTRODUCTION ............................................. ............................... .... 3
H ISTORY ................................................................................. ....... 4
DISTRIBUTION OF AVOCADOS IN FLORIDA .........................................--- 4
Distribution factors, distribution by districts
USES OF THE AVOCADO ..........................-.....-............... .. ..--- ------. 6
B OTA N Y ........................... ...... .............................................................. ......... 7
Description, relationship, races
VARIETIES CULTIVATED COMMERCIALLY IN FLORIDA ....................................---.... 10
Competition, characteristics of desirable variety, relative impor-
tance of commercial varieties, varietal descriptions
Varieties, cold protection, treatment of cold injured trees
POLLINATION AND INTERPLANTING ....... ............................ ..................... 24
Flower behavior, insects and pollination, interplanting
observations, hand pollination
PROPAGATION .... .......... .... ...................................... ...-- ..--.....--- ...............--...... 28
Stocks, growing the seedlings, grafting, budding, growing
grafted trees in the nursery, topworking
GROVE MANAGEMENT ................. ..--... ...........-- ....-- .......---- --. ------... --------- 41
Planting distances, preparation of land for planting, time of
planting, interplanting and variety selection, planting,
weed control around young trees, soil management in bear-
ing groves, irrigation, cover crops, pruning, fertilization,
non-bearing trees, bearing trees, windbreaks, treatment of
storm-damaged trees
PRODUCTION AND MARKETING ......................................-----.. ...----. ..--------. 67
Yields, thinning, harvesting, packing, marketing, storage
D ISEASES .......................... .......... ..................................................................... 74
Cercospora spot or blotch, anthracnose or blackspot, avocado
scab, stem-end rot, nematodes, avocado root rot, minor
pathogenic diseases, physiological diseases
GENERAL SPRAY PROGRAM FOR DISEASE CONTROL .......................................... 88
INSECTS AND OTHER PESTS ....... ........................................ ....... ..... ....- ....... 89
Avocado red mite, scale insects, avocado tree girdler, thrips,
mirids, pumpkin bug, avocado leaf roller, minor insect pests
LITERATURE CITED ........................ ......................- -------........ 99

Printed November 1958 and Revised slightly May 1963


The Late Vice-Director in Charge, Sub-Tropical Experiment Station,
Homestead, Florida

Avocado production in Florida has increased steadily during
the past 25 years and has doubled approximately during the past
five years. In 1930-31 the total production was only about 33,000
bushels. During the 1955-56 season Florida shipped more than
500,000 55-pound bushels, or about 28 percent of the avocados
sold in the United States during that season (2)2. California
supplied over one million bushels, or about 63 percent of the avo-
cados sold in the United States in the 1955-56 season, with Cuba
supplying most of the balance. A few avocados are produced
commercially in the Rio Grande Valley of Texas.
While for centuries the avocado has been an important food
crop of Central America and adjacent territory, where it is in-
digenous, only a small percentage of the population in the United
States is acquainted with it. It is still considered as a luxury
crop by a large percentage of the consuming public, although it
now is obtainable at reasonable prices considering its high food
Production costs have been rising steadily. Increase in cost
of land, taxes, cost of labor and nearly everything the grower
must buy have helped to slow down the planting of new acreage
to avocados in recent years. It is increasingly important for
the Florida avocado grower to make every tree produce its ut-
most. Unproductive trees within the variety must be eliminated.
Production costs must be reduced wherever possible without
sacrificing quality and yield. The grower should concentrate on
the production of quality fruit within practical limits, including
the appeal which the fruit makes to the consumer.
With the better varieties, no finer quality avocados are pro-
duced anywhere than in Florida, if they are well grown, allowed
to mature before being picked and handled properly in transit.
The adoption of a Federal Avocado Marketing Agreement by the
avocado industry in 1954 has done much to eliminate immature
and poorly graded fruit from the market. If continued, this
SThis bulletin represents a complete revision of Extension Bulletin
141 by H. S. Wolfe, L. R. Toy, A. L. Stahl and G. D. Ruehle, published in
December 1949. Bulletin 141 was previously issued as Extension Bulletins
112 and 129 and Experiment Station Bulletin 272, and was used as a basis
for this bulletin.
2 Figures in parentheses refer to Literature Cited.

Florida Agricultural Experiment Stations

agreement unquestionably will benefit the entire industry (23).
Not only will the consumer obtain better avocados from Florida,
but grades, packs and varieties will tend to be standardized.
Many problems of avocado culture are still under investiga-
tion and much remains to be learned about the avocado and its
requirements in Florida. There is still no general agreement
as to the most satisfactory varieties, although numerous seed-
lings have been named and propagated as varieties.
Invaluable assistance has been received from many individ-
uals in the preparation of this bulletin. Grateful acknowledg-
ment is made to John D. Campbell and David M. Biggar for mak-
ing some of the data contained herein as complete and accurate
as possible and to John C. Noonan for many of the photographs.

It is probable that the avocado was introduced into Florida
by the Spaniards, but the first recorded importation was in 1833
(39) by Henry Perrine. The fate of this early plantation and
the type of avocado introduced by Perrine are not known. West
Indian type seedlings were found growing wild in hammocks
by the first permanent settlers in the Miami region. By 1900
there were several commercial groves of West Indian seedlings
established near Miami.
Late in the 1890's successful budding of the avocado was ac-
complished in Florida, and in 1900 George B. Cellon began com-
mercial propagation in the first avocado nursery (63). The first
commercial planting of budded trees was 20 acres of the Trapp
variety established near Miami in 1906 (63). Budwood of a
Mexican variety was imported from California in 1902. Seed-
lings of the first Guatemalan varieties in Florida, from seeds
the USDA introduced in 1904-06, fruited in 1911-12 (63). Im-
portations have continued from California, Mexico and Central
America, and varieties have been selected from seedlings of
these importations. An estimated three-fourths of the acreage
in Florida is now planted to varieties of Florida origin, mostly
West Indian, Guatemalan or hybrid selections.

The avocado is distinctly tropical and subtropical in its re-
quirements, and therefore is limited in its distribution. Mature

The Florida Avocado Industry

trees of some West Indian varieties may be killed by tempera-
tures of 240 F. and injured considerably at 270, while some Guat-
emalan varieties may withstand 210 and some very hardy Mex-
ican varieties endure lower than 180. Young trees are less cold-
resistant than old trees, and those in their first year cannot en-
dure temperatures within six or seven degrees of what mature
trees can stand. Duration of injurious temperatures and condi-
tion of the tree mostly determine the severity of cold injury.
Because of their sensitivity to cold, adequate air drainage
must be provided for avocados, particularly in the Ridge section
of Florida. Avocados should be planted only on the best situated
land, preferably on a slope from a lake shore. In the flatlands of
southern Florida, air drainage is less important.
The avocado is grown successfully on a wide range of soils,
including the sands of the Ridge, the limerock of Dade County
and the muck of the Lake region. It is intolerant of poor drain-
age, and cannot endure standing water for more than a day or two.
Care must be taken to set the trees in mounds in low portions of
a grove, and to avoid planting in areas subject to standing water
during the season of heavy rains. The avocado has low salt tol-
erance and should not be planted on land subject to salt intru-
sion nor irrigated with water of high salt content.
Exposure to strong winds of either the prevailing or occa-
sional type damages the fruit and tree. It is both possible and
desirable to plant a windbreak of trees, where there is no natural
protection from the winds. The extreme brittleness of avocado
wood is a sound reason for establishing windbreaks. Of more
importance, they also reduce evaporation loss during dry seasons,
especially during the blooming and fruit setting period, and re-
duce loss from bruising and dropping of nearly mature fruit.
Rainfall is not a limiting factor in Florida except on some of
the keys. For one or two years after planting, avocado trees
often need supplementary irrigation during dry periods, but
mature trees seldom require irrigation for thrifty growth and
fruit production. The fruit is not harmed by rains during ripen-
ing, and the flowering period is extended sufficiently that occa-
sional rains usually do not affect adversely the setting of fruit.

Under the Federal Avocado Marketing Agreement, the pro-
duction area of the state is divided into two districts. District
1, or Dade County, produces more than 90 percent of the Florida

Florida Agricultural Experiment Stations

avocados, and District 2 includes the rest of the state where avo-
cados are grown commercially. Bearing trees six years of age
or older in District 1 comprise 8,753 acres, with approximately
245 acres of non-bearing trees (4). An estimated 500 acres of
commercial plantings are in District 2, located mostly in High-
lands, Pinellas, Hillsborough and Polk counties. Occasional small
groves are scattered throughout the remaining counties of south-
ern Florida, but most trees in those counties are in dooryard
plantings, especially in Tampa, Fort Myers, West Palm Beach,
Orlando and other cities. There are also scattered single trees,
mostly seedlings of the hardy Mexican race, in nearly every
county in the peninsula, as far north as St. Augustine and Gaines-
Considerable land where air drainage is good enough to per-
mit successful avocado growing is still available in the ridge sec-
tion and in other counties with sand soils. Southern Dade County
still has thousands of acres of potential avocado land. In this
district, however, high land prices and taxes have practically
stopped the planting of new avocado acreage in recent years.

The avocado is used widely as a salad fruit in the United
States, where it is prized for its delicate, nut-like flavor and its
smooth, buttery consistency. In its tropical home it is one of
the most important items of the daily diet when in season. As
its high food value becomes better known, it should find a much
larger place in the diet of the American public.
A popular way of serving the avocado in Florida is "on the
half shell," or sliced, with the addition of line juice, salt or salad
dressing. Because of its oil content it need not be served with
an oil dressing. The fruit is frequently used as an ingredient
of salads of the Waldorf type, or mixed with grapefruit or orange.
Mashed and seasoned avocados are used as a sandwich filling
or as a spread on salted crackers, and in the tropics they are
often added to soups just before serving. Fully ripe fruit has
been used successfully to make avocado ice cream, and this offers
some promise as a minor outlet for disposal of good quality fruit
but unsalable because of poor appearance.
Cooking detracts from both flavor and appearance of the
avocado, but satisfactory frozen products can be prepared in
several ways. Cruess (11) and his co-workers in California
have reported that the macerated flesh can be mixed with either

The Florida Avocado Industry

sugar or vinegar, frozen quickly and stored successfully in glass
containers at 150 F. or lower. The sweetened, frozen pulp can
be used in ice cream and the vinegar and pulp mixture can be
used in sandwich spreads and salads. Macfie and Stahl (28)
recently reported satisfactory storage of quick frozen macerated
avocado pulp mixed with salad dressing, salt and spices in enamel
lined cans which were frozen immediately after filling and seal-
The avocado possesses nutritive values far exceeding other
fresh fruits. Compared with the dessert fruits, the avocado is
higher in ash and protein and lower in sugars. It is much higher
in oil content than other fruits, with the exception of the ripe
olive. There is wide variation in the fat content of the fresh
pulp, ranging from 3 to 30 percent. The high caloric value and
gery low carbohydrate content of avocado pulp makes it espe-
cially suitable as a source of energy for diabetics.
Mattill (32) found avocado fat as digestible as butter-fat,
although not similar in composition. Weatherby and his associ-
ates in California (59) have confirmed the findings of Santos
(46) that avocados are an excellent source of vitamin B and have
further shown the vitamin A content to be good (61). They
reported traces of C, fair amounts of D and good amounts of E
to be present. These tests have been made only on the Fuerte
variety, and the vitamin content is likely to vary with the oil
content of the variety (60).
Expressed oil of the avocado is very similar in chemical com-
position to olive oil, and is adapted to similar uses. Its high
cost makes its use in soap manufacture impractical, but in recent
years there has developed in California a small commercial utili-
zation of avocado oil in cosmetic manufacture.

The avocado tree varies, depending on variety, from short and
spreading to tall and slender. The wood is light in weight and
quite brittle, so that the tree is injured easily by storms. While
classed as evergreen, trees of some varieties lose all or nearly all
of their leaves at the time of flowering. New leaves appear very
soon from the terminal shoot of the inflorescence. Trees of other
varieties tend to shed their old leaves gradually over a longer
period during the spring and are never completely leafless.

Florida Agricultural Experiment Stations

Young leaves are often finely pubescent and more or less
reddish or bronze, but at maturity are smooth and green. Ma-
ture leaves are leathery, from a few inches to 16 inches in length,
and vary in shape from oval to lanceolate or elliptic. Leaf tips
usually are acute, but are obtuse in a few varieties, and the
base cuneate.
The pale green or yellowish green flowers, about one-half
inch across when fully open, are produced on a much branched
terminal inflorescence, the central axis of which usually is ter-
minated by a shoot. The flower is perfect, with 12 stamens, 9
of which are functional, each having 4 pollen chambers. The
single pistil has one carpel with one ovule. Calyx and corolla are
distinguishable only by position and are in reality perianth lobes.
All parts of the flower are more or less covered with fine hairs.
The fruit is a berry consisting of a single large seed with a
thick, fleshy pulp. Skin thickness and texture vary according
to variety. The fruits of cultivated varieties vary in size, shape,
color and other characters. They usually are pyriform to round
or oval, but sometimes are cucumber- or gourd-shaped. In weight
the fruits vary from a few ounces to a few pounds. The color
ranges from light yellowish green, through dark green, maroon,
brown to purplish black.
The pulp of the edible portion when ripe is of a buttery con-
sistency, creamy to bright yellow in color, often greenish near
the skin. It contains a considerable amount of vegetable oil, from
3 to 30 percent in different varieties. The matured seed has
2 parchment-like coats and 2 large hemispherical cotyledons en-
closing the small embryo, located at the end of the seed farthest
from the stem but directed toward it. The seed is tight in the
cavity in some varieties and loose in others. It may be round,
oblate, spherical or conical. The cotyledons may be smooth or
roughened on the surfaces according to variety.

The avocado is Persea americana Miller (P. gratissima
Gaertn.) of the laurel family, Lauraceae. It is native to Central
America and adjacent portions of North and South America.

Three general groups of avocados have been recognized for
horticultural purposes. These are loosely termed West Indian,
Guatemalan and Mexican races, with the latter considered by

The Florida Avocado Industry

some botanists as a distinct botanical variety, P. americana var.
drymifolia Mez. The races were classified by Popenoe (39) in
accordance with the following key:
1. Leaves anise-scented; skin of fruit thin (rarely more
than 1/32 inch in thickness)..........................Mexican race
2. Leaves not anise-scented; skin of fruit thicker (from 1/32
to 1/4 inch in thickness)
a. Fruit summer and fall ripening; skin usually not
more than 1/16 inch thick, leathery in texture
---------------------....................................West Indian race
b. Fruit winter and spring ripening; skin 1/16 to 1/
inch thick, woody in texture-...........Guatemalan race
This scheme of differentiation is becoming constantly less use-
ful, both because of the increasing number of hybrid varieties
and because intermediate types between Guatemalan and Mexi-
can groups have been found growing wild in Mexico. Popenoe
and Williams (40) report finding wild forms with the hard skins
of the Guatemalan race, but with anise odor in the leaves.
Schroeder (47) on the other hand reports the findings of wild
avocados of an otherwise Mexican type but lacking anise odor
in the leaves. It thus appears that wild forms exist which can-
not be classified in accordance with Popenoe's scheme and that
gradation in characters can be found between the Mexican and
Guatemalan groups.
Among cultivated varieties, fruits of the West Indian va-
rieties ripen six to nine months from blossoming, are borne on
short stems and have smooth leathery skins, which in green-
skinned varieties tend to be yellowish green at maturity. The
seed is comparatively large and somewhat loose in the cavity,
with the seed coats usually separated and the cotyledons rough
on the surface.
Fruits of the Guatemalan varieties ripen nine to 12 months
after blossoming, are borne on long stems and have thick rather
woody, usually roughened skins, which in green-fruited varieties
usually is dark green. The seed tends to be small and tight in
the cavity and the cotyledon surfaces are smooth or nearly so.
Fruits of the Mexican varieties are smaller than that of the
other races, rarely exceeding one pound and in most varieties
averaging eight ounces. The skin is nearly always thin and
smooth. The seed is comparatively large with thin seed coats
and smooth cotyledon surface.

Florida Agricultural Experiment Stations

The selection of a variety or varieties for planting in com-
mercial groves is of primary importance. The problem has be-
come of increasing perplexity in recent years.

Florida avocados must meet the competition of Cuban and
California avocados. Cuban avocados mature mainly from July
to September. California avocados are available throughout
the year, but supplies are heaviest from January until July.
From the competitive viewpoint, the best market period for Flor-
ida avocados is from August to January. The planting of va-
rieties maturing fruit from October to January has been so
heavy during the past decade, however, that surpluses with
attendant low prices occur in Florida during this period. The
advisability of planting more avocados maturing fruit during
this period is doubtful until a better system of distribution in
the marketing of Florida avocados is developed.

The general characteristics of both tree and fruit should be
considered in choosing a variety for commercial planting. The
tree should be a regular and heavy producer, but yields should
not be so heavy that expensive thinning is required and tree
exhaustion and dieback are caused. The tree should be readily
propagated, make satisfactory growth in both the nursery and
the grove and develop a spreading, reasonably low head without
excessive pruning. It should be fairly tolerant to cold and rea-
sonably resistant to diseases and insects.
In considering the fruit itself, market preference and ac-
ceptance can scarcely be overemphasized. Thus far the markets
appear to prefer medium sized fruits weighing. from 10 to 18
ounces, but there is a limited demand, especially in certain mar-
kets, for large-fruited varieties. The fruits should be fairly
uniform in shape, preferably broadly pyriform or oval, with flesh
free of objectionable fibers and with a small seed which is tight
in the cavity. Good edible quality appears to be of less impor-
tance than attractive appearance. The skin should be smooth
to slightly pebbled and preferably dark green rather than purple
or black. The fruit should not be harmed by refrigeration or
by temperatures near the freezing point in the grove.

The Florida Avocado Industry

No single variety now grown in Florida possesses all of the
desired ideal characters.

Lula is the leading variety and accounts for one-fourth to
one-third of the total commercial production. From 70 to 85
percent of the total production is accounted for by Lula and nine
other varieties, namely Booth 7, Booth 8, Booth 1, Booth 3,
Waldin, Pollock, Hickson, Taylor and Trapp. Seedlings and some
40 other varieties make up the remainder.
Of the leading 10 varieties, Lula, Booth 7, Booth 8, Waldin
and Pollock probably will maintain their present positions, with
some of them increasing in importance during the next decade.
Other varieties now of commercial importance and likely to
remain so for some time, but with gradual decrease in produc-
tion, include Hickson and Taylor. Others in the same category
but less likely to maintain their present position include Fuchsia,
Booth 3 and Collinson. These varieties are not being planted
now and in some groves are being topworked to other varieties.
Varieties still of commercial importance but being eliminated
by topworking include Booth 1, Trapp, Linda and Wagner.
Varieties not now grown extensively but likely to increase
in importance during the next decade include Monroe, Hall and
Choquette. These varieties produce large fruits which find
acceptance in special markets. Tonnage, which matures its fruit
between the peak of Waldin and Booth 8 production, probably
also will increase in importance.

Varieties likely to maintain their present position or to in-
crease in importance during the next decade are described be-
low. The reader is referred to checklists of avocado varieties
published in earlier bulletins for descriptions of other varie-
ties (63).
Season of fruit maturity in any variety varies somewhat
with the locality, time of fruit setting and weather during fruit
development. Consequently, only an approximate season of
maturity is given.
The figures given under each variety for the oil content indi-
cate, wherever known, the normal average values for the be-
ginning and end of the given season of maturity. Where there

Florida Agricultural Experiment Stations

are no data as to seasonal range, the oil content value is the aver-
age of a number of analyses of mature fruits.

Fig. 1.-Fruit of the Lula variety.

Lula (Fig. 1).-Originated on the place of George B. Cellon,
Miami, Florida, from a seed reported to be from the original
Taft tree at Orange, California. The Taft tree is Guatemalan,
but the pollen parent is unknown. Fruited first in 1919. Propa-
gated commercially in 1921. Fruit pyriform or occasionally
necked, medium large, 14 to 24 ounces, skin nearly smooth, light
green. Flesh pale to greenish yelow, of very good flavor. Seed
large but tight. Oil content 6.0 to 15.0 percent. Season October
10 to January 15. The variety is precocious and productive.
The tree is a rapid and thrifty grower with a tall upright habit.
Both foliage and fruit are highly susceptible to avocado scab,
but this can be controlled by timely applications of fungicides,
if trees are spaced far enough apart to permit effective spray
coverage. The fruit has a tendency to sunburn if held on the
trees late in the season. The variety has proved satisfactory
both in the ridge section and on the lower East Coast, in spite

The Florida Avocado Industry

of its susceptibility to disease and its tall habit of growth. The
fruit can be refrigerated successfully.

Fig. 2.-Fruit of the Booth 7 variety.

Booth 7 (Fig. 2).-Originated in grove of Will Booth, Home-
stead, Florida, as a seedling of an unknown Guatemalan parent
in mixed planting with West Indian avocados. Seed planted in
1920. Fruited first in 1927. Propagated commercially in 1935.
Fruit rounded obovate, medium size, 10 to 20 ounces. Skin
glossy, bright green, slightly pebbled, thick, woody. Flesh light
yellow, of good flavor. Seed medium, tight. Oil content seven to
14 percent. Season October 15 to December 15. The tree is a
thrifty grower with a spreading habit. The variety is prolific
with most of the fruits borne singly. Many trees will overload
and show exhaustion and the variety tends to alternate bearing.
A serious fault is the pronounced tendency of the fruit to drop
following a drop in temperature to 38 F. or lower. The fruit
will withstand moderate refrigeration.
Booth 8 (Fig. 3).-Origin and history the same as Booth 7.
Fruit oblong-ovate, small to medium large, nine to 28 ounces.
Skin rather dull, medium green, slightly roughened, rather thick
and woody. Flesh light cream color, of fair flavor. Seed medium
large, tight. Oil content six to 12 percent. Season October 1
to December 15. The tree is a thrifty grower with a spreading
habit. The variety is precocious and prolific with much of the

Florida Agricultural Experiment Stations

Fig. 3.-Fruit of the Booth 8 variety.

Fig. 4.-Variations in size and shape of Booth 8 fruits of the same age
from the same tree. Fruits on the left are from a cluster of five; fruit on
the right was borne singly.


The Florida Avocado Industry

fruit borne in clusters. The trees often bear so heavily that they
show exhaustion with loss of foliage, sunburned bark and die-
back of branches, and require from one to three years to re-
cover. Young trees allowed to bear much fruit may die. The
tendency to alternate bearing is strongly developed. Heavily
laden branches are subject to breakage by wind and produce
smaller than average fruit (Fig. 4), unless carefully propped and
the fruit drastically thinned. Propping by itself is often unsatis-
factory, since many of the fruits do not meet size restrictions
early enough to be harvested in time to avoid defoliation, sun-
burn and dieback of branches. Trees exhausted from carrying
an excessive crop are very susceptible to cold damage. The har-
vested fruit will withstand a moderate amount of refrigeration.

Fig. 5.-Fruit of the Waldin variety.

Waldin (Fig. 5).-Originated on the place of B. A. Waldin,
Homestead, Florida, from a seed planted in 1909. Fruited first
in 1913. Propagated commercially, in 1917. Fruit oblong to
oval, with a characteristic flattening on one side of the blossom
end, medium to large size, 14 to 28 ounces. Skin smooth, pale
green to greenish yellow. Flesh pale to greenish yellow, of good
flavor. Seed medium to large, fairly tight. Oil content five to
10 percent. Season September 1 to November 1. The tree is
precocious, productive, with a somewhat spreading habit, and is
hardier than most varieties of ,he West Indian race. There is a

Florida Agricultural Experiment Stations

marked tendency toward bearing the fruit in clusters, and such
fruit usually is small and sunburned. Fruit should be thinned
carefully to encourage normal development. Despite the tend-
ency to overbearing and consequent sunburning of exposed fruit
and dieback of limbs, this variety probably is the best for its
season. The fruit does not refrigerate well.

Fig. 6.-Fruit of the Pollock variety.

Pollock (Fig. 6).-Originated on the place of H. S. Pollock,
Miami, Florida, some time prior to 1896, in a seedling plantation.
Propagated commercially in 1901. Fruit oblong to pyriform,
large to very large, 18 to 40 ounces, but often larger if allowed
to remain on the tree after the normal shipping season for the
variety. Skin smooth, light glossy green. Flesh a rich yellow,
blending with green near the skin, of very good quality. Seed
large, often loose in the cavity. Oil content three to five percent.
Pollock is generally considered to be the best variety now avail-
able for its season, July 15 to September 1. The tree is a vig-
orous grower with a spreading habit, but is rather shy in bearing
and lacks cold resistance. Although too large for general mar-

The Florida Avocado Industry

ket purposes, the fruit usually has commanded a good price be-
cause of its good quality and early season. It ships better than
Fuchsia and the tree is thriftier than Simmonds with less tend-
ency to shed fruit. Fuchsia and Simmonds are large-fruited
West Indian varieties with approximately the same season of
maturity as Pollock. Pollock fruit will withstand some refrig-
eration at 420 F.

Fig. .-Fruit of the Hickson variety.
Fig. 7-Fruit of the Hickson variety.

Hickson (Fig. 7).-Originated in grove of J. R. Hickson, Nar-
anja, Florida, as a seedling of unknown parentage. Fruiting first
observed in 1932. Topworked in 1933 and since then in many
groves. Nursery propagation in 1938. Fruit obovate, medium
small, 16 to 20 ounces. Skin green, slightly rough, thick, brittle.
Flesh light yellow, of fair to good quality. Seed small, tight.
Oil content eight to 10 percent. Season November 1 to December
15. The tree is vigorous and upright in habit. It is heavy bear-
ing in alternate years with fruit hanging well in windstorms but
lacking cold resistance. Considerable trouble from bruising,
fruit spotting and decay in transit has been reported by some
shippers. The fruit does not refrigerate well.

Florida Agricultural Experiment Stations

Taylor (Fig. 8).-Originated from a seed of the Royal planted
in 1908 at the USDA Plant Introduction Garden, Miami, Florida.
Fruited first in 1913. Propagated commercially in 1914. Fruit
obovate to pyriform, rarely necked, small to medium size, 12-18
ounces. Skin rough-pebbled, dark,green. Flesh light yellow,
rather moist, of very good flavor. Seed medium size, tight. Oil
content 12 to 17 percent. Season November 15 to January 15.

r i

J __

Fig. 8.-Fruit of the Taylor variety.

One of the first Guatemalan varieties to be planted commercially.
The tree comes into bearing early, is vigorous and is the most
resistant to cold of the commercial varieties in Florida. The
tree has a tall, slender habit which can be modified by judicious
pruning when the trees are young, if proper spacing between
trees is provided at planting. Usually the fruit is borne singly
and is well distributed over the tree, but yields have been di-
appointing in some groves on the Lower East Coast. Tipburn is
common on this variety during drought periods. The fruit,
when free of disease, usually brings a good price and can be re-
frigerated successfully.

The Florida Avocado Industry

Fig. 9.-Fruit of the Tonnage variety.

Fig. 10.-Fruit of the Monroe variety.

Florida Agricultural Experiment Stations

Tonnage (Fig. 9).-Originated in grove of Dan Roberts,
Homestead, Florida, as seedling of Taylor. Seed planted in 1916.
Fruited first in 1921. Propagated extensively by topworking
in grove of originator, and on a small scale in other groves from
about 1930. Fruit pyriform with stem placed obliquely, often
necked, medium large size, 14 to 24 ounces. Skin dark green,
pebbled, rather glossy, thick. Flesh pale yellow, of good flavor.
Seed medium, fairly tight. Oil content 8 to 10 percent. Season
September 15 to November 1. The tree is a vigorous grower,
with rather tall upright growth, but not as slender as Taylor.
Monroe (Fig. 10).-Originated in grove of J. J. G. Phillips,
Homestead, Florida, as seedling of unknown parentage in mixed
planting of Guatemalan and West Indian varieties. Fruited
first in 1935, and topworked extensively in Phillip's grove in
1935 and 1936. Plant patented No. 261 in 1937. Fruit large,
elliptical, flattened obliquely toward apex on one side, weight 24
to 40 ounces. Skin glossy, dark green, finely pebbled, mod-
erately thick and pliable. Flesh light yellow, of good quality.
Seed medium size, tight in cavity. Oil content 10 to 14 percent.
Season November 1 through January. The tree has a spreading
habit. The variety bears heavily and the fruit is handsome.
The first Florida variety to be patented, it was not offered to
other growers until 1956.
Hall (Fig. 11).-Originated on place of Willis Hall, Miami,
Florida, as seedling of unknown origin. Fruited first in 1937.
Propagated in 1938.. Fruit pyriform, 20 to 30 ounces. Skin
nearly smooth, dark green, fairly thick. Flesh deep yellow of
good flavor. Seed medium large, tight in cavity. Oil content
10 to 16 percent. Season November 1 to January 15. The tree
is a vigorous grower with a rather spreading habit, moderately
hardy to cold. The variety bears heavily in alternate years
and tends to bear fruit in clusters. It is susceptible to avocado
scab. The handsome fruit is too large for general market pur-
pose, but commands a good price on special markets that prefer
large fruit.
Choquette (Fig. 12).-Originated on place of R. D. Choquette,
Miami, Florida, from seed of unknown origin. Planted in Janu-
ary 1929. Fruited first in 1934. Propagated in 1939. Fruit
oval, somewhat flattened obliquely toward apex on one side,
24 to 40 ounces. Skin nearly smooth, glossy, dark green to light
green late in season, somewhat leathery. Flesh thick, yellow,
of very good flavor. Seed medium size, tight to fairly tight in

Fig. 11.-Fruit of the Hall variety.

Fig. 12.-Fruit of the Choquette variety.

Florida Agricultural Experiment Stations

cavity. Oil content eight to 13 percent. Season December 1 to
February 1. The tree is vigorous with a spreading habit and
is fairly regular in bearing.

Observations have been made on the relative hardiness of
most of the commercially grown avocado varieties in Florida.
While it is not possible to give definite limiting temperatures for
each variety, it is possible to rank them comparatively.
1. The group hardiest to cold includes Taylor, Lula and
Winter Mexican.
2. Next hardiest are Nabal, Hall and Tonnage.
3. A third group includes Booth 7, Booth 1, Booth 3, Monroe,
Wagner and Choquette.
4. Closely following group 3 are Booth 8, Waldin, Hickson,
Collinson and Linda.
5. Least hardy are most West Indians, such as Pollock,
Fuchsia and Trapp.
Exceptions are always found in this ranking, because of pe-
culiar local conditions and variations in dormancy, age and vigor
of trees. Observations of 12-year-old Booth 8 trees in 1956 in a
commercial grove illustrate the effect of tree condition on cold
damage. A freeze occurred on January 15, with an official tem-
perature of 270 F. recorded at the Experiment Station 1.5 miles
away. Trees which had carried a heavy crop to maturity and
showed pronounced evidence of exhaustion were severely killed
back, in some cases to framework branches four to six inches
in diameter. Adjacent trees which had borne a very light crop
and were in good condition when the freeze occurred lost some
leaves and terminal buds but sustained no wood damage and
recovered in time to produce a crop in the next season. In the
same grove a low of 29 F. was recorded on February 6, 1947.
Trees of the fairly hardy Taylor variety, less than two years of
age, in a flush of new growth, were severely damaged or killed
outright; but dormant trees of the same age and variety lost
only some leaves and terminal buds and recovered quickly.
Fruits of some varieties are on the tree during the period of
greatest frost hazard and may show more or less freeze injury.

The Florida Avocado Industry

Varieties differ in the degree to which the fruit withstands low
temperatures. Fruits of Booth 7 and Booth 1 usually fall very
soon after a severe frost. The fruit stems may freeze and cause
the fruit to drop. Fruits of Booth 8, Lula and Taylor show less
tendency to dorp. The flesh of the fruit itself may show frozen
areas which turn gray to brown, develop undesirable flavors and
may fail to soften normally after the fruit is harvested.

Young trees of all varieties are easily injured by low tempera-
tures and should be protected during the winter for several
years after planting. Numerous observations have shown that
clean cultivation during the first year or two helps prevent cold
damage. Bare ground absorbs more heat during the day than
ground shaded by weeds or trash.
Newly established trees the first winter usually are small
enough to be covered with a bean hamper during cold periods
for protection against frost. Trees too large for the use of bean
hampers may be covered with slatted tripod screens or shades.
The trunks can be protected against mild freezes by wrapping
with a suitable insulating material or, in sandy areas, by banking
with soil. In unheated groves, even where the top may be killed,
the insulated or banked trunk is often saved for the development
of a new top the following year.
Where available in quantity, dried grass has proved a satis-
factory insulating material. Fiber glass, rock wool and similar
insulating materials held in place with kraft paper may be used.
The insulation must be provided before cold weather occurs,
preferably early in November.
To insulate the tree trunks with dead grass, tie a fairly tight
layer of grass two or three inches thick around the trunk, from
the ground well into the lower framework of branches. As soon
as danger of frost is over, the grass should be removed and it
can be used as mulching material.
Orchard heaters are effective protection for the tops of both
old and young trees. Freezes in District 1 are not frequent or
of long duration, however, and the practicality of investing in
heating equipment is questionable. In much of District 2, where
freezes are more severe and of more frequent occurrence, frost
protection should be considered a necessary part of the grove
management program. Oil burning heaters should be provided
at the rate of about one heater per tree.

Florida Agricultural Experiment Stations

Trees which lose all foliage, but sustain little or no injury
to the branches, usually recover very quickly without special
care. A new crop of leaves develops quickly and, if extensive
damage to buds has not occurred, the trees may even blossom
and set some fruit in the season after the frost injury.
Cutting out dead wood from trees which have suffered heavy
damage should not be done until late spring or summer following
the injury, to make certain that all dying back has ceased. All
cuts should be made into living wood and, where possible, at
crotches or at buds to avoid the development of dead pruning
stubs. Cuts of three inches or more in diameter should be
treated with a water-repellant paint. In case the damage is
very severe and it appears necessary to remove the frozen wood
back to the main framework branches later, whitewashing the
bark of the trunk and larger branches soon after the freeze will
aid in preventing sunburn.
The vigorous shoots appearing after heavy pruning should
be thinned out to form a framework of well-spaced branches in
the new top. Well-balanced fertilizers should be applied at the
regular,time with amounts reduced in accordance with the ex-
tent of the damage.

The avocado has perfect flowers, each capable of producing
pollen and of developing into a fruit. An exception, the Collin-
son variety, has flowers unable to produce any pollen at all
(55). Nirody (37) in 1922 and Stout (53, 54) in 1923 and 1924
showed that the stamens and pistils of the avocado flower nor-
mally mature at different times and flowers of a particular
variety have two distinct periods of opening instead of one.
A new set of flowers opens for the first time every day on every
tree, but in some varieties this occurs in the morning (Class A)
and, in the other varieties, in the afternoon (Class B).
On a tree of an A variety, under favorable weather conditions,
Stout found the flowers opening for the first time at some hour
of the morning, depending on the variety, as Nirody had re-
ported. All these flowers closed without shedding pollen and at
some time during the afternoon another set of flowers opened
and shed pollen; the first set did not open for pollen shedding

The Florida Avocado Industry

until the afternoon of the second day. Thus, every day on every
A variety tree in the morning a fresh set of flowers opened to
receive pollen and in the afternoon a set to shed pollen, which
had opened the morning of the day before. In B varieties the
cycle was the same as Nirody had reported, the same flowers
opening in the morning to shed pollen which had opened the
previous afternoon to receive it.

WAGNER ... ..
TAFT **.*..
PuNlES o .. ........._
C 1 6cHMMj4D _* __* **
M N LD D *N *.
5cHMIDT *r ***
WINSLOWsoN __ ...
To 7v tm/c6 ...... -
LiNDA ............

tA "PP
Fig. 13.-Normal daily bloom openings for various varieties of avocados.
--- =first or "female" opening of flowers-receptive to pollen.
-- second or "male" opening of flowers-no pollen being shed.
.. = second or "male" opening of flowers-pollen being shed.

In Fig. 13 the normal midseason behavior of a number of
varieties with respect to flower opening, adapted largely from
Nirody and Stout, is summarized.
Stout (53) found that in California sudden changes in tem-
perature and cloudiness may cause much irregularity in flower
behavior, so that overlapping of sets of flowers often occurs.
Clark (5, 6, 7) also reported extensive overlapping of flower sets
in the coastal areas of California and that Fuerte proved self-

Florida Agricultural Experiment Stations

fruitful even when tented by itself. Stout and Savage (55)
found that in Florida overlapping occurs, as in California, as a
result of sudden drops in temperature and cloudy and rainy
periods. They pointed out, however, that, while such overlap
seemed favorable to close-pollination, conditions inducing it were
unfavorable to pollination because insects were more sluggish
and pollen and stigmas were less able to function normally.
The following list contains varieties likely to be found in Florida
groves, classified with respect to flower behavior.
Class A varieties Class B varieties
Avon Ajax
Booth 1 Booth 3, 5, 7, 8
Choquette Hall
Collinson Hickson
Fuchsia Linda
Lula Nabal
Monroe Pollock
Peterson Tonnage
Taylor Trapp
Wagner Winter Mexican

A number of insects are helpful in pollinating the avocado.
Various flies, wasps and other insects have been observed at
work in Florida avocado groves. Bees are very fond of avocado
nectar. Probably hives of bees in the avocado grove would assist
in obtaining a set of fruit. Investigations by Wolfenbarger
(65) indicate that this is true. Good crops are set consistently
in groves considerable distances from any bee hives, but it is
probable that a hive of bees per acre, with five set in the middle
of each five-acre tract, would materially increase production.
There are few large groves planted uniformly enough to make
adequate testing possible.

Because of the impossibility of getting a reasonable percent-
age of fruit from hand pollinations and the length of time and
amount of equipment required for extensive tenting of recipro-
cal varieties, few data are available on compatibility of different
varieties for pollination. Observations reported on the general
need of interplanting to provide pollination are somewhat con-
Clark and Clark (7) reported no benefit to either variety in
California, when Fuerte and Challenge, Fuerte and Blakeman,

The Florida Avocado Industry

Fuerte and Spinks, or Tingley and Harman were top-worked to-
gether, although these pairs are all reciprocal in flower behavior
and abundant insect activity was manifest. Likewise a test
made by Mrs. W. J. Krome at Homestead in 1932, where Linda
and Taft were tested together with a hive of bees at one end
of the test, indicated no mutual benefit.
Robinson and Savage (41) observed in Florida marked in-
crease in yields when a reciprocating variety was provided. A
solid block of Fuertes had been unfruitful for several years.
When Spinks trees came into flower on one side of the Fuerte
planting, there was a definite gradation in Fuerte crop setting,
highest next to the Spinks trees and diminishing steadily with
distance from them.
Lula was reported as an effective pollinizer for Pollock in
one commercial Florida grove (12), but Pollock has remained a
shy bearer in some other groves although interplanted with Lula.
In Florida the varieties Lula, Taylor, Waldin and Trapp are
reported to have fruited about as well in solid block plantings as
in mixed plantings. The Sub-Tropical Experiment Station has
endeavored to collect accurate data on the need for interplanting
reciprocating varieties, by actual counts of fruits of hundreds of
trees in various groves. From these data and from extensive
observations it seems clear that, while maximum yields cannot
always be expected from solid plantings, nevertheless there is
not necessarily an advantage in having reciprocating varieties
closer than three or four rows. There seems also to be some
difference between varieties with respect to their ability to set
crops in more or less solid plantings. Waldin, Trapp, Lula and
Taylor seem to be able to do this; while Wagner, Linda and the
self-unfruitful Collinson have more need for interplanting. If
pollination requirements of a variety are not known, it prob-
ably should not be planted more than 100 feet from trees of
another variety that sheds pollen at the right time. Since
Florida growers must plant several varieties to extend the mar-
keting period, interplanting of reciprocating varieties is no handi-
cap. Probably certain varieties will cross-fertilize more readily
than others, but at present there is not sufficient evidence to
recommend combinations which will guarantee unqualified suc-
In California Hodgson (21) has concluded that interplanting
with varieties of two reciprocal groups to provide for cross-
pollination has not been necessary or beneficial.

Florida Agricultural Experiment Stations

Several investigators have attempted to obtain fruit from
controlled hand crosses (37 41, 53, 56). From the many thou-
sand hand pollinations made, only about a half dozen new hybrids
of definitely known origin were obtained and none of these
has proven desirable for general planting. Hand pollination as
a means of developing new varieties does not appear to be a
promising field of activity.

The avocado was propagated exclusively by seed until about
60 years ago, and in tropical countries that is still the usual
method; but as seedling groves have great variability in quality
and quantity of fruit produced by different trees, vegetative
propagation must be employed to insure reproduction of any
given variety. Cuttings of avocado have proven difficult to
root, although experiments along this line have not been very
extensive. Grafting and budding are the methods of propaga-
tion employed for the avocado by nurserymen in Florida.

Little information is available concerning racial and varietal
compatibilities of stock and scion. Interest in stocks in Florida
is based mainly on the uniformity and vigor of growth of the
seedlings for grafting, as well as vigor of growth they support
in the mature tree. There is no accurate information to show
that Mexican or Guatemalan stocks impart more cold resistance
to mature trees than West Indian stocks. Seedlings of the
Mexican race are the preferred stocks in California, because of
their greater resistance to frost (21). If the temperature be-
comes low enough at the surface of the soil to kill wood of the
stock, these young trees in the nursery or the grove may be in
slightly less danger than trees on West Indian stocks. Some
Mexican seedlings are of exceptionally vigorous growth, while
others are not. Guatemalan and hybrid scions appear to suc-
ceed equally well on all three races, but West Indian varieties
usually are unsuccessful on Mexican stock.
For the present, seeds of the West Indian race, either ob-
tained from locally grown fruit or imported from Cuba, are used
widely to provide nursery stocks, although seeds of any cull
fruit of any variety produced locally are also used. Experience
has shown that the relatively large seeds of Waldin and Lula

The Florida Avocado Industry

provide a high percentage of sturdy, uniform stocks and seeds
of these varieties are usually available in quantity.

The seeds should be taken from rot-free fruit, and abnormal
and undersized ones discarded. They are viable for two or three
weeks after removal from the fruit, if kept in a cool, dry place,
and they may be stored for several months at cool temperatures
in a medium which prevents them from drying out. They will
germinate sooner if the seed coats are removed. Removal of
seed coats is facilitated by first wetting the seeds and then dry-
ing them rapidly in the sun. They are planted with the part
farthest from the stem downward. Seeds should never be cov-
ered more than half an inch and commonly the apex is left pro-
truding slightly above the soil.
Seeds may be sprouted in shallow beds of sawdust, vermicu-
lite or coarse sand and transferred to containers as soon as they
have sprouted, or may be planted at once in the containers.
Plant boxes may be used as containers, but at present, practically
all stocks are grown either in paper cylinders made from 30-
pound roofing paper or in tin cans, of No. 10 to 5-quart sizes.
There are several sizes of cylinders used, the commonest being
6 to 61/2 inches in diameter and 9 to 10 inches tall. This is made
from a 22 x 12 inch sheet of paper stapled together and about
two inches of the bottom notched and turned in to form a loose
bottom which allows drainage. Holes are punched in the bot-
tom or on the sides near the bottom of cans to provide drainage.
The containers are filled with a sandy loam containing a light
sprinkling of commercial fertilizer. Good drainage and regular
watering are essential to growing seedlings successfully.
Seeds may also be planted 18 inches apart in nursery rows
in the open, when the soil is of sufficient depth and irrigation
can be provided; the seedlings are then budded or grafted in
the nursery row. This system is no longer practiced by com-
mercial nurserymen in Florida. However, some nurserymen
grow seedlings in the open successfully in plant containers where
irrigation is provided. Seedlings thus grown are shorter and
usually stockier than those grown under slatshade, but can be
topworked as successfully. Young seedlings are more suscept-
ible than older trees to injury from low temperatures and some
protection must be given to stock grown in the open when dam-
aging frosts or freezes are experienced.

Florida Agricultural Experiment Stations

Seeds begin to germinate within a few weeks. The seedlings
are usually ready for grafting two to four months after the seed
are planted (Fig. 14). Somewhat older seedlings are used for
budding, since the bark of very young seedlings is easily torn in
the process of inserting the bud.

Fig. 14.-Seedlings grown under slat shade in gallon cans three months
after the seed were planted; these still have pliable stems and are suitable
for side-grafting.

The Florida Avocado Industry

Grafting has largely superseded budding in Florida avocado
nurseries. The time required for producing a tree of planting
size is lessened considerably by grafting, and some varieties
difficult to bud-especially Taylor, Wagner and Tonnage-are
grafted easily. Also, it is much easier to find suitable graft
scions than suitable budwood, and there is less danger of buds
being shed. Stocks of lead-pencil size may be grafted by whip-
grafting and even more slender ones by side-grafting. Terminal
scions or "tips" 2 to 21/2 inches long are used and they should
be from the last matured growth, just as the terminal bud be-
gins to swell to initiate a flush of growth. However, there is
more latitude regarding maturity in selecting twigs than in se-
lecting budwood. The buds and leaves along the lower part of
the scion are cut off closely so that the wraps will fit tightly.
Either very thin waxed cloth or rubber budding strips are used
for wraps. The rubber strips give adequate pressure and ex-
clude water without danger of injurious binding of the tender
stock and scion.

Fig. 15.-Side-grafting avocados. A, seedling suitable for grafting;
B, making the incision; C, scion inserted; D, the graft wrapped with rub-
ber band.

. :. I. r

I 1

Florida Agricultural Experiment Stations

Side-grafting is an easy and popular form of nursery propa-
gation. For this it is especially important that the tissues of
the stock be succulent and the stems pliable, since the scion is
inserted in the center of the stock. It is important also that
the scion be practically free of pith. A long sloping downward
cut is made diagonally a few inches above the ground level, half
way or less through the stem (Fig. 15 B). The base of the
scion is cut to a long slender wedge by tapering the cuts on each
side. The wedge is inserted quickly into the incision in the
stock so that no cut surfaces are exposed and is held firmly by
wrapping (Fig. 15 C, D). The tops of the stock plants can be
headed back as soon as union is completed. The stock should
not be cut back completely until the graft shoot is growing vig-
orously. The final cut should be made smoothly, close to the
shoot and sharply slanting to permit complete healing without
leaving a dead stub (Fig. 16). Painting the cut with grafting
wax or other harmless impervious preparation is desirable. Dur-
ing the growth of the new graft shoot, it may be tied when five
or six inches long to the stock or to a stake to encourage upright
growth and prevent wind damage. In case the graft fails to
take, the stock still may be cleft-grafted lower down (Fig. 17).
Veneering-grafting is practically as easy as the simple side-
graft and offers a better chance for successful unions between
stock and scion under a variety of conditions. It can be employed
either on stocks of lead-pencil size with stems still soft and pli-
able or on older stocks which have passed their prime for side-
grafting. Terminal scions suitable for use in side-grafting are
commonly used on stocks with pliable stems, but wood from the
first or second growth cycles below the terminal flush appear to
be better for use on older stocks.
In preparing the stock a cut about 11/2 inch long is made
downward in much the same manner as with the side-graft,
except that is not cut so deeply. The piece is removed by a di-
agonal cut at the base, thus leaving a small notch. The scion,
which should be approximately the same caliper as the stock, is
cut to fit the notch and the edges of the cut in the stock. Tying
with rubber budding strips or with very thin waxed cloth com-
pletes the operation. Covering the exposed cut surfaces with a
soft grafting wax is advisable when older stock and scions are
used, but is unnecessary when terminal scions are grafted to
pliable stocks. After the stock and scion have united, the seed-
ling top is treated as for side-grafted stocks.

The Florida Avocado Industry

A modification called the cinchona veneer-graft method was
described and reported to have some advantages over the older
methods of side- and veneer-grafting (9).




Fig. 16.-Side-grafted avocado tree. Stock cut back to grafting site.
Note sloping cut which will heal over completely without dieback.

' i.

4W. B;^^
W )'I,,

Florida Agricultural Experiment Stations

Fig. 17.-Cleft-grafting avocados. A, making the incision; B, scion
inserted; C, completed and wrapped graft.

Formerly the avocado was propagated in Florida nurseries
usually by shield-budding, the same method used in citrus nurs-
eries. The method now is used very little in Florida, but is still
preferred in California and some other regions. Shield budding
requires much care and experience for success, but when prop-
erly done, a high percentage of avocado buds unite with the
stock. Important factors are the condition of budwood and
stock. Seedlings for budding should be in a thrifty growing con-
dition and any weak ones should be discarded. They may be
budded at any time after they have attained a caliper of %8/ to
1/2 inch, provided the cambium is actively dividing. Summer
and fall budding is the standard nursery practice in California

The Florida Avocado Industry

(21). In Florida the dry period from December through March
appears better than summer or fall for budding. Budwood
should be selected from well-matured terminal growth, prefer-
ably from twigs which are just ready to put out a new flush of
growth. The best buds are plump and show no sign of having
started growth. These are found usually on the next-to-the-last
or the last flush of growth. Budsticks should be cut from healthy
trees known to bear satisfactory crops.
The bud is cut in the form of a shield from 1 to 11/ inches
long, depending on size of stock, with a single sliding motion.
A very sharp, thin-bladed knife should be used. Success is de-
pendent largely on uniform, smooth straight cut which will make
good contact with the stock. The cut is made in the stock, when-
ever the bark slips readily, in the form of a T or inverted T,
within two or three inches of the ground. The bud should be
inserted immediately after the slit is cut and allowed to complete
the opening of the slit by its own entrance. The cut surface of
the bud should not be touched by the budder. After the bud is
inserted it should be held in place securely by wrapping with
rubber bands or strips, leaving the budeye exposed. Waxed
cloth or raffia may be used for wrapping but must be loosened
in three to five weeks to prevent binding. The tops of the stock
plants can be headed back as soon as union is completed, and
the wrapping removed entirely about six weeks after budding.
Subsequent treatment is the same as for grafted plants.
Whether budwood or scions are being cut, the propagating
materials should not be allowed to dry out. Budsticks can be
held for a time by sealing the cut end in paraffin and keeping
the stick in slightly moist sphagnum or peat moss. The more
tender tip scions should be put into this medium at once to keep
them from wilting.

Young grafted trees are fertilized with light applications of
a fertilizer rather high in nitrogen and phosphoric acid and low
in potash, such as 5-7-3 or 6-6-3 mixtures. Occasional applica-
tions of liquid manure also are beneficial. Frequent watering is
necessary for trees growing in small containers.
Overcrowding of young grafted trees under slatshade should
be avoided. During periods of heavy rain and high humidity,
foliage diseases, such as powdery mildew or Phytophthora blight,
may occur. Two or three sprayings of the foliage with a mild

36 Florida Agricultural Experiment Stations

copper fungicide, spaced several weeks apart, usually will pre-
vent losses from such diseases.
Grafted trees require little further attention until they are
large enough to plant, any time after the new top has matured one
or two flushes of growth. It is well to harden young trees by
placing them outside the slathouse for two to three weeks before
transplanting them to the field.

As has been the case in many other fruit industries, the
earlier plantings of avocados in Florida contained many unpro-
ductive and inferior seedlings and many of the propagated va-
rieties have proved unsatisfactory after extended trial. No con-
sistent, organized interest has been maintained in the study and
standardization of varieties and consequently growers are now
resorting to extensive top-working of the unsatisfactory varieties
and seedlings. No attempt should be made to top-work trees in
poor condition, unless due to a temporary lack of fertilizer. It
is more economical in the long run to replace unhealthy trees
with thrifty nursery trees than to attempt to work them over.
Formerly it was considered very difficult to graft avocado
trees of bearing age and budding was the only method used for
top-working. A method of cleft-grafting, first suggested by
Krome (24) and amplified by Elliott (14), has been very success-
ful in Florida, when the work was done during the cooler season
of the year. December and January are the best months for
grafting in Florida, but the process may be carried on until
April, although the percentage of success decreases steadily as
the season advances. Grafts have been made with success in
July, but the chances of success are too small to do this except
in emergencies. Grafting is a more detailed and slower process
than budding and involves a larger outlay for equipment. Thus,
shield-budding is still used in top-working by some growers. It
usually takes longer, however, to produce a bearing tree by bud-
ding than by grafting, and so the latter method has steadily
gained favor (Fig. 18).
For cleft-grafting scions should be chosen from more mature
growth than is used for budwood, preferably from the second or
third flush back from the tip. They should have a diameter of %/8
inch or more and preferably are cut about six to eight inches
long. The upper portion should have at least two plump, dor-
mant buds and the lower half must be fairly straight. Trees

The Florida Avocado Industry

to be grafted are sawed off one to four feet above the ground,
depending on the diameter of the trunk. If several large limbs
are present with diameters of three inches or more, the cuts
should be made on these limbs a foot or more above their junction
so that they form a framework for the new tree. Some propaga-
tors prefer to leave several "nurse" limbs, but this is not es-
sential. Experiments in which one third, two thirds and all of
the limbs have been removed at the time of top-working have
shown no striking differences in results. The cleft is made with
a saw (Fig. 19 A) and should be three to five inches deep, ac-
cording to the length of the scion. The portion of the stump
through which the cut is made should be straight and free from
knots at the sides of the cleft. A hard-wood wedge is driven into
this cleft until the trunk beneath the cut begins to split (Fig.
19 B). The sides of the cleft should be pared thinly to remove
sawdust and to make a smooth surface so that the cambium lay-
ers of stock and scion fit snugly and make close contact (Fig.
19 C).
The scions are trimmed to a wedge shape along the lower por-
tion (Fig. 19 D), leaving them slightly thicker on the edge which

Fig. 18.-Top-worked trees 12 months after the cleft-grafts were made.

-' i-.~

U."' *
Li __

, D.E
Fig. 19.-Top-working avocados by cleft-grafting. A, making the cut;
B, opening the cleft; C, trimming the cleft; D, inserting the scions; E,
removing the wedge. (See Fig. 20 for final steps in the process.)

The Florida Avocado Industry

will be on the outside of the cleft so that the cleft will not be
held open by the inner edge, preventing the cambium layers
from coming close together. A scion is placed in the cleft at each
side (Fig. 19 D) and fitted carefully into place, so that the cam-
bium layers of scion and stock coincide as fully as possible. It
must be remembered that the bark of the stock will be much
thicker than that of the scion and that it is the juncture of wood
and bark which must be brought in contact in both cases. When
the scions are tightly fitted, the wedge is withdrawn entirely
in the case of small stocks of less than three inches diameter,
leaving the pressure of the closing cleft to hold the scions in place
(Fig. 19 E). In large stumps, however, this pressure will be
too great for the scions and the wedge is withdrawn only far
enough to insure a tight holding of the scions by the cleft. The
top of the wedge may then be sawed off flush with the stump.
To prevent decay all surfaces must be coated with a grafting
compound (Fig. 20 F) and the cleft filled also (Fig. 20 G). The
scions also should be coated where they are in the cleft. A
strip of heavy paper is then tied around the stub, (Fig. 20 H)
projecting several inches above the top of the scions, and filled
with a mixture of sand and peat moss, peat moss alone or saw-
dust (Fig. 20 I). This may be watered lightly at intervals to
prevent the scions from drying out before they have united with
the stock. It is advisable also to bend a small strip of paper
over the open cylinder to give additional protection against the
sun. Holes for ventilation should be made at the top and bot-
tom of the cylinder also. These cylinders may be left in place
indefinitely, as they afford protection against cold as well as
against sun and wind.
Another method of top-working occasionally employed is to
cut off the branches a foot or two from the main crotch, or to
cut off the trunk, and to bud, veneer-graft or side-graft three or
four of the new shoots that develop near the cut end of each
stub. This method is often used to rework stumps where cleft-
grafting was unsuccessful.
In California, according to Hodgson, the standard method
of cleft-grafting is now being superseded rapidly by sawkerf
grafting. In this method, after the branch to be grafted is cut
off, the scions are inserted in clefts made by sawing so that a
thin narrow wedge of wood is removed. The scion is cut wedge-
shaped and trimmed to enable it to fit closely into the cleft so
that the cambium layer of stock and scion coincide as closely

Florida Agricultural Experiment Stations

as possible. Budding cloth strips or strong cord are then wrap-
ped around the end of the limb. Precautions are taken to cover
all cut surfaces with wax or other protective compounds and to
prevent the scions from drying out. This method should prove
successful in Florida and might be more desirable for top-work-



Fig. 20.-Final steps in top-working by cleft-grafting. F, painting the
scions and exposed stock surface; G, filling the cleft with a grafting com-
pound; H, protecting the complete graft from sun and wind; I, putting
damp peat moss in the graft collar to keep the scions moist.

The Florida Avocado Industry

ing very large stumps than the standard cleft-graft. Other
methods little used in Florida, but reported as successful in
California, are bark-grafting and side-grafting for top-working
old trees.
Whatever the method of top-working, it is necessary to ex-
amine the trees at frequent intervals to remove suckers and
prune back the rapidly growing scions to form a well-branched
and mechanically strong framework for the new top. It is ad-
visable to provide supports for the growing scions to prevent
them from breaking out. Stubs and other large cuts occasion-
ally will need retreatment to prevent the entrance of wood rot-
ting fungi.
The grafting compound commonly used in Florida is a mix-
ture of 4 parts beeswax and 1 part paraffin. Some prefer to add
a small piece of carnauba wax, about an inch in diameter, to
each pint of the above mixture to raise the melting point. This
is desirable, especially when the work is done in the warmer
spring months, but is not essential. Results equal to those ob-
tained with beeswax-paraffin mixture also have been obtained
by use of a proprietary asphalt emulsion (Treeseal). This is
no more expensive and requires no heating.
The following tools and materials are required in top-work-
ing: Carpenter's saw, adjustable pruning saw (known as Cali-
fornia saw), hand pruning shears, hardwood wedges, roll of
heavy building paper 15 inches wide, heavy twine, labels, pencil,
whetstone, mallet or hammer, very sharp thin-bladed knife, a
mixture of sand and peat-moss or of sawdust, and the grafting
compound. If the beeswax-paraffin mixture is used, a wax melter
must be added. Patented wax melters are on the market and
are well worth their cost when much grafting is being done.
Care should be taken not to apply the wax too hot.

Spacing of trees in the grove should be governed by varieties
planted, type of soil and to some extent by the shape of the
grove tract. Trees tend to grow larger in rich soil and should
be spaced more widely in such areas. Although some varieties
grow tall and slender and may be spaced closer than varieties
with a wide spreading habit of growth, it is unwise to use several
spacings in the same grove, since, if top-working to other va-

Florida Agricultural Experiment Stations

rieties is desired later, the close spacing of originally slender
trees may cause inconvenience.
In District 1 spacing of 25 to 30 feet is recommended for
trees planted on the square. Many of the older groves were
planted with closer spacing. Where the trees are planted closely,
the ground is shaded earlier and more completely. This pre-
vents humus burning so rapidly, with resultant benefits to the
roots and the water-holding capacity of the soil in young groves.
However, trees planted closer than 25 feet apart begin crowd-
ing when about 12 years old. Such crowding shades out lower
branches and gradually diminishes and raises bearing wood.
Spraying effectively for pest control becomes more difficult and
costly and harvesting costs are increased. There is no evidence
that close planting reduces the severity of hurricane damage;
in fact in the 1945 hurricane, damage was more severe in some
closely planted groves than in others with wider spacing. Grow-
ers with old groves of badly crowded trees are now confronted
with the problem of how best to correct this condition. Removal
of some of the trees and heading back the remaining ones to re-
new bearing wood closer to the ground appears to be the best
In District 2, wherd the trees usually attain larger size,
spacings of 35 to 40 feet are better than closer spacings. In
this section spacings of 171/2 by 35 feet are desirable, where
the grower is willing to remove the alternate trees when crowd-
ing in the closely spaced rows begins to occur. If windstorms
were not frequent, wider spacings than the above would be bet-
ter for all avocado sections in Florida.

In District 1, where the bulk of Florida avocados is grown,
the soil consists of a more or less hard oolitic limestone contain-
ing numerous pockets of sand or red clay. In the Redland dis-
trict the oolite crops out over the surface, so that the only till-
able soil is in potholes. Farther north sand often overlies the
rock from two inches to two feet. Early plantings were made in
deep sand or in potholes of soil. Later the practice of blasting
holes with dynamite came into vogue. Earlier groves, set among
stumps of pine forest often bore excellent crops but cultural op-
erations were difficult. It has now become general practice to
clear and scarify the land before planting. The scarifiers break
up the rock to a depth of four to six inches and leave a surface

The Florida Avocado Industry

layer of mingled rock and soil in which the roots can spread and
feed (Fig. 21). It is usually necessary to scarify the land four
times to get it into good condition. In recent years an improve-
ment in the process of scarification has been widely adopted.
First the land is cleared by a specially developed form of bull-
dozer, which cuts just below the ground level and lifts out large
pine trees with ease. The pines and other debris are pushed
into piles and burned. Then a heavy triangle scarifier is mounted
in place of the bulldozer blade on the front of the huge track-
type tractor. The rock plowed out by this scarifying plow is
ground beneath the treads of the tractor. After four or more
cuts are made, tree and palmetto roots are raked out, gathered
into piles and either burned or hauled away. Down the tree
rows loose rock and soil are then scraped aside, and plowing
is repeated until 12 inches or more of loose material are finally
achieved in the tree rows when the field is leveled, with four to
six inch depth between rows.
Trees may be planted at once, but there is some advantage
in preparing the land a year in advance of planting and growing
a cover crop or a winter vegetable crop before planting. This
gives opportunity for reprecipitation of the free lime resulting
from breaking up the lime rock, as well as increasing the organic

Fig. 21.-Preparation of limestone Rockdale soil in District 1 for plant-
ing avocados. At left (where man is standing), two cuts have been made
east and west; at right, a third cut has been made north and south. Usually
a fourth cut is made and the land leveled before the tree rows are plowed
out to more depth.


Florida Agricultural Experiment Stations

content of the soil. Increase in weed growth from such crop-
ping may offset these advantages somewhat.
On the deeper sandy soils in District 2, the same method of
land preparation is followed for avocados as for citrus (3).

Avocados may be planted successfully in Florida at any sea-
son, in sections where commercial culture is practiced, provided
precautions are taken against certain seasonal weather condi-
tions after the trees are set. More expense is entailed in set-
ting at some seasons than at others. The best season for plant-
ing avocado trees is from early April to early June. Tempera-
tures at this season favor growth and the amount of watering
required is likely to be small. The cost of watering newly
planted trees may become great when trees are planted at the
beginning of the normally dry season.
Weather conditions in September and October are likely to be
good for setting trees, since usually abundant rains occur dur-
ing these months and temperatures are lower than during the
summer. The winter months normally are the driest of the
year and watering must be done fairly regularly all winter. The
grower planning to set trees in late autumn should be prepared
to protect them from cold. In central and northern sections of
District 2, avocados should be banked like citrus trees during
winter. Fall planting is somewhat more hazardous in these
areas. Trees planted in midsummer require more care in shad-
ing than trees set in the fall or spring, but even in September
shading should be provided.

There is no reliable evidence regarding which reciprocating
varieties are most helpful mutually, and so the only general rule
which can be given is that formulated by Nirody: See that varie-
ties are planted together which will provide pollen for each other
at the time when each is ready to receive pollen. Stout has
shown that almost any B variety sheds pollen at a time of day
suitable for almost any A variety to receive it, provided the va-
rieties are not too widely separated in their seasons of bloom.
Many B varieties can also be pollinated by almost any A variety,
but some of them open too late in the afternoon for any A variety
to pollinate them with certainty. The observed normal season
of bloom is given in Fig. 22 for a number of the older standard

The Florida Avocado Industry

varieties. There are few varieties whose seasons are so widely
separated that they do not have a few days or weeks of simultan-
eous blooming, but it is better not to trust brief periods of com-
mon bloom. Early varieties ought to have early or midseason
varieties as reciprocals, and late varieties should have late or
midseason reciprocating varieties. The following classification
may be of assistance.
Early Blooming Midseason Blooming Late Blooming
Pollock Hall Booth 7
Fuchsia Lula Booth 8
Simmonds Monroe Choquette
Waldin Sdg. 27-1 Tonnage Taylor
The method to assure pollination in planting a grove is not
wholly established, but the considerable data amassed at the
Sub-Tropical Experiment Station show that alternating several
rows of one variety with an equal number of ones from the re-
ciprocating group provides adequate opportunities for pollina-
tion. As many as six or eight rows of one variety may be


WALDIN ____ ____
WA G 1owon
5CHMIDT _____ _____ _____

Fig. 22.-Normal blooming season for various varieties of avocados.
Fig. 22.-Normal blooming season for various varieties of avocados.




46 Florida Agricultural Experiment Stations

planted together the length of the grove, with three or four
rows of a reciprocal variety at either side. In this system a tree
of one variety is never more than three or four rows distant
from a tree of the reciprocal variety. Formerly, when fertilizing
and spraying were done by hand, wider middles were recom-
mended every six rows for a roadway. With the wide use of
mechanical fertilizer spreaders and with the trend to the use of
mechanical spray devices to distribute pesticides, such road-
ways are no longer needed.
Some consideration should be given to variations in disease
resistance and fungicide requirements of the different varieties
in laying out the grove. Varieties highly susceptible to avocado
scab, such as Lula, Nabal and Hall, should be planted as far as
possible away from varieties resistant to this disease. The lat-
ter require two or three fungicidal applications during the year,
whereas scab-susceptible varieties usually require twice as many
fungicidal sprayings. If resistant and susceptible varieties are
intermixed in a haphazard manner, usually all trees in a block
must be sprayed alike to save time, especially if mechanical
spray booms are used. Resistant varieties are oversprayed when
enough applications are made to susceptible varieties for ade-
quate disease control.

Fig. 23.-Planting gauge in use.

The Florida Avocado Industry

On the limerock soils of District 1 in Dade County the
trees are set higher than in other sections because of the slow
surface run-off in rainy weather. They are planted in a mound
so that the crown roots are about six inches above the general
ground level. In other cases the trees usually are set only
slightly above soil level. The improvised planting gauge shown
in Fig. 23 is made of light 1 x 11/2 inch wood and is very useful
in planting trees properly.
When trees are planted from paper cylinders, the turned-
under flaps are removed and the tree is set in place in line with
sighting stakes. The paper is slit on one side with a knife and
removed. The soil is packed firmly around the undisturbed ball
of roots and the tree watered well. When trees are planted
from cans, the cylinder of soil containing the roots can be freed
by cutting the can away with tin shears or with a special can
cutting tool (Fig. 24). Special care must be taken to avoid
leaving air pockets in the soil about the roots. A basin should

Fig. 24.-A specially made can cutter in use, left; can separated
from the ball of soil and roots, right.

Florida Agricultural Experiment Stations

be made about the tree for watering. The tree should be mulched
heavily with grass and weeds or sawdust to prevent the soil
about the new roots from drying out and heating. This mulch
should be pulled away when there is danger of frost, unless the
tree is to be covered as frost protection. Young trees with a
mulch, or with weeds growing around the base, are injured
more readily by frost than those with bare soil around them.
Frequent waterings are necessary until the trees are estab-
lished if rainfall does not keep the trees adequately supplied.
Four gallons of water per tree should be applied every three or
four days during the first two weeks, and every week for two or
three months thereafter. One inch of rain probably will give
the tree the equivalent of this amount. With a thick mulch the
loss of water is less rapid and these recommendations assume
a fair amount of mulch around the tree. By the end of three
months the trees should be able to take care of themselves,
especially if well mulched, unless a long dry period occurs dur-
ing the first year.
Shading of newly planted trees is desirable when plantings
are made at any season, but especially for summer plantings.

Fig. 25.-Drag used for clean cultivating Rockdale limestone soil
in District 1.

The Florida Avocado Industry

An economical and convenient method for shading is to drive
stakes on the south, east and west sides of the tree, just clearing
the branch spread. A burlap feed or fertilizer sack is split down
one side and stretched in V-form around these three stakes,
which should stand about four feet high. These shades give
some protection from wind as well as sun.

On newly scarified pineland, not previously cropped, weed
competition is seldom serious until the trees are three or four
feet tall, or the second or third year after planting. Weed con-
trol is easily obtained by mulching the watering basins and by
occasionally hoeing out weeds which grow through or around
the mulch. The middles can be kept relatively clean by drag-
ging with a specially built drag (Fig. 25).
On limerock pineland previously cropped to vegetables, or
which has previously supported citrus, avocado or other tree
crops, to obtain rapid growth in young avocado trees it is essen-
tial to control weeds around the trees from the start. Harkness
(18) studied weed control about young avocado trees planted in
shallow limerock and concluded that, when normal amounts of
fertilizer are used, weed competition appears to be mainly a
competition for water. Mowing did not give satisfactory con-

Fig. 26.-Typical trees from weed control test at Sub-Tropical Experi-
ment Station. A, weeds controlled by hand hoeing; B, weeds controlled by
mulching with grass; C, bermudagrass around tree mowed periodically.
The thin leaves on the top permitted sunburning of the trunk and the tree
eventually died.

Florida Agricultural Experiment Stations

trol around young trees. Bermudagrass was much worse as a
competitor than Spanish-needle, which usually is the dominant
broadleaf plant in fertilized groves. Differences between mulch-
ing and clean cultivation by hoeing was less important than
differences between good and poor weed control (Fig. 26).
While Bermuda appears to be the worst weed, other vigor-
ously growing grasses are almost as harmful. Johnsongrass, if
left unchecked, may cause the death of very young avocado
trees. Paragrass and St. Augustinegrass also may seriously
affect the growth of young trees. Dalapon used as a foliage
spray, at the rate of 20 to 25 pounds in 100 gallons of water per
acre, is highly effective in controlling these grasses. For spot
spraying of small areas use 4 ounces of dalapon per gallon of
water and wet all foliage thoroughly. It is most effective when
the grasses are growing vigorously, but before seed stalks or
seed heads are formed. It should not be applied during severe
drouth or when the grasses are dormant. The addition of a
wetting agent such as Triton B-1956 will increase the effective-
ness of the herbicide. A single application usually does not kill
out all the grass completely. For complete kill a second applica-
tion should be made. Spraying the chemical on avocado leaves
should be avoided and the spray should not be applied to the
soil within a foot of the trunks of small trees, since the chemical
may be taken up by the tree roots. In the middles, dragging will
not kill Johnson and Bermuda grasses, but instead may spread
them over the entire grove.
In limited tests to date using black polyethylene plastic films
as a semi-permanent mulch around young avocado trees in pre-
viously farmed weedy land showed considerable promise. A six
foot square piece of film placed around newly planted trees was
effective in conserving moisture and preventing weed growth
during the first year (Fig. 27). The tree obtains fertilizer and
water through numerous small slits cut in the film.
On sandy soils clean cultivation usually is practiced during
the dry season, followed by cover crops in the middles during the
summer rainy season. Mulching with sawdust or other ma-
terials supplemented by hand hoeing is practiced to control the
weeds immediately around the trees.

On the sandy soils of District 2 soil management systems in
avocado orchards differ little, if any, from those in citrus

The Florida Avocado Industry

orchards. Clean cultivation is commonly employed during the
dry winter and spring seasons to conserve moisture and elim-
inate fire hazard. With the beginning of the rainy season about
mid-May, cultivation is abandoned and a ground cover of native
weeds or grasses or of a planted cover crop is established to pro-
tect the soil from the heat of the sun and to maintain the organic
matter content of the soil. During the fall after the rains nor-
mally are over the cover crop is disked into the topsoil.

Fig. 27.-Black polyethylene plastic film placed around recently
planted tree for weed control.

On the limerock soil of District 1 such a system of soil man-
agement is impractical and unwise. After the first year or
two when more or less clean cultivation by dragging may be
practiced to reduce frost hazard, the common practice is to per-

Florida Agricultural Experiment Stations

mit a volunteer growth of grasses and various weeds to estab-
lish a permanent ground cover. This growth is mowed several
times during the summer and fall to facilitate grove operations
and is kept mowed very short during the normally dry winter
and spring. Where cover crops can be established, they are han-
dled the same way. After planting, the land never needs to be
disked or dragged, except for sowing cover crops, fire protec-
tion or reducing frost hazard to very young trees. Because the
shallow soil necessitates the roots feeding near the surface, it is
unwise to drag the soil after the trees are large enough that the
roots occupy the middles, although fire hazard may make it ad-
visable to drag a row or two around the margin of the grove.
Limerock soil is deficient in humus and every effort should
be made to add organic matter. Permanent mulching gives ex-
cellent results with young trees. Grass and weeds growing in
the row middles are piled about the trees to shade the roots and
conserve moisture, but unless the middles are fertilized to pro-
duce a heavy mulch, it is rarely that enough material can be
produced in the grove itself. The feeding roots grow into and
just beneath the mulch and as it decays more must be added so
that the roots will not be injured by exposure. As the trees
reach full bearing age, a natural mulch begins to accumulate in
appreciable quantity on the soil under the branches and an or-
ganic layer is gradually built up on the soil surface in the ab-
sence of tillage. Mulching is of extreme importance in the
growing of avocados in warm, humid climates and particularly
so when the soils are as light and shallow as are those of southern
Mature avocado leaves are leathery and do not indicate soil
dryness as citrus leaves do by curling and wilting. Young avo-
cado trees need abundant moisture for vigorous growth and
non-bearing trees should be supplied with water during the nor-
mally dry winter and spring months.
Conclusive data are lacking regarding the need for irrigating
bearing trees, particularly those growing in the limerock soils
of District 1, to maintain fruit production. Moderate drouth in
November and December may be beneficial and some varieties
set very heavy crops in "on fruit" years without supplemental
irrigation, even in very dry springs. Malcolm and Harkness
(31), studying the effects of irrigation on tomatoes growing on
Rockdale limestone soil, reported that considerable moisture is

The Florida Avocado Industry

made available to the crop by capillary action from the per-
manent water table. Additional water is needed for optimum
growth on tomatoes and likely is needed on avocados as well,
particularly during severe drouths when drying winds occur.
However, many successful growers do not irrigate bearing trees,
whereas other apparently equally successful growers regularly
practice irrigation. Data are lacking on which to base definite
recommendations for irrigating bearing trees, but experience
by those who practice irrigation suggests that at least two acre-
inches of water should be applied every two weeks during the
blooming and fruit setting period, unless rainfall supplies it.
Various methods of irrigation are in use, most of them sup-
plying water in spray form either through perforated pipe or
through risers and sprinkler heads. Water for irrigation in
District 2 is obtained from artesian wells in some coastal areas,
or pumped from lakes or deep wells in interior areas. In Dis-
trict 1 irrigation water is pumped from shallow wells drilled in
the porous limestone. Usually centrifugal or turbine pumps with
motors burning gasoline or distillate are used, but some electri-
cal installations are in use.

On sandy soils a cover crop is allowed to grow only during
the summer rainy season. The cover crop usually is volunteer
growth of grasses and broadleaf weeds, but it may be a planted
leguminous crop. Hairy indigo appears to be the most satis-
factory legume. Crotalaria, cowpea, Alyce clover, beggarweed
and other legumes have been used, but these usually grow well for
only a few years and then fail to make satisfactory stands.
Usually the volunteer stands of grasses and weeds will give
good cover and a considerable quantity of green manure.
No satisfactory permanent leguminous cover crop has been
found for the limestone soils of District 1. Crotalaria spectabilis
Roth. has proven satisfactory as a cover crop on newly scarified
land but it does not volunteer well year after year, unless the
ground is dragged each year to cover the seed and reduce weed
competition. In tests during recent years hairy indigo, white
;sweetclover and Alyce clover have made satisfactory cover crops
in some groves but have failed in others. White sweetclover and
Sarawak bean (Dolichos hosei Craib.), if established in good
stands, are better able to compete with volunteer plants than

54 Florida Agricultural Experiment Stations

hairy indigo or the crotalarias. Soybeans are being grown in-
creasingly as a cover crop to follow vegetables and these may
be grown in the middles of young groves. The requirements of
this crop are not known sufficiently, or how long it will remain
satisfactory as a grove cover crop.
A mixture of volunteer Natal grass and native grasses,
legumes and broadleaf weeds offers the best permanent cover
crop for District 1. If given a good start and allowed a little
fertilizer, such a mixture makes a good supply of mulch annually.
Whatever the cover crop grown, it should not be allowed to com-
pete seriously with the trees for moisture during the dry season.


No systematic pruning methods for the avocado are followed
generally. Young trees which have been well cared for and
have not received a setback require little pruning, except to
train or modify the top, during the first four or five years. Such
training should aim to develop strong framework branches and
a desirable form.
In bearing trees sometimes small limbs which have carried
a very heavy crop are devitalized and these should be cut back
to strong healthy growth. Likewise, trees dying back from root
troubles may be benefited greatly by severe pruning, giving the
tree a chance to develop new roots without heavy transpiration
demand for a season. Limbs very close to the ground should be
removed if they interfere with mowing and other cultural op-
erations. Fruit on such limbs is often more or less scarred from
contact with the soil or with the cover crop and much of it is un-
Some varieties, such as Lula and Taylor, tend to grow very
tall. These should be pinched or pruned back periodically while
young to encourage a more spreading habit. However, even
when pruned, 12- to 15-year-old trees are likely to be so tall
that most of the fruit produced in the extreme tops is unsalable
because of blemishes from ineffective disease control or limb
scars. Low spreading trees are sprayed, pruned and harvested
more economically and are less liable to wind damage. It is
desirable to top tall trees to 18 to 20 feet (Fig. 28). The bearing
area can be lowered by such topping in trees with sufficient
spacing to permit vigorous growth of the lower branches. With
trees planted too closely the new growth following topping tends

The Florida Avocado Industry

to be vigorous only at the extreme top of the tree and within
a year or two the trees are as tall as before pruning and the op-
eration must be repeated. Dead, broken or diseased limbs should
be removed, the cuts being made close to the limbs from which
these branches originated.


Fig. 28.-Topping Taylor trees to 18 feet.

Florida Agricultural Experiment Stations

The best time to prune is in the dormant winter period after
the fruit has been harvested. All cuts more than two inches
in diameter should be painted with some protective material
soon after they are made. Carbolineum and several commercial
pruning paints having an asphalt base are satisfactory for this
work. Very large cuts may be protected from wood decaying
fungi by painting the dried cut surface with a water suspension
of cuprous oxide (1 pound per 1 gallon of water) and 24 to 48
hours later with a commercially prepared water emulsion of as-
phalt or with a pruning paint. When cuts are left unpainted,
wood borers are sometimes found deep in the wood. These insects
gradually burrow downward and weaken the limb to such an ex-
tent that it rarely recovers and wood-decaying fungi follow them.

In general avocados in District 1 are fertilized rather heavily,
the fertilizer carrying a high percentage of nitrogen, much of
which is derived from organic sources. Good results have fol-
lowed rather widely varying treatments, provided the quantity
used was sufficient, and it is not possible to outline any one pro-
gram which is best for all conditions. There are no long-time
experiments that might be used as a basis for making such rec-
ommendations. Some valuable information has been obtained
from research studies, made mostly on young trees. Tests
started in the late 1930's were terminated by the 1945 hurricane
and experiments started since 1945 have not continued long
enough to define clearly the nutritional needs of old bearing
Rockdale limestone soils of both the sandy phase near Miami
and the red clay phase near Homestead have a high pH (7.2-8.3)
and very low levels of most plant nutrients and of organic mat-
ter. Newly scarified soil has a tendency to fix phosphates as
well i;:} copper, zinc, manganese and iron. Water-soluble com-
pounds, especially nitrates and potash, leach away rapidly from
the shb:llow topsoil during periods of heavy rainfall. When the
trees have reached full bearing age, a natural mulch of fallen
leaves begins to accumulate in appreciable quantity on the soil
under the branches, provided tillage is not practiced. This
mulch is in itself a source of nutrients and as it decays signifi-
cant amounts of most of the elements required for plant growth
are made available to the tree.

The Florida Avocado Industry

Studies in the changes in fertility of avocado soils in District
1, by means of periodic soil analyses made in recent years, have
contributed much to knowledge of what effect various fertilizer
components have on nutritional levels over long periods.
Malcolm (30) found that 35 years of avocado grove culture
brought about marked changes in the sandy phase of Rockdale
soil. The level of plant nutrients in a virgin soil studied for
comparison was very low. While this soil contained sufficient
calcium and magnesium for normal plant growth, even these ele-
ments were present in small quantity, considering its limestone
origin. Manganese was the only element which was relatively
high. In contrast, the acid organic layer accumulated above the
surface of the mineral soil contained high concentrations of most
of the elements requisite for normal growth. Practically all of
the changes had taken place in the top six inches of soil, where
the highest concentration of avocado feeder roots occurs. In
the virgin soil there was practically no nitrate nitrogen at any
depth; in contrast, the grove soil contained a good supply con-
centrated mainly in the surface three inches of soil and decreas-
ing rapidly with depth. In this same horizon the water-soluble
phosphorus content was very high, whereas in the virgin soil
there was insufficient phosphorus for the growth of cultivated
crops. Reserves of phosphorus were found rather high also in
the 18- to 24-inch horizon. Potassium, however, had not accum-
ulated to any appreciable extent, although added for years in
equal or larger quantity. Copper was not detectable in the virgin
soil, but in the grove soil copper from spray residues and ferti-
lizer amounted to approximately 850 pounds per acre, concen-
trated in the acid zone.
Malcolm (29) also studied the water-soluble phosphorus and
potassium levels in Rockdale soils of other avocado groves in
Dade county. The average water soluble content of phosphorus
was twice that found in orange and grapefruit groves by Peech
and Young (38), and was about the same in all seasons. A rela-
tively low and uniform supply was maintained in some of the
groves and where this was done the trees were in excellent condi-
tion. Malcolm states that probably phosphate fertilizers could
be applied less frequently and at lower rates than at present.
Water-soluble potassium was not maintained at a uniform level
in any grove. High as well as moderate concentrations disap-
peared rapidly during periods of heavy rains, but remained in the
soil during periods of dry weather. Potassium levels were high-

Florida Agricultural Experiment Stations

est in late winter and lowest at the end of summer. Malcolm
concluded that potash should be applied when there is enough
rain to make it available to the trees and after excessive rains
leached the potash away.
Response to nitrogenous fertilizers has been consistent in
tests performed to date (62, 26) and is observed readily in com-
mercial groves. Applications of quickly available nitrogen or of
nitrogen- and potash-bearing materials, intermediate to appli-
cations of complete mixtures, have been shown to increase yields,
especially on Lula, Waldin, Booth and Pollock varieties.
In a study of nitrate levels in Rockdale grove soils, Gallatin
(16) found it very difficult to maintain fairly uniform levels of
nitrate nitrogen (25-60 ppm), when readily available chemical
materials were used. Such materials were nearly completely
leached within a few days after application if rains of more than
three inches occurred during this period. Fairly uniform levels
could be maintained with cyanamid and urea-formaldehyde resins
of low solubility. To obtain maximum benefits from the latter
and from natural organic, maintenance of adequate moisture
levels was necessary.
When not used excessively, animal and poultry manures are
beneficial to the avocado, adding humus besides being valuable
as fertilizers. They probably are more valuable for use on young
trees than in old groves where an organic layer has accumulated
above the mineral soil. The presence of this organic layer sug-
gests that the need for high percentages of high priced natural
organic nitrogenous materials in fertilizer mixtures may become
gradually less as groves become older. Experimental data,
from trees in an old bearing grove prior to its destruction by
the 1945 hurricane, tend to confirm this. Yields in plots ferti-
lized with the synthetic nitrogen fertilizers urea and calcium
cyanamide were as high as in plots fertilized with mixtures con-
taining natural organic sources of nitrogen.
In the early years of the industry in Florida most nitrogen,
phosphorus and potash were supplied to the trees as natural
organic, which contained many of the minor elements in suffi-
cient quantity that mineral deficiencies usually were not a prob-
lem. With the present day use of more or less pure chemicals
in fertilizer mixtures, deficiencies of copper, zinc, manganese,
magnesium and iron have been recognized. Copper deficiency
has been found in commercial groves only on very young trees,
since an excess of copper is available to the trees as soon as cop-

The Florida Avocado Industry

per fungicides are used for disease control. Zinc deficiency has
been severe only where trees have been overfertilized with nitro-
genous fertilizers and especially with cyanamide. Manganese
applied as a spray is beneficial to young trees and to trees grow-
ing in soil containing marl. Response to magnesium has been
demonstrated in some groves and probably the requirements
of the avocado tree for this element approach those of the orange.
Sand culture studies by Furr, Reece and Gardner (15) indicate
that boron is necessary for normal growth of the avocado, and
there is evidence that high-yielding varieties such as Booth 8
growing in Rockdale soils benefit from boron applications.
In general, symptoms of zinc, manganese and iron deficien-
cies are seen more frequently on avocados growing in calcareous
soils and symptoms of copper and boron deficiencies are found
more commonly on those growing in light sandy soils of the
central and coastal areas. With the exception of iron deficiency,
symptoms of micro-element deficiencies can be corrected readily
by foliage sprays containing the deficient elements, but it is more
practical to supply these elements from the beginning to pre-
vent the development of symptoms.
The fertilizer tests and soil studies made to date, together
with observations on tree condition and yields in many commer-
cial groves, afford a fairly reliable basis for making recommen-
dations that will satisfy most general requirements for avocados
growing on Rockdale soils. It should be emphasized that the
general condition of the trees and size and quality of any one
fruit crop are not determined solely by the ratio or poundage
of any single fertilizer application or even of a single year's fer-
tilization, but may reflect practices for a period of several years.
Experimental data are not available as a guide for fertilizing
avocados growing on the soils of District 2. Commonly citrus
fertilizer mixtures are applied three or four times a year and
this appears in general to have given good results. Observations
suggest that the nitrogen needs of avocado trees in these deep
soils may be satisfied by smaller applications than are used in
District 1. One grower at Lake Placid has improved the physical
structure of the light sandy soil and the vigor of the trees on his
property by making heavy applications of poultry manure to
supplement the commercial fertilizer applied.
The fertilizer program to maintain thrifty growth of non-
bearing trees should include frequent and fairly liberal applica-

Florida Agricultural Experiment Stations

tions of nitrogen, phosphate, potash and water-soluble mag-
nesium fertilizers; plus supplying small amounts of copper, zinc,
manganese and boron by means of foliage sprays.
Trees planted in newly scarified soil without disturbing the
roots should be fertilized at planting time with 2 or 3 ounces
of a low analysis fertilizer mixture. Such mixtures as 4-9-3-1.5,3
4-7-5-1.5 or 5-10-5-2, with about 25 percent of the nitrogen from
organic sources, are satisfactory. About four ounces of the
same mixture should be applied four to six weeks later and the
application repeated at 30- to 40-day intervals during the sum-
mer rainy season and at about 60-day intervals thereafter to
the end of the first year. The amount per tree should be in-
creased gradually to about 8/4 pound per tree by the end of the
During the second and third years the same analysis may
be used about every 60 days and the amounts gradually increased
to 11/2 pounds per tree by the end of the second year, and three
pounds per tree by the end of the third year of a mixture contain-
ing five percent nitrogen. During the fourth and fifth years the
mixture may be changed to 5-5-5-3 or 6-6-6-3 and the number
of applications reduced to three or four per year. The rate per
tree may be adjusted in accordance with the amount of growth
and general appearance of the tree, but probably should not
exceed 1.25 pounds per each year of age of the tree per applica-
tion three times a year, or 1.0 pound per each year of age per
application four times per year.
There is a tendency to use higher analysis mixtures as a
matter of economy, but it is easier to avoid root damage due
to high concentration of soluble salts in localized areas with the
lower analysis mixtures. Young avocado trees have been se-
verely damaged in some instances by the excessive use and
careless spreading of fertilizers. For young trees fertilizers
should be spread fairly uniformly over the root area, which
extends from near the trunk to two or three feet beyond the
canopy of branches, in trees more than one year in the field.
There is no necessity to work the fertilizer into the soil, which
preferably should be moist at time of application. For applica-
tions during the summer rainy season, the percentage of nitro-
gen derived from natural or synthetic organic should be at
least 40, but for spring and fall applications may be 25 or less.
S 4(nitrogen)-9(phosphoric acid)-3(potash)-1.5(magnesium oxide).

The Florida Avocado Industry

Trees planted in May or June should be sprayed a month or
six weeks after planting and again in September with a spray
containing two pounds of tribasic copper sulfate or its equiva-
lent in some other neutral copper, 1 pound of neutral zinc, 1
pound of neutral manganese and 1/2 pound of borax per 100 gal-
lons of water. The nutritional spray should be applied twice
during the second year and once a year thereafter until the trees
reach bearing age.
Most varieties, if not set back by freezes, begin to bear suffi-
cient fruit by the fifth or sixth year to warrant the expense of
a complete spray program for control of fruit-spotting fungi.
Zinc, manganese and boron can then be supplied by adding the
recommended amounts to one of the copper fungicidal sprays.
Fertilizer mixtures such as 6-6-6-3 may be continued at the
same frequency of application with amounts gradually increased
with age and size of the trees up to about 12 years. As a rough
guide the formula of 1.25 pounds of a 6 percent mixture for each
year of the trees' age should be sufficient for each of three appli-
cations. If four applications are made during the year the
amount per application may be reduced to 1 pound of the 6 per-
cent mixture for each year of the trees' age.
Tree condition and size of crop should be considered also in
determining the quantity of fertilizer to be used. Trees should
have sufficient fertilizer for normal development. Undernour-
ished trees produce fruit which is sub-normal in size, pale in
color and generally lacking in attractiveness. If considerable
variation is apparent in the grove, the trees should be divided
into two classes with respect to condition of growth and ferti-
lized accordingly. Special attention should be given to the fer-
tilization of trees carrying heavy crops of fruit. When an un-
usually heavy crop has been set it is well to make one or two
light to moderate applications of nitrogen intermediate to the
regular complete mixtures. Amounts of nitrogen per tree in
these intermediate applications should be 1/2 to 3/4 of the amount
of this element supplied at a regular application. A tree kept in
vigorous growing condition with full foliage matures its fruit
with less injury to itself and with less sunburning and dwarfing
of the fruit than if it is undernourished. In some heavy pro-
ducing varieties, such as Booth 8, it is necessary also to thin the
fruit, as well as to fertilize heavily to avoid these injuries in
the "on fruit" years. On the other hand trees which fail to set

62 Florida Agricultural Experiment Stations

a crop need not be fertilized nearly as heavily as those loaded
with fruit, and need only be kept in good healthy condition for
the next year's crop.
At 12 years of age many trees grownig in the shallow Rock-
dale limestone soils extend their roots laterally far enough to
meet in the middles of 25-foot rows. When this occurs, ferti-
lizers are applied efficiently from trunk to trunk by means of
mechanical spreaders. At this age most of the ground is shaded,
and in the absence of tillage, the accumulation of an acid organic
layer above the mineral soil is well under way. Since this layer
contains appreciable quantities of most of the nutrients neces-
sary for growth, there is little need for increasing the poundage
of fertilizer used per acre as the trees become older. The amount
of phosphorus and expensive natural forms of nitrogen may be
reduced. Mixtures such as 6-2-6-3, 7-2-7-3 and 8-4-8-4 will usually
satisfy the actual requirements. A mixture such as 10-0-10-4
has been used for one or two applications per year with good
results. For use on old trees these mixtures may be made up
mostly from chemical sources, with about half of the nitrogen
derived from slowly available and half from readily soluble ma-
terials. Small amounts of natural organic materials may be
incorporated in the mixtures to prevent caking so that they can
be spread efficiently by mechanical spreaders. The poundage
per acre should be varied somewhat according to tree condition,
size of crop and rainfall, but should fall in the range of 150 to
200 pounds of nitrogen per acre per year for good bearing groves.
This amount may be split more or less evenly into three applica-
tions per year at about the following periods; February or March,
May or June and October; or some of the nitrogen may be ap-
plied as intermediate applications in April and August.
Nitrogen may be supplied also by means of foliage sprays.
In 1950 and 1951 urea, added to the fungicidal sprays applied
from February to July on the Lula variety at the Sub-Tropical
Experiment Station, gave an annual increase of 11.5 pounds of
fruit per tree. On Booth 8 in the same years urea added to
fungicidal sprays in May, June and July gave an annual increase
of 31 pounds of fruit per tree. This appears to be an economical
method of supplying additional nitrogen to the tree during the
critical fruit-setting period. Urea known to have a low biuret
content should be used; 5 pounds per 100 gallons of spray is

The Florida Avocado Industry

Bearing trees may receive larger amounts of fertilizer with-
out apparent injury to the tree, but overstimulation of the tree
is wasteful and undesirable. Like most tree fruits, vigorous
growth and fruitfulness do not exist at the same time in the
same tree.
The tolerance of the avocado tree to extreme conditions of
soil acidity and alkalinity has not been investigated thoroughly.
Where the soil is known to be far from average in reaction, the
safer plan is to use, so far as possible, materials which will tend
to bring about a more nearly normal soil reaction. Sulfate of
ammonia and most of the natural organic nitrogen fertilizers,
will tend to correct high alkalinity, while nitrate of soda, bone
meal, basic slag and
Where avocados are grown commercially in Florida winds
may cause more or less injury to tree and crop. During the
dry season the trees are unable to obtain sufficient water to
compensate for increased evaporation losses from winds. The
mature avocado leaf seldom shows water deficit by wilting, as
citrus leaves do, but the rapid shedding of fruit when rain comes
again, if not before, indicates the former deficit. Windbreaks
reduce air movement and so lower evaporation loss. Strong
winds are injurious to the fruit, either causing bruising or scar-
ring or breaking the fruit stem and causing it to fall. Sudden
windstorms of gale force occasionally occur during the fall and
winter and cause considerable dropping and bruising of mature
or nearly ripe fruit and breaking of limbs heavily loaded with
fruit. A source of entry for fungi is afforded by bruising. Scar-
ring spoils the appearance and causes fruit to be down-graded
in the packinghouse. These sources of loss are greatly reduced
by good windbreaks.
While a single row of windbreak trees affords considerable
protection, it is desirable to plant a double row in staggered
formation. The windbreak should extend entirely around the
grove and in large groves it is well to separate each tract of
about five acres from the next by a single line of windbreak.
The space sacrificed for the windbreak will be a valuable invest-
ment in improved fruit. Where the contour of the land is un-
even, as in the Ridge section, of District 2, openings in the wind-
breaks should be left at one or more of the lowest places to
permit drainage of cold air out of the grove.

Florida Agricultural Experiment Stations

The most satisfactory trees for windbreaks in southern
Florida are the casuarinas. Two species are commonly grown,
the tall, slender Australian-pine (Casuarina equisetifolia Forst.)
and the shorter, thicker Brazilian-oak (C. lepidophloia F. Muell.).
Both species grow rapidly and become tall enough to be effective
windguards. They both have nitrogen-fixing bacteria in root
nodules, as Mowry (33) has shown. The second species usually
suckers freely on limerock soil, which is objectionable, but it
makes a lower and denser growth than the Australian-pine, is
hardier to cold and provides better wind protection. C. lepi-
dophloia scions can be topworked readily onto 1/4 inch caliper
seedlings of C. equisetifolia by using the side-veneer graft (45).
Grafted trees made in 1946 at the Sub-Tropical Experiment Sta-
tion have grown as vigorously as self-rooted trees. Thus the
main objection to the use of C. lepidophloia is removed, since
the root system of C. equisetifolia shows little tendency to sucker.
Other satisfactory windbreak species are the jambolan
(Syzygium cuminii Skeels) for District 1 and the Australian silk-
oak (Grevillea robusta A. Cunn.) in the Ridge section of Dis-
trict 2. The species of Eucalyptus have, in general, not made
satisfactory windbreaks in Florida. No species of tree, that
grows tall enough in southern Florida to be an effective wind-
break, has been tested that can be expected to withstand hur-
ricane winds in excess of 100 miles per hour without serious
breakage or uprooting.

A few avocado trees are blown down nearly every year in
District 1, as a result of small occasional windstorms. When
larger storms with winds of hurricane force pass close to an
avocado production area, breakage and uprooting of trees may
be extensive. Infrequently, the center of a hurricane passes
directly over the production area of District 1; this occurred in
September 1945, when winds of 125 to 150 miles per hour were
experienced over practically the entire area devoted to commer-
cial growing of avocados, mangos and citrus fruits in southern
Dade County. All groves of bearing trees were severely dam-
aged and in many, every tree was either blown over or the tops
were more or less completely broken off or blown away (Fig. 29).
The avocado tree is remarkable in its ability to recover from
such storm damage, if prompt and proper rehabilitation meas-
ures are taken. For trees with the root system intact and

The Florida Avocado Industry

where the bark of the trunk and framework branches has not
been loosened by the twisting force of the wind, treatment con-
sists of cutting back to good sound wood. The cuts should be
smooth and should be treated as described under "Pruning" and
the exposed bark of trunk and framework branches whitewashed
to prevent sunburning. For trees that have been blown down,
drying out of exposed roots and sunburning of the trunks must
be prevented until the tree can be replanted. This may be ac-
complished by piling the leafy branches of the top over the
roots, trunk and main framework branches to shade them. The
trees should be replanted as soon as possible. Broken roots
are trimmed back by pruning, and the tree is reset at the proper
level in the original hole after the latter has been cleaned out
either by hand tools or a scraper or a small bulldozer blade
powered by a tractor. Loose soil is then tamped in carefully
and firmly around the roots and the tree watered liberally. The
top is cut back, the cuts treated and the bark of the dehorned

Fig. 29.-Avocado grove at Sub-Tropical Experiment Station
after 1945 hurricane.

' / I

I', ..' .

Florida Agricultural Experiment Stations

tree whitewashed. The loose leaves and small branches may
be piled around the trunk to complete the operation (Fig. 30).
Periodic irrigation and applying mixed fertilizer such as is
used on young trees, are important measures for recovery. Un-
der favorable conditions a new, top and root system will develop
during the first year and a crop of fruit will be produced the
second year after the storm.
A durable whitewash may be made from quicklime 50 pounds,
water 10 gallons and salt 41/2 pounds. The lime is slaked in
such a way as to prevent burning and then the salt is added.
The mixture is then diluted to suitable consistency for applica-
tion. If it is to be applied with a sprayer, it should be strained
carefully and diluted more than would be necessary for brush
An equally satisfactory whitewash can be made using a
fine grade of hydrated lime and adding zinc sulfate to it. A
ratio of 1 pound of zinc sulfate to 10 pounds of lime will be sat-
isfactory. The zinc sulfate should be dissolved in water first and

Fig. 30.-Whitewashing dehorned and replanted Pollock avocado trees
after 1945 hurricane. These trees bore a good crop in 1947.

^ ILl. IA AJk

The Florida Avocado Industry

then the lime added and the mixture diluted to the desired con-
sistency for application. Normally it is desirable to make two
applications if the whitewash is applied with a sprayer. The
numerous shoots that grow from the drastically pruned trees
should be thinned somewhat in order to direct growth into a
smaller number of well-spaced branches.


Seedling avocados usually start to bear some fruit in five to
seven years. Budded trees sometimes bear one or two fruits the
year after being planted. Any fruit set the first, second and
third years should be removed. By the fourth year the budded
tree should be able to set and mature a small crop, and in the
fifth year it may carry a moderate crop safely. Fruits set on
young trees of precocious varieties such as Booth 8 should be
carefully thinned to prevent tree exhaustion.
Even in well-fertilized groves the yield of a given variety
may vary tremendously from one season to the next, depending
on such factors as size of crop carried the previous year, condi-
tions during the blooming season and rainfall, winds, diseases
and insects during fruit development. After hurricanes and
severe freezes, trees may require several years to come back
into normal production.
Such heavy and consistent bearers as Waldin cannot be ex-
pected to produce more than three 55-pound crates per season,
as the average over a period of years, although occasionally in-
dividual trees may produce over 10 crates; The average safe
expectancy for long-time production would be nearer two crates
per tree. Lula, Hall, Booth 8, Booth 7 and Choquette may be
counted on for average production nearly equal to Waldin, while
Taylor, Tonnage and Hickson are not quite so productive. Pol-
lock has not averaged much more than 1/ bushel per tree in
commercial groves. Some unusually well cared for groves have
exceeded these figures, but not the average grove.

Most varieties of avocado tend toward alternate bearing,
i.e., a heavy crop one season is followed by a light crop or none
at all the next. The question has been raised about the possi-
bility of bringing about more regular bearing by proper thin-

Florida Agricultural Experiment Stations

ning. Extensive experiments have not been performed and
so far as is known no grower has ever practiced avocado thin-
ning extensively. Limited tests have been made on the Booth
8 variety. These tests indicate that even drastic thinning will
not control alternate bearing in this variety. Removal of 3/ of
the fruit from some trees did not prevent signs of crop ex-
haustion and the production of a very light crop the following
season. However, proper thinning will reduce the severity of
crop exhaustion and cause some increase in size of the harvested
fruit. Breakage of limbs and sunburning of fruit and of limbs
can be reduced to a minimum by proper thinning. With most
of the commercial varieties the cost of thinning is not justified
by the benefits obtained, but thinning should be considered for
such varieties as Booth 8, Booth 7, Hall and Waldin. The follow-
ing suggestions for thinning are made, based on the limited
experience available.
The best time for the first thinning is when the fruit is about
one inch in diameter. Heavily'laden trees should receive atten-
tion first. The number of fruits to be removed should be gov-
erned by the condition of the particular tree and the ability of
the variety to carry heavy crops. All fruit which is badly
blemished, bruised or mis-shapen should be removed first, since
such fruit would be of little or no value even if the tree could
mature it, and the food and moisture which it would use may
better go to develop better quality fruit. Small branches car-
rying very heavy loads of fruit should be relieved of most of it.
Such branches are unable to produce sufficient foliage and the
fruit is likely to be dwarfed and sunburned. Four to six weeks
later a check-up should be made and additional fruit removed
where necessary.
Avocados should always be picked with the stem attached
to the fruit. When the fruit can be reached by hand, the usual
methods are either to take the fruit in the palm of the hand
and tip it sharply sidewise or upward to snap off the fruit stem
which is trimmed short later with clippers or a knife to avoid
stem punctures to other fruit, or grasp the fruit in one hand and
cut the stem to the desired length with orange clippers. The
avocado is very susceptible to decay if the skin is broken, and
careful handling at all times will prevent loss.
For reaching fruit in the top of tall trees a long bamboo
picking pole is used. To the end of the pole is attached a sharp-

The Florida Avocado Industry

bladed hook and below this is fastened a small canvas bag to
catch the detached fruit. The picker carries a galvanized iron
picking bucket to receive the fruit on transfer from the pick-
ing pole receptacle, or to pick fruit into directly by hand. A
picking platform, raised and lowered by a hydraulic lift oper-
ated by a tractor, is often used to facilitate picking the fruit
from tall trees (Fig. 31).
It is difficult to determine the proper period for harvesting
the fruit, especially of the green-skinned varieties. The season
of maturity varies slightly from year to year for each variety,
according to time of bloom and seasonal conditions during fruit
growth, and so no calendar date can be given for maturity of
any variety. To create a market and maintain the demand nec-
essary for the profitable culture of a little-known fruit such as
the avocado, it is imperative that the consumer be offered fruit
which has attained maturity sufficient to be palatable upon
ripening. Because mature and immature fruits appear similar
externally, usually purchasers cannot differentiate between them
and the result is too often disappointment and prejudice against
the avocado.

Fig. 31.-Picking platform raised and lowered by hydraulic lift,
for picking avocados from tall trees.

70 Florida Agricultural Experiment Stations

The avocado may remain in perfect condition on the tree
for some time after reaching maturity, a few fruit often hang-
ing on for months and never becoming soft enough on the tree
for eating. However, the harvesting and shipping of fruit
which has reached the fullest possible degree of maturity on
the tree is not recommended. The seed may sprout while the
fruit is on the tree or the fruit may ripen so rapidly after har-
vest that it cannot be shipped satisfactorily. The importance
of standardizing a relatively simple and accurate test of ma-
turity for Florida avocados can scarcely be overemphasized.
In California the oil content of the fruit is considered a satis-
factory measure of maturity and establishing a state maturity
standard of 8 percent oil content has greatly benefited the
industry in that state (21).
Although much work has been done to develop a test based
on oil content for Florida avocados, nothing satisfactory has
been worked out. Harkness (19) concluded that for the present
no physical or chemical method can be recommended as a test
of the maturity of Florida avocados. Harding (17) and Soule
and Harding (48) found that maturity of fruit within a variety
could be fairly well defined on the basis of specified picking
dates in conjunction with minimum weight. Large fruit picked
on a specified date tended to be more palatable than smaller fruit.
On the basis of the maturity studies and of past experience with
the various varieties, maturity standards under the Federal
Avocado Marketing Agreement are regulated by imposing a
schedule of arbitrary shipping dates by variety and sizes within
a variety. The larger sizes of a variety are permitted to be
harvested at specified dates before size restrictions are com-
pletely removed. The schedules are subject to constant revision
and the system has undoubtedly kept much immature fruit from
being marketed. It has not worked as well with some varieties
as others, probably because the best schedule has not yet been
determined for all varieties. Satisfactory ripening of a few
representative fruits and the presence of a brown seed coat are
considered evidence of proper maturity for seedling avocados.
When the fruit is adjudged mature, the first pickings from
the more heavily loaded limbs should include all that are per-
mitted under the schedule of sizes. Unless this is done the loss
of vitality occasioned by the overcrop may induce a dying back
of such branches and the loss of all fruit on them. Of course

The Florida Avocado Industry

if the fruit had been thinned judiciously, this situation would
not arise at harvesting. From parts of the tree where the fruit
is well distributed the largest fruit should be picked each time.

At the packinghouse, the fruit is graded by hand according
to specific standards established by the Avocado Administrative
Committee, and then sized mechanically over automatic sizers.
United States standards for Florida avocados (1) have been
established but as yet have not been adopted by the Florida
Florida avocados are now packed in containers which fall
into two general types: (1) the lug or flat holding a single layer
of fruits and (2) the crate or box holding several layers of
fruits (Fig. 32). The latter is used for avocados weighing
at least 20 ounces or measuring 37/8 inches in diameter and may
be either of fiberboard or wood with inside dimensions of 11 x
16%/4 x 10 inches. Avocado flats have inside dimensions of
131/2 x 161/2 inches, but may be 31/4, 3/4 or 41/2 inches in depth.
The larger two flats may be of fiberboard or wood; the smaller
one is of fiberboard. The trend at present is toward the use of
wooden containers. Flats hold approximately 131/2 pounds net
weight of avocados.
In packing avocados in wooden flats and boxes, layers of
slightly moistened excelsior or wood wool are placed on the in-

Fig. 32.-Containers used for shipping Florida avocados. Left, lug of
Taylor fruits; right, fiberboard box with layers of Booth 1 fruits separated
by soft paper dividers.

nan,. *04sI~v,4.n 1, a. f.... f

Florida Agricultural Experiment Stations

side surfaces of the package as well as between the fruits, so
that each fruit is nested. Soft paper dividers are used in some
packinghouses to separate layers of fruit in packed boxes. In
paper flats excelsior usually is placed only on the bottom and
sides of the container. The bulge pack used for citrus is quite
impractical for avocados, as they have no resiliency and cannot
withstand pressure without injury. Using tissue wraps for avo-
cados is also undesirable, as wrapped fruit tends to ripen too
quickly. The pack, or number of fruits per lug, varies with the
variety and also within the variety.
Packed fruit is subjected to inspection by the Federal-State
Inspection Service and must conform to the maturity, grade
and pack regulations before it can be shipped to market.

While formerly the bulk of the avocado crop moved to mar-
ket by express train and boat, from 90 to 95 percent of the crop
is now moved by refrigerated truck. Trucks have greatly in-
creased the range of distribution of avocados and have opened
new markets all through the South and East.
The crop is marketed by some 10 or 12 shippers, for the most
part, ranging from growers who ship only what they produce
to shippers who grow nothing. No one of these marketing
agencies handles more than 35 percent of the crop. Competi-
tion by the independent shippers to sell their fruit during periods
of heavy supply tends to depress the price. The entire industry
and especially the grower would benefit if a better system of
distribution and marketing of Florida avocados were used. At
present most of the crop is sold on consignment, but f.o.b. sales
have increased considerably in recent years.
Consumer preference in size, color and texture of fruit varies
widely in different markets and with fluctuating price levels.
Considerable experience in marketing avocados and constant
contact with principal markets are necessary to obtain maximum
returns. The general tendency seems to be toward fruit of
small to medium size (12, 14, 16 and 18 packs), green in color
and fairly smooth-skinned or pebbled. Some special markets
accept or prefer larger fruit. It should be pointed out also that
the Florida avocado industry has made little effort to market
purple or black-skinned avocados, although some seedlings or
varieties of this type produce handsome fruit of good quality.

The Florida Avocado Industry 73

It is sometimes desirable to put avocados in cold storage to
hold them in firm condition during a temporary market glut.
It has not been found practical to keep them in storage for long
periods. Practically all of the avocado packinghouses in Florida
have installed or have access to cold storage facilities. Studies
by Lynch and Stahl (27) and Mustard (35) have shown that
avocados of different races and also varieties of the same race
behave differently under cold storage. Table 1 shows results
of storage tests by Lynch and Stahl on seven commercial varie-
ties, stored at different times throughout their season and at a
wide range of temperatures. Trapp was the most satisfactory
West Indian variety for holding in storage and Taylor the best
of the Guatemalan or hybrid varieties. Mustard concluded from
her studies that Waldin, Trapp, Lula and Schmidt are not well
adapted to refrigerated storage, but Booth 7, Lula and Nabal
can be satisfactorily refrigerated for one week and Booth 8 and
Taylor for two weeks. Storage temperatures studied ranged
from 40-57 F.

Optimum Days held Days to Days fruit Gain in
tempera- at soften was market-
Variety ture of optimum thereafter marketable able
storage in tempera- at room after life due to
degrees F. ture temp. softening storage
Pollock ........ 42 21 3 3 17-19
Trapp .......... 42 21 4-6 2-5 18-20
Waldin.......... Cold injury noted on skin even at 48o F. storage.
Collinson .... 42-48 14 4-6 3-4 7-8
Booth 8 ...... 42 14 4-6 1-4 7-9
Lula ............ 37 28 3-6 2-3 20-24
Taylor ........ 37 28 4-6 2-3 23-26

Investigation (49) has shown the importance of preventing
accumulation of carbon dioxide in the storage room. Keeping
the C02 concentration below three percent has permitted longer
storage at all temperatures and reduced the development of
brown discoloration on the fruit skin. Early studies of various
wrapping materials seemed to show that wrapping with pliofilm

74 Florida Agricultural Experiment Stations

reduced wilting and prolonged the storage life of avocados (50).
In further investigations with pliofilm and other wraps, how-
ever, the wraps had no appreciable effect in retarding the break-
down of stored avocados (36) and a satisfactory method for use
of avocado wraps has not been developed.
The USDA Agricultural Marketing Service recommends that
most varieties of avocado be held at 450 F. during transporta-
tion by truck, except the West Indian varieties, which should be
transported at 550 F. It is also recommended that shipments
during subfreezing weather be heated (22).


The more important avocado diseases cause annual loss to
Florida avocado growers. With the recent adoption of grade
standards and the elimination from the market of most of the
cull fruit in recent years, the control of diseases causing fruit
blemishes and decay has become more important.

Cercospora spot is caused by a fungus, Cercospora purpurea
Cke., which is able to infect previously uninjured fruits and
Individual spots on leaves are angular in shape, generally
less than 1/16 inch in diameter, and are brown to chocolate
brown in color. They may be distinct and scattered or may
coalesce to form irregular patches (Fig. 33). Successive crops
of spores are produced on either surface of the spots during
moist periods throughout the year. The grayish spore-bearing
tufts may be seen readily with the aid of a hand lens.
On fruits mature spots generally are 1/4 inch or less in di-
ameter, light to dark brown, irregular in shape and slightly
sunken (Fig. 33). The surface usually becomes cracked or fis-
sured with the grayish spore-bearing tufts present during or
immediately after moist periods. The fungus is confined to the
rind tissue. The dead, cracked tissues on the surface of the
spots afford means of entrance for anthracnose and other decay
organisms, which may cause the fruit to spoil as it approaches
maturity or after it is harvested.
The disease is carried over from one season to the next on
old leaf infections and, if control measures are not taken, tends
to become progressively more serious. Leaf infection may be-

The Florida Avocado Industry

come severe enough to cause some defoliation. Fruit infection
will take place from May to September, but the most critical
period appears to be from May 15 to July 1. All varieties now
grown in Florida are susceptible, but the disease is less severe
and more easily controlled on Collinson, Fuchsia and Pollock
than on other commercial varieties.




Fig. 33.-Cercospora blotch on avocado leaf and fruit.
Cercospora spot can be controlled readily by timely applica-
tions of copper sprays, as Stevens (51) first showed. An appli-
cation in early May followed by another in early June gives ef-
fective control on varieties maturing in summer and autumn.
Both foliage and fruit should be well covered by these sprays.
On winter-maturing varieties a third spray application about
mid-July is desirable, directed chiefly to covering the fruit. A
4-4-100 bordeaux, wettable cuprous oxide 1 pound to 100 gallons
of water, or other neutral copper fungicides used at about equal

Florida Agricultural Experiment Stations

metallic copper content in the spray, are effective fungicides.
An efficient spreader should be added to the spray mixture un-
less the manufacturer has incorporated one in the fungicide.

Anthracnose is caused by the common fungus Colletotrichum
gloeosporioides Penz., which lives as a saprophyte or weak para-
site on many plants in Florida. On the avocado the fungus is
commonly found growing on dead twigs and in dead spots on
leaves and fruits. It is unable to grow actively in healthy, un-
injured, growing fruits, but may establish latent infections on
them, especially at the lenticels (58). The latent infections re-
main inactive until the fruit is mature and develop rapidly as
the fruit softens. Latent infections generally are of little im-
portance on well sprayed fruit harvested at the right stage
of maturity and handled properly until it reaches the consumer.
The fungus establishes itself actively in avocado fruits as
they approach maturity through dead areas or cracks in the
rind caused by other fungi, mechanical injuries or insects, es-
pecially the larger plant bugs. Cercospora spot is the most com-
mon avenue of entrance in unsprayed fruit.

Fig. 34.-Anthracnose. Left, on Waldin fruit;
right, following scab on Booth 8 fruit.

The Florida Avocado Industry

Anthracnose infections are slightly sunken and nearly cir-
cular in outline, are dark brown to black, and vary from tiny
spots to 1/2 inch or more in diameter. As the fruit ripens the
infection spreads rapidly into the flesh, causing a greenish-black,
fairly firm decay which eventually involves a large portion to
all of the fruit. The surface of the lesion may develop radial
cracks and during moist periods will show pinkish spore masses
of the fungus (Fig. 34). All varieties grown in Florida are
subject to attack if conditions are favorable to infection.
Where Cercospora spot is adequately controlled there is usual-
ly little trouble from black spot. On late-maturing varieties,
such as Nabal, Taylor and Choquette, a fall application of a cop-
per fungicide in addition to the recommended Cercospora spot
program sometimes may reduce anthracnose infection on wind-
bruised and insect-stung fruit. The Fuerte variety is very sus-
ceptible to anthracnose infection late in the season and this is
one of the chief reasons why this variety is not grown success-
fully in Florida. Harvesting fruits in an immature condition
appears to contribute to anthracnose development in storage and
transit, especially at small bruises and skin breaks caused in
harvesting and packing.

Avocado scab is caused by the fungus Sphaceloma perseae
Jenkins, which is carried over from one season to the next on leaf
and stem lesions. When moisture and temperature conditions
are favorable, the fungus readily infects young succulent tis-
sues of avocado leaves, twigs and fruits, forming the character-
istic scab lesions in which spores are produced. The spores are
spread to other parts of the tree or to other trees by wind, rain,
dew and possibly by insects. Leaves are most susceptible in
the very early stages of growth and become entirely resistant
by the time the leaf tissues harden. Fruits are extremely sus-
ceptible just after the petals fall and become gradually more
resistant as they develop, but do not acquire immunity until
they have attained over half their normal size.
Individual spots on leaf blades generally are less than 1/8
inch in diameter, and are purplish to dark brown, fading to
grayish brown with age. They are visible on both sides of the
leaf and eventually the centers may weather away, leaving small
irregular holes fringed with grayish brown tissue. In case of
severe infection the leaves. become variously distorted and
stunted. Infections on the under side of the leaf are confined

78 Florida Agricultural Experiment Stations

mainly to the midrib and veins. Spots on the veins, leaf petioles
and the bark of twigs are slightly elevated and are oval to elon-
gate in shape (Fig. 35).
On fruits the spots are at first raised, circular to oval and
dark brown to purplish brown. They are scattered or may coa-
lesce to form irregular extended areas sometimes involving prac-
tically the entire surface of the fruit. Eating quality of the
mature fruit is not impaired by the disease, but its appearance
is very unattractive and in severe cases the fruit may be de-

Fig. 35.-Avocado scab on Lula fruit and on seedling leaves and stem.

The Florida Avocado Industry

formed or dwarfed. The black spot fungus may gain access to
the flesh through cracks in scab blemishes.
Avocado varieties and seedlings vary in susceptibility to
scab. Lula is the most susceptible commercial variety; Hall,
Fuerte, Nabal, Booth 8, Booth 7 and Taylor are among those
moderately susceptible; Fuchsia, Pollock, Booth 1 and Waldin
are quite resistant.
Stevens first showed.that scab can be controlled on the fruit
by proper spraying with 6-6-100 bordeaux, and Ruehle (44) has
obtained equally good control with 11/2-100 wettable cuprous
oxide and other neutral copper fungicides which leave less resi-
due than bordeaux. Spraying for cercospora spot controls scab
satisfactorily on resistant varieties and usually on moderately
susceptible ones not interplanted with Lula or susceptible seed-
lings. Lula avocados require three sprays in addition to the cer-
cospora spray program. These should be timed as follows: (1)
just as the bloom buds begin to open, usually late in January;
(2) near the end of the main bloom period, when many small
fruits have set, usually from the middle of February to March 1;
(3) three to four weeks after (2), when all the fruit has set
during a normal season. Later sprays should be timed for con-
trol of cercospora spot in May, June and July. Moderately sus-
ceptible varieties which start the season with many old scabby
leaves will need the first two applications to free them from
scab. If the hold-over foliage of such varieties is practically
free of scab but they are interplanted with Lulas or scabby seed-
lings, it is advisable to apply the first spray recommended for
Lulas to trees of the moderately susceptible varieties.

Considerable loss of fruit sometimes occurs in shipments
from rots which start at the stem end and develop as the fruit
softens. Both Diplodia natalensis Pole-Evans and Diaporthe
citri Wolf, the common fungi causing stem-end rot in citrus
fruits, have been isolated from these rots. Contributing factors
appear to be shipping immature fruit and using inadequately
ventilated containers. No special control measures appear to
be necessary, when mature fruit is shipped in well ventilated
containers and kept refrigerated within safe limits.
Roots of avocado trees may be infested by migratory semi-
endoparasitic nematodes. The burrowing nematode, Radopholus

Florida Agricultural Experiment Stations

similis (Cobb) Thorne, and a meadow nematode, Pratylenchus
pratensis (de Man) Filipjev, were reported on avocados by Du-
Charme and Suit in 1953 (13). Where avocado trees were in-
fested by the burrowing nematode, they observed symptoms of
decline comparable to those on citrus affected by spreading de-
Later, studies of the distribution, numbers and possible soil
preference of these nematodes were made in the important avo-
cado growing areas of the state (67, 68). Burrowing and meadow
nematodes were widespread and frequently numerous in the
light sandy soils of District 2. Meadow nematodes alone (Praty-
lenchus spp.), usually in relatively small numbers, were found in
many avocado groves on sandy loam and sandy muck soils along
the East Coast and in the limerock soils of District 1. The bur-
rowing nematode was not found on avocados growing in lime-
rock soils. No good correlation appeared to exist between the
observable incidence of these nematodes and tree condition. Al-
though decline symptoms in the top were more prevalent in a few
groves infested with both species than in those where only
meadow nematodes or no nematodes were found, there was little
evidence of a progressive spread of decline as in citrus. In sev-
eral groves receiving normal care, trees remained productive
despite rather heavy infestation by both nematodes.
Brown to black root lesions caused by infestation of either
species may be small localized areas or extensive depressions
which girdle and may kill the roots. Side roots frequently form
above the dead portion. Fungi found in the lesions probably
contribute to the death of root tissue.
While the presence of large numbers of parasitic nematodes
and especially burrowing nematodes on avocado roots undoubt-
edly adversely affects the growth of the trees and constitutes a
threat to avocado growing in the light sand soils of District 2,
at present these pests do not appear to present a serious problem
to avocado growers in District 1. Investigations are under way
to obtain additional information about these nematodes and their
effect on avocado production.

Trees which have endured a few days of water standing high-
er than the crown roots are likely to die quickly. Studies in
California by Wager (57) and later by Zentmyer and Klotz

The Florida Avocado Industry

(70) and in Peru by Crandall (10) showed that a soil fungus,
Phytophthora cinnamomi Rands, attacks and rapidly kills avo-
cado roots in poorly drained soils. These observations have been
confirmed in Florida, as well as Wager's finding that avocado
seedling trees in soil free of the fungus are not injured by sub-
mergence of the soil for two or three days. In District 1 where
a few trees have been killed in low areas following a flash flood,
the disease has slowly spread outward in subsequent years, al-
though actual flooding did not occur in these years. The water
table frequently rises to within a few inches of the ground sur-
face during the rainy season in areas with less than 10 feet
elevations and this serves to spread the infection.
Severely infected trees show sparse foliage with pale green,
often wilted leaves and dieback occurs in advanced stages. Feed-
er roots become blackened and decayed and may be almost en-
tirely absent in advanced stages of the disease.
In Florida no control measures other than providing good
drainage and avoiding planting avocados in areas that are sub-
ject to flooding are suggested. In California, where the disease
is a serious problem, establishment of barriers to prevent spread
of the fungus through soil and along roots and fumigating the
soil in small infected areas are among measures suggested for
combating the disease (71).
Occasionally isolated avocado trees growing among healthy
trees in well drained soils will suddenly wilt. Such trees may
die outright, although sometimes they have recovered following
severe pruning. The cause of the trouble in Florida is not defi-
nitely known. A similar trouble in California (69) has been at-
tributed to parasitism of the fungus Verticillium albo-atrum
Reink. and Berth.

Powdery mildew (Oidium sp.) frequently occurs on avocado
foliage. It may become sufficiently serious in nurseries on un-
sprayed trees to warrant spraying for its control. In commer-
cial groves sprayed regularly for control of fruit diseases pow-
dery mildew, although usually present on leaves near the ground,
is never of sufficient importance to justify the expense of special
sprays for its control. Infections on young expanding leaves
appear as dark green areas with a powdery, white, spore-bearing
growth on the under sides. The under surface of infected areas
on mature leaves generally is at first purplish brown and more

Florida Agricultural Experiment Stations

or less covered with the white powdery growth. With age the
white surface growth may disappear, leaving conspicuous net-
like markings (Fig. 36) at the sites of infection. The latter
appear from the upper side of the leaf as conspicuous yellow or
yellowish green areas.

Fig. 36.-Powdery mildew markings on avocado leaf.

An algal spot (Cephaleuros virescens Kze.) sometimes occurs
on avocado leaves and needlessly alarms the grower. The prom-
inent circular spots are greenish-gray, becoming reddish brown
when the alga is fruiting (Fig. 37). They may be found on a
large number of leaves, but no permanent damage is known to
result. A fruit blemish caused by the alga has been observed
infrequently on unsprayed fruits of the avocado (43). This
alga is of economic importance as a pest on some other fruits
in Florida.
A seedling blight caused by species of Phytophthora some-
times occurs in avocado nurseries. A severe outbreak occurred in
a nursery near Princeton, Florida, in 1948 (8) during a period of
heavy rain and high humidity. No serious recurrence of the dis-
ease has been observed, but a few infected seedlings and young
grafted trees occasionally have been observed in nurseries.
Spacing the trees to provide adequate circulation of air and oc-
casional spraying with a mild copper fungicide usually prevents
the disease.

The Florida Avocado Industry

Fig. 37.-Algal spot on avocado leaf.

Sun-blotch, caused by a virus, has been observed only a few
times in Florida. The disease was reported by Stevens (52) in
1929 and several other cases have been observed. The only
known method of transmission is through use of budwood, graft-
wood or seeds from infected trees. It probably was brought to
Florida from California as diseased propagating material. Symp-
toms include yellowish or white depressed markings on green
fruit (Fig. 38), depressed streaks on green twigs, slightly dis-
torted growth of leaves and stunting. Control consists of com-
plete destruction of diseased trees.
Cankers occasionally occur and probably are caused by a
number of species of fungi. Symptoms include dead, sunken,
cracked areas in the bark, accompanied by reddish-brown stain-
ing of affected tissues, sometimes with exudation of white, sug-
ary material. Treatment consists of cutting away the diseased
bark well into healthy bark and, after the cleaned area dries,
painting it with a water suspension of cuprous oxide or freshly
prepared bordeaux paste.

Several troubles causing an unhealthy or diseased appear-
ance of the foliage are due wholly to environmental conditions
and not to any organism.
Little-leaf or mottled-leaf is a nutritional trouble caused by
a deficiency of zinc available to the plant. A mottled or frenched
condition of the foliage has been observed for years, principally
on the calcareous rock of District 1. It develops occasionally
during the long winter dry season, but usually the symptoms

84 Florida Agricultural Experiment Stations

Fig. 38.-Sunblotch symptoms on young avocado fruits.

Fig. 39.-Zinc deficiency of avocado. Left, leaves with
severe symptoms; right, normal leaf.

The Florida Avocado Industry

have disappeared with the coming of spring rains. Occasionally
severe symptoms have developed in some groves. Usually this
follows the injudicious application of cyanamide or of large
amounts of other relatively pure chemical fertilizers. In more
severe cases, in addition to the chlorotic areas between the veins,
the leaves are small, narrow, recurved and stiff. Also the nodes
are shortened to form a rosette type of terminal growth and
defoliation and dieback occur (Fig. 39). Fruit normally pyri-
form in shape may be oval to round and undersized (Fig. 40).

Fig. 40.-Zinc deficiency of avocado. Left, normal Lula fruit;
right, Lula fruit with symptoms of severe zinc deficiency.

Use of 10-5-100 zinc sulfate-lime spray (10 pounds zinc sulfate,
5 pounds hydrated lime, 100 gallons of water) has corrected se-
vere cases, while a 5-21/2-100 spray seems adequate for correc-
tion of milder cases (42). For maintenance a 2-1-100 spray is
recommended or if neutral zinc is used a 1-100 spray will be
adequate. Spray applications are most efficient when made just
as a flush of growth is starting. They may be combined with
a fungicidal spray by adding the recommended amounts of zinc
and lime to 100 gallons of copper fungicide.

Florida Agricultural Experiment Stations

Chlorosis caused by a deficiency of available iron has become
an increasingly serious problem in recent years on avocado trees
growing in calcareous soils of District 1 (20). Chlorosis may
be evident on individual branches or over entire trees and chlor-
otic trees usually are scattered among other trees showing a
normal green color. In mild cases the tree fails to set fruit or
produces yellow fruit, the leaves are generally chlorotic with
veins remaining green, but there is little reduction in size of
leaves. Many of these milder cases recover without any special
treatment. In more severe cases the leaves are reduced in size,
show burning of the tips and margins and may fall prematurely.
In extreme cases dieback of branches occurs.
Chlorosis due to iron deficiency can be corrected by soil ap-
plications of certain iron chelates. Response has been obtained
from treating bearing trees with 50 grams of iron per tree in
the form of NaFe-DPTA or Fe-HEEDTA, using 50 gallons of
water to flush the material into the soil. Treatment of large
trees is expensive and probably should be used only in severe
cases. Chlorosis on young trees can be corrected by using less
of the chelate. Quicker and more complete correction has been
obtained from the use of certain experimental iron chelates.
Ground applications of iron sulfate and sprays of iron sulfate
or iron chelates have been ineffective and sometimes cause injury.
Boron deficiency may be of importance in the growing of
avocados in Florida. In addition to the shortened internodes
and greatly swollen axillary buds, accompanied by tip burning
of leaves and premature defoliation, reported (15) on young
trees in sand soils, there is good evidence that abnormal seed
development in some varieties is a manifestation of boron de-
ficiency (Fig. 41). Internal browning and under-development
of seed frequently occurs when the boron content of the dried
leaves is less than 40 ppm, but does not occur at 50 ppm (25).
Boron toxicity symptoms are seen when the boron content ex-
ceeds 100 ppm. On Rockdale limestone soils a pound or more of
borax per tree per year generally is required to cause toxicity.
Borax leaches from these soils and it appears from tests to date
that the application of 20 pounds per acre is required each year
on such soils to eliminate the brown seed condition.
In abnormally dry periods during the summer fruit of Booth
8 sometimes wilts on the trees, leaves and fruit drop, and feeder
roots die. Such trees may die if the fruit is not removed when
wilting starts. A high percentage of the fruit of such trees has

The Florida Avocado Industry

brown seeds and the boron content of the leaves is always low.
Probably boron deficiency contributes in large part to the wilt.
Borax may be applied with fungicidal sprays in amounts up
to 11/2 pounds per 100 gallons, but on limestone soils more than
one application is required to produce a noticeable effect on bo-
ron-deficient trees. On acid sandy soils there is more danger of
reaching toxicity levels with boron and not more than one spray
application per year is suggested.

Fig. 41.-Brown seed in Waldin fruit, symptomatic of boron deficiency.

While copper and manganese deficiencies occasionally have
been observed on young avocado trees, they are of little impor-
tance and can be avoided readily (see fertilization of non-bearing
Tipburn is the term applied to the irregular dying of tips and
margins of mature leaves. While it may occur on all varieties,
it is more common on some varieties, such as Taylor. The dead
areas have a scorched appearance and become infected with
Colletotrichum gloeosporiodes and other fungi.
Tipburn usually appears during the normally dry winter
months, especially following dry windy periods. It may be
caused by an accumulation of salts at the leaf tips. High chloride
content in the soil or irrigation water may cause tipburn. The







Algal Spot

Varietal Susceptibility
Very Susceptible
Many seedlings

Moderately Susceptible
Booth 7, 8, 1, 3, 5
Fuerte Tonnage
Hickson Seedlings

Slightly Susceptible
Most other varieties
and some seedlings

All susceptible

Cercospora and An-
thracnose control
more difficult on
late maturing va-

Cuprous oxide(80% Cu)
1% lbs.-100 gallons

Tri-basic copper sulfate
4 lbs.-100 gallons

Bordeaux 6-6-100

Other forms of neutral
copper which have
proved effective; use
equivalent in metallic
copper to tri-basic cop-
per sulfate
4 lbs.-100 gallons
Cuprous oxide
(80% Cu)
1 lb.-100 gallons
Tri-basic copper sulfate
3 lbs.-100 gallons
Bordeaux 4-4-100
Other forms of neutral
copper; use equivalent
in metallic copper to
tri-basic copper sulfate
3 lbs.-100

(4) May 1 to May 15.

(5) June 1 to June 15.

(6) July 10 to July 15.

(7) September 1 to 15.

Timing of
(1) When bloom buds
just begin to swell;
usually late in Jan-
uary but later in
some years.
(2) During late bloom
when many fruits
have set; Mid-Feb-
ruary to March 15.

(3) Three to four
weeks after (2).

Pollock and other very early
varieties require application (4)

Lula, Waldin, Trapp, Booth 7,
Booth 8 and other midseason
varieties require applications
(4) and (5).

Taylor, Nabal, Choquette and
other late varieties require ap-
plications (4), (5) and (6). In
some years (7) will be beneficial
for control of anthracnose at
bruises and insect stings.

Varietal Requirements
Very susceptible varieties re-
quire applications (1), (2) and

Moderately susceptible varieties,
if interplanted with Lulas or
scabby seedlings, require appli-
cation (1) and in years favor-
able to scab, (1) and (2). If
not interplanted with suscept-
ible varieties, applications (4),
(5) and (6) usually keep scab
under control for moderately
and slightly susceptible varie-
ties and (1), (2) or (3) are not

Compatibility with
Arsenate of lead
Wettable sulfur

Neutral Coppers
Benzene hexa-
Arsenate of lead
Wettable sulfur

Since avocado fruit
and leaves are wet
with difficulty, the
most effective
spreaders should
be added to the
sprays at concen-
trations required
to provide ade-
quate wetting,
without excessive
run-off of the par-
ticular brand of
spreader used.

The Florida Avocado Industry

trouble may also develop as a result of fertilizer injury (Fig. 42)
to the roots and usually accompanies severe deficiencies of iron
or zinc.

Fig. 42.-A type of tipburn on avocado leaves following
heavy application of cyanamide to trees.

Most of the early work on avocado pests in Florida was done
by Moznette, whose bulletin (34) was written in 1922. Since
then new pests of the avocado have appeared, additional biologi-
cal information has been obtained on several species and many
new insecticides and acaricides have become available, resulting
in improved control measures. This new information has re-
cently been compiled in a bulletin by Wolfenbarger (66) and
the reader is referred to it for a more comprehensive discussion
of insect pests of the avocado in Florida.
The avocado serves as host for a considerable number of
insect pests and mites, but only a half dozen of these cause in-
juries severe enough to warrant the application of pesticides.
All have natural enemies which tend to keep populations con-
trolled and most avocado pests appear sporadically. Trees should
be examined frequently for infestations. When pests increase

Florida Agricultural Experiment Stations

to proportions where extensive tree or crop damage is likely to
occur, the appropriate pesticide should be applied. Usually the
best control program is to apply pesticides only when they are
urgently needed.
Most pesticides are poisonous and should be handled with
caution. The labels on pesticide packages should be read and the
safety precautions carefully followed. It is important that all
workmen as well as foremen and supervisors understand and
follow all safety precautions when working with parathion, mala-
thion and other highly toxic insecticides.

During warm, dry weather from December to March this mite
(Oligonychus yothersi (McG.)) is often found in large numbers
on avocado trees. The mite is very small, but its bright red
color makes it easily visible to the naked eye when it moves. It
is a sucking type pest, feeding on the upper leaf surface only
and causing the foliage to appear reddish-gray to rusty brown.
This discoloration is first evident along each side of the midrib
and larger veins, but it widens as the mites extend their feeding
area until it finally may cover the entire leaf. By this time the
leaves are badly devitalized and soon may be shed. The grower
should be on the lookout for the first signs of injury during dry
weather and not wait for brown leaves to appear. Wolfenbarger
suggests that control measures be applied when the population
reaches six or more mites per leaf.
Control is readily accomplished by applying 4 to 8 ounces of
sulfur dust per tree (or 30-40 pounds per acre), depending on
the size of the trees, from either a crop dusting plane or a ground
machine. Wettable sulfur, 10 pounds per 100 gallons, as a fo-
liage spray also will give satisfactory control. Some of the new
acaricides, including demeton, Kelthane and ovex have been found
more effective than sulfur. Usually commercial control is ob-
tained with one thorough application.

Dic t y o s p e r m u m scale, (Chrysomphalus dictyospermi
(Morg.)), is the most injurious scale insect attacking the avo-
cado in both nursery and grove in Florida. In the grove the
scale is found on twigs, young vigorous limbs, and occasionally
foliage and fruit. Heavy infestations weaken and often kill
smaller branches. First indication of injury is a cracked and

The Florida Avocado Industry

darkened appearance of the normally green bark. The bark of
badly infested trees may be almost concealed by the scales in the
crotches of small branches. Inert residue from bordeaux and
other sprays is a factor in increasing and maintaining this pest
in groves. The insects are more abundant in the summer and
early fall. The scale covering is about 1/25 inch in diameter,
brownish-gray and circular. In the crawler stage the insect is
yellow. Injury caused by this species, in common with other
scales, is through loss of sap sucked from the plant tissues.
Pyriform scale (Protopulvinaria pyriformis Ckll.) is not so
destructive in District 1 as the foregoing species, but the fruit
appearance is marred by a sooty-mold fungus which grows on
the honeydew excreted by this insect. In District 2 it is some-
times a serious pest. It is found on the under surface of leaves
and occasionally on the fruit. The adult is pear-shaped, reddish
brown with a whitish margin and about 1/8 inch in length.
Other scale insects frequently observed infesting avocado
leaves are Florida red (Chrysomphalus aonidum (L.)), latania
scale (Aspidiotus lataniae Sign.) and Florida wax scale (Cero-
plastes floridensis Comst.). Still other species occasionally in-
fest avocados. Usually these species are of minor importance
and are controlled by parasites or may be controlled by scalecides
recommended for the more important scale insects.
Oil emulsions usually are effective in controlling scale in-
sects, if applied at high pressures and in sufficient strength.
Oil emulsions with actual oil content of 11/4 to 11/2 percent in
summer and 11/2 to 12 percent in winter are recommended. A
combination of oil and parathion, using 1 gallon of emulsive oil
and 1 pound of 15 percent wettable parathion per 100 gallons
of spray, will give excellent control. Malathion at 2 to 3 pounds
of 25 percent wettable powder, instead of parathion, may be
combined with oil. Parathion at 11/. pounds of 15 percent wet-
table or malathion at 4-5 pounds of 25 percent wettable powder
per 100 gallons of water will give effective control of scale in-
sects, but if applied alone during the dry season tends to give
rise to injurious infestations of avocado red mite. Oil emulsions
may injure or defoliate trees if applied during a very dry period,
when temperatures are above 90 F., or just before or during a
cold period.
This native insect (Heilipus squamosus Lec.) was first rec-
ognized as injuring or killing avocado trees by Wolfenbarger


Fig. 43.-Avocado tree girdler. A, young avocado fruit feeding holes made by adult beetle (enlarged). B, pupa (below)
and larva removed from avocado tree (about 3 X). C, avocado graft showing type of injury caused by adult. (Photographs
by D. O. Wolfenbarger.)

The Florida Avocado Industry

(64) in 1947, although there is evidence that it had been infest-
ing avocado trees for at least eight years before then. Eight
percent of the trees in one planting were killed by this pest in
1947-48, but it has not been so injurious since that time.
The adult is a snout-nosed weevil about 3/ inch in length,
predominantly black with irregular white areas and spots on its
wing covers (Fig. 43A). Eggs are deposited in the inner bark
generally at or near ground level. The whitish larvae (Fig. 43B)
burrow in the inner bark or in the wood of small trees, feeding
and pushing out reddish brown frass as a sign of infestation.
Young trees may be girdled so completely that they die or make
only feeble growth. Later the adults emerge and feed on avocado
buds, twigs, blossoms or young fruits (Fig. 43C). All stages
of the beetle are present throughout the year, but adults are
more abundant in April to June and again in September and Oc-
Periodically examine tree trunks near the ground level for
frass, indicative of larval infestation. Excision of larvae, fol-
lowed by painting the wounds with a good commercial pruning
paint, is recommended for control.

In recent years the greenhouse thrips (Heliothrips haemor-
rhoidalus (Bouche)) has caused much discolored, cracked and
scarred fruit. Loss from cullage of such fruit has been serious
on certain varieties in some groves. These insects tend to con-
gregate and feed on leaves and on fruit during the late summer
and fall. Frequently they are found where two or more fruits
contact one another or where a leaf contacts a fruit. Varieties
which tend to bear their fruit in clusters, such as Booth 8, Waldin
and Hall, are likely to sustain more severe thrips injury than
varieties which bear fruit singly.
Thrips injury appears first as silvery or ashy-gray patches
on fruit and on upper leaf surfaces, changing later to brownish
with the epidermis becoming thickened and cracked (Fig. 44).
Brown to black small drops of hardened excrement are visible
and all stages of the insect may be found on the infested parts.
Nymphs are yellowish white in color with red eyes. Adults
are black with fringed wings and are about 1/20 inch in length.
Periodic examinations of fruit beginning in August and con-
tinuing until fruit harvest is recommended to detect infestations
before serious damage occurs.

94 Florida Agricultural Experiment Stations


Fig. 44.-Typical injury by greenhouse thrips to Hall fruit and foliage.

The Florida Avocado Industry

If thrips control is necessary 40% nicotine sulfate at 1 pint,
parathion at 1 pound of 15% wettable powder or malathion at
3 pounds of 25%/ wettable powder, or lindane at 1 pound of 25%
wettable powder to 100 gallons of water may be used.
The red-banded thrips (Selenothrips rubrocinctus (Giard.))
is sometimes found attacking avocado leaves and fruit and at
times may cause as much damage as the greenhouse thrips. The
nymphs are generally yellowish with two red bands on the ab-
domen. The adults are quite similar in size and appearance to
adult greenhouse thrips. Type of injury and control are the
same for both species.
Flower thrips (Frankliniella cephalica (Craw.)) are some-
times abundant in avocado flowers. There is no conclusive evi-
dence that they cause injury severe enough to warrant the use
of insecticides.
Occasionally avocados have suffered serious crop injury from
the attack of certain plant bugs known as mirids, closely re-
lated to the tarnished plant bug. One or more species of these
sucking insects may attack the opening buds and recently set
fruits in large numbers for short periods in the spring and may
cause much dropping of young fruit or development of malformed
fruit. Apparently they migrate from Spanish-needle or other
herbaceous plants growing in or near the grove. Fortunately,
such migrations do not occur every year and, when they do, are
of short duration. In most years these bugs are so few in num-
bers that they require no insecticidal treatment.
The insects are approximately 1/8 inch in length with color
patterns of varying mixtures of green and brown. They are
shy, hiding when possible or, in the adult stage, flying away when
Because of the seriousness of the sporadic attacks, the grower
should make frequent examinations of expanding buds, blossom
panicles and young fruits. If more than three mirids per blos-
som cluster are found, prompt spraying with benzene hexa-
chloride, 2 pounds of 10% gamma-isomer per 100 gallons of
water, is advisable.
The pumpkin bug (Nezara viridula (L.)) may feed on bloom
panicles and fruit stems and on mature or nearly mature fruit,
where the stings permit entrance of fungi and development of
decay. Although these insects cause only minor damage in
most years, occasionally they cause serious losses on late-matur-

Florida Agricultural Experiment Stations

ing varieties such as Taylor, Lula and Choquette. The greenish-
colored bugs are a little more than 1/2 inch long and slightly more
than half as wide, being broadly oval in outline. The young are
smaller, nearly circular in outline and are bluish with some
red or brown markings. Pumpkin bugs emit a strong, disagree-
able odor.
Parathion, applied as a spray at 2 pounds of 15% wettable
powder per 100 gallons of water, will give practical control if
these pests become abundant on fruit approaching maturity.
During spring and summer young expanding leaves may be
infested with the larvae of a small gray moth (Gracillaria per-
seae Busck). These larvae roll the leaves by drawing them in-
ward from the margin. They are chewing insects and when
abundant give the foliage a very ragged appearance. The actual
damage, however, is usually not sufficient to warrant spraying.
Spraying with arsenate of lead at the rate of 3 pounds per 100
gallons of water will control the pest. If a copper fungicide is
applied at the time the leaf-roller is active, the lead arsenate
may be added to it.
The following insects at times have become troublesome on
avocados, but seldom cause much damage in well-kept groves.
Infestations are usually localized and sporadic, or, if present
more or less continuously, cause so little damage that the expense
of control is impractical.
Mealybugs (Pseudococcus sp.) are sometimes found on avo-
cado foliage, especially under shaded conditions in nurseries.
When infestations are troublesome under slat-shade, the insects
may be washed off with water applied with force from a hose
or scrubbed from the leaves with a suds of laundry or fish-oil
soap. Spot spraying of infested trees or branches with para-
thion, as used for pumpkin bug and scale control, will also con-
trol mealybugs.
Ambrosia beetles of one or more species sometimes burrow
into branches of large avocado trees or into the trunks of small
trees. The portion beyond the burrow may wilt and die from
infection carried into the burrow by the beetle. A tiny hole sur-
rounded by a white crystalline deposit of dried exuded sap is
the characteristic sign of ambrosia beetle attack. Removal and
destruction of infested portions is recommended for control.

The Florida Avocado Industry

Avocado lace-bug (Acysta perseae Heid.) occasionally may
infest the undersides of leaves. The reddish to brownish-black
bugs with lace-like wings are about 1/12 inch long. Benzene
hexachloride at 2 pounds of 10% gamma isomer or 1 pint of
40% nicotine sulfate or 1 pound of 15% parathion wettable
powder to 100 gallons are effective sprays if control measures
become necessary.
Avocado whitefly (Trialeurodes floridensis (Quaint.)) occa-
sionally may be found in appreciable numbers on avocado, espe-
cially on young, vigorously growing trees. They cause an injury
very similar to that caused by pyriform scale and sooty-mold de-
velopment also follows infestations of this pest. The adult is a
pale yellow insect with white wings and is about 1/25 inch in
length. Pupae are circular in shape with fringed margins. Con-
trol is seldom necessary, but may be accomplished with insecti-
cides recommended for scale control.
Moth larvae of several species feed on avocado fruit, twigs
or foliage. Natural enemies usually control these pests and in-
festations rarely become serious. They are also controlled more
or less by pesticides applied to control the more important pests.
Beetles of several species with general feeding habits some-
times feed on avocado foliage. Such feeding is generally local-
ized, often limited to a few trees or to branches of a tree.

Florida Agricultural Experiment Stations


The U. S. Food and Drug Administration has established
tolerances on amounts of certain pesticide residues that can
remain on avocado fruit. These are given in the chart below
along with the recommended waiting periods between last appli-
cation and harvest.
Tolerances and waiting periods as of March 1, 1963 for ma-
terials recommended in this bulletin follow:

Tolerance Minimum Days
Pesticide (ppm) Before Harvest
Benzene Hexachloride (BHC) 5 Use just before blossom-
ing or just after fruit
DDT 7 30 days
Lead Arsenate 7 Remove excess residues
(Combined lead) at harvest time.
Lindane 10 Use just before blossom-
ing or just after fruit
Malathion 8 7 days
Nicotine Sulfate 2 No time limitations
Oil emulsion Exempt No time limitations
Parathion 1 21 days
Sulfur, wettable No time limitations
Sulfur, dust No time limitations
These materials are generally recognized as safe and require no limiting tolerances
when used as recommended.


Insecticides are poisons and should be handled as such. Read
the label on each pesticide container before each use and heed
all cautions and warnings. Store pesticides securely in original
labeled containers away from food and feed. Dispose of empty
containers safely.
Parathion is especially toxic and is not recommended for
dooryard avocado trees.

The Florida Avocado Industry


1. Agricultural Marketing Service. USDA United States Standards for
Florida avocados. F. R. Doc. 57-6382. Aug., 1957.
2. Avocado Administrative Committee. Ann. Rept. of the Avocado Ad-
ministrative Committee, 1955-56 season 1-20. Processed.
3. CAMP, A. F. et al. Citrus industry in Florida. Fla. State Dept. Agr.
Tallahassee. 1957.
4. CAMPBELL, JOHN D. Letter of Oct. 30, 1956.
5. CLARK, O. Avocado pollination and bees. Calif. Avoc. Asso. Rept.
1922-23: 57-62. 1923.
6. CLARK, O. Avocado pollination tests. Cal. Avoc. Asso. Rept. 1923-24:
16-22. 1924.
7. CLARK, O., and A. CLARK. Results of pollination and other experi-
ments on the avocado at Point Loma Homestead. Cal. Avoc. Asso.
Rept. 1925-26: 85-94. 1926.
8. CONOVER, ROBERT A. A seedling blight of avocado caused by Phytoph-
thora palmivora. Phytopath. 38: 1032-1034. 1948.
9. COOPER, WILLIAM C., and J. R. FURR. The cinchona veneer-graft meth-
od of propagating subtropical fruit trees. Proc. Fla. State Hort.
Soc. 58: 176-180. 1945.
10. CRANDALL, BOWEN S. Phytophthora cinnamomi root rot of avocados
under tropical conditions. Phytopath. 38: 123-130. 1948.
11. CRUESS, W. V., and E. HARROLD. Investigations on the utilization of
cull avocados. Yrbk. of the Cal. Avoc. Assoc. 1927: 34-38. 1928.
12. DAVIS, SAM J. Avocado growing in the Redland District. Proc. Fla.
State Hort. Soc. 52: 71-73. 1939.
13. DUCHARME, E. P., and R. F. SUIT. Nematodes associated with avocado
roots in citrus spreading decline areas. Plant Dis. Rept. 37: 427-
428. 1953.
14. ELLIOTT, J. M. Practical pointers on budding and grafting avocados.
Cal. Avoc. Asso. Rept. 1922-23: 23-24. 1923.
15. FURR, J. R., PHILIP C. REECE, and F. E. GARDNER. Symptoms exhibited
by avocado trees grown in outdoor sand cultures deprived of vari-
ous mineral nutrients. Proc. Fla. State Hort. Soc. 59: 138-145.
16. GALLATIN, M. H. Fertilizer, irrigation studies on avocados and limes
on the Rockdale soils of the Homestead area. Proc. Fla. State
Hort. Soc. 66: 228-230. 1953.
17. HARDING, P. L. The relation of maturity to quality in Florida avo-
cados. Proc. Fla. State Hort. Soc. 67: 276-280. 1954.
18. HARKNESS, ROY W. Weed control studies around young avocado trees.
Proc. Fla. State Hort. Soc. 63: 251-261. 1950.
19. HARKNESS, ROY W. Chemical and physical tests of avocado maturity.
Proc. Fla. State Hort. Soc. 67: 248-250. 1954.
20. HARKNESS, ROY W., and J. L. MALCOLM. Iron chlorosis in avocados.
Proc. Fla. State Hort. Soc. 70: 297-300. 1957.

100 Florida Agricultural Experiment Stations

21. HODGSON, R. W. The California avocado industry. Cal. Agr. Ext. Serv.
Cir. 43, 1930. Revised 1947.
22. JOHNSON, H. D., and P. G. BREAKIRON. Protecting perishable foods
during transportation by truck. USDA Agr. Handb. No. 105. 1956.
23. KROME, W. H. The Federal Marketing Agreement. Proc. Fla. State
Hort. Soc. 67: 268-271. 1954.
24. KROME, W. J. The Medora method of top working avocados. Proc.
Fla. State. Hort. Soc. 29: 149-151. 1916.
25. LINCOLN, F. B., and RoY W. HARKNESS. Ann. Rept. Fla. Agr. Exp.
Sta. 1952: 274 and 1953: 314. 1952, 1953.
26. LYNCH, S. J., and S. GOLDWEBER. Some effects of nitrogen, phosphorus
and potassium on the yield and tree growth of avocados. Proc.
Fla. State Hort. Soc. 69: 289-292. 1956.
27. LYNCH, S. J., and A. L. STAHL. Studies in the cold storage of avocados.
Proc. Fla. State Hort. Soc. 52: 73-78. 1939.
28. MACFIE, G. B., JR., and A. L. STAHL. Investigations on the utilization
of cull avocados. Proc. Fla. State Hort. Soc. 68: 136-137. 1955.
29. MALCOLM, JOHN L. Water soluble phosphorus and potassium in the
soil of lime and avocado groves in Dade County. Proc. Fla. State
Hort. Soc. 64: 285-292. 1951.
30. MALCOLM, JOHN L. The effects of longtime avocado culture on the
composition of sandy soil in Dade County. Proc. Fla. State Hort.
Soc. 69: 313-324. 1956.
31. MALCOLM, JOHN L., and ROY W. HARKNESS. Irrigation experiments
with tomatoes on a Rockdale soil. Proc. Fla. State Hort. Soc. 70:
126-133. 1957.
32. MATTILL, H. A. Avocado digestibility. Cal. Avoc. Asso. Rept. 1916:
93-95. 1916.
33. MOWRY, H. Symbiotic nitrogen fixation in the genus Casuarina. Soil
Science 36: 409-424. 1933.
34. MOZNETTE, G. F. The avocado: Its insect enemies and how to combat
them. USDA Farm Bul. 1261. 1922.
35. MUSTARD, MARGARET J. Effect of cold storage on some Florida avo-
cados. Proc. Fla. State Hort. Soc. 65: 180-186. 1952.
36. MUSTARD, MARGARET J., and A. L. STAHL. Packaging and storage of
mangos and avocados. Proc. Fla. State Hort. Soc. 62: 226-232.
37. NIRODY, B. S. Investigations in avocado breeding. Cal. Avoc. Asso.
Rept. 1921-22: 65-78. 1922.
38. PEECH, M., and T. W. YOUNG. Chemical studies on soils from Florida
citrus groves. Fla. Agr. Exp. Sta. Bul. 448. 1948.
39. POPENOE, WILSON. Manual of Tropical and Subtropical fruits. Mac-
millan. 1920.
40. POPENOE, WILSON, and LouIs O. WILLIAMS. The expedition to Mexico
of October, 1947. Yrbk. of the Cal. Avoc. Soc. for the year 1947:
22-28. 1947.