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AVOCADO

Persea species

Lauraceae

Common Name: Avocado, Alligator Pear (English); Aguacate, Palta (Spanish)

Origin: The avocado probably originated in southern Mexico but was
cultivated from the Rio Grande to central Peru before the arrival of
Europeans.

Species: Guatamalan (Persea nubigena var. guatamalensis L. Wms.), Mexican
(P. americana var. drymifolia Blake), West Indian (P. americana Mill.
var. americana). Hybrid forms exist between all three types.

Related species: Coyo (Persea schiedeana Nees), Anay (Beilschmiedia anay
Kosterm)

Adaptation: Avocados do well in the mild-winter areas of California,
Florida and Hawaii. Some hardier varieties can be grown in the cooler
parts of northern and inland California and along the Gulf Coast. The
northern limits in California is approximately Cape Mendocino and Red
Bluff. Avocados do best some distance from ocean influence but are not
adapted to the desert interior. West Indian varieties thrive in humid,
tropical climates and freeze at or near 32° F. Guatamalan types are
native to cool, high-altitude tropics and are hardy 30 - 26° F. Mexican
types are native to dry subtropical plateaus and thrive in a
Mediterranean climate. They are hardy 24 - 19° F. Avocados need some
protection from high winds which may break the branches. There are dwarf
forms of avocados suitable for growing in containers. Avocados have been
grown in California (Santa Barbara) since 1871.

DESCRIPTION

Growth Habit: The avocado is a dense, evergreen tree, shedding many
leaves in early spring. It is fast growing and can with age reach 80
feet, although usually less, and generally branches to form a broad tree.
Some cultivars are columnar, others selected for nearly prostrate form.
One cultivar makes a good espalier. Growth is in frequent flushes during
warm weather in southern regions with only one long flush per year in
cooler areas. Injury to branches causes a secretion of dulcitol, a white,
powdery sugar, at scars. Roots are coarse and greedy and will raise
pavement with age. Grafted plants normally produce fruit within one to
two years compared to 8 - 20 years for seedlings.

Foliage: Avocado leaves are alternate, glossy, elliptic and dark green
with paler veins. They normally remain on the tree for 2 - 3 years. The
leaves of West Indian varieties are scentless, while Guatamalan types are
rarely anise-scented and have medicinal use. The leaves of Mexican types
have a pronounced anise scent when crushed. The leaves are high in oils
and slow to compost and may collect in mounds beneath trees.

Flowers: Avocado flowers appear in January - March before the first
seasonal growth, in terminal panicles of 200 - 300 small yellow-green
blooms. Each panicle will produce only one to three fruits. The flowers
are perfect, but are either receptive to pollen in the morning and shed
pollen the following afternoon (type A), or are receptive to pollen in
the afternoon, and shed pollen the following morning (type B). About 5%
of flowers are defective in form and sterile. Production is best with
cross-pollination between types A and B. The flowers attract bees and
hoverflies and pollination usually good except during cool weather.
Off-season blooms may appear during the year and often set fruit. Some
cultivars bloom and set fruit in alternate years.

Fruits: West Indian type avocados produce enormous, smooth round, glossy
green fruits that are low in oil and weigh up to 2 pounds. Guatamalan
types produce medium ovoid or pear-shaped, pebbled green fruits that turn
blackish-green when ripe. The fruit of Mexican varieties are small (6 -
10 ounces) with paper-thin skins that turn glossy green or black when
ripe. The flesh of avocados is deep green near the skin, becoming
yellowish nearer the single large, inedible ovoid seed. The flesh is hard
when harvested but softens to a buttery texture. Wind-caused abrasion can
scar the skin, forming cracks which extend into the flesh. "Cukes" are
seedless, pickle-shaped fruits. Off-season fruit should not be harvested
with the main crop, but left on the tree to mature. Seeds may sprout
within an avocado when it is over-mature, causing internal molds and
breakdown. High in monosaturates, the oil content of avocados is second
only to olives among fruits, and sometimes greater. Clinical feeding
studies in humans have shown that avocado oil can reduce blood
cholesterol.

CULTURE

Location: Avocados will grow in shade and between buildings, but are
productive only in full sun. The roots are highly competitive and will
choke out nearby plants. The shade under the trees is too dense to garden
under, and the constant litter can be annoying. In cooler areas plant the
tree where it will receive sun during the winter. Give the tree plenty of
room--up to 20 feet. The avocado is not suitable for hedgerow, but two or
three trees can be planted in a single large hole to save garden space
and enhance pollination. At the beach or in windy inland canyons, provide
a windbreak of some sort. Once established the avocado is a fairly tough
tree. Indoor trees need low night temperatures to induce bloom. Container
plants should be moved outdoors with care. Whitewashing the trunk or
branches will prevent sunburn.

Soil: Avocado trees like loose, decomposed granite or sandy loam best.
They will not survive in locations with poor drainage. The trees grow
well on hillsides and should never be planted in stream beds. They are
tolerant of acid or alkaline soil. In containers use a planting mix
combined with topsoil. Plastic containers should be avoided. It is also
useful to plant the tub with annual flowers to reduce excess soil
moisture and temperature. Container plants should be leached often to
reduce salts.

Irrigation: Avocado trees may not need irrigation during the winter rainy
season, but watch for prolonged mid-winter dry spells. Over irrigation
can induce root which is the most common cause of avocado failure. To
test to see if irrigation is necessary, dig a hole 9 inches deep and test
the soil by squeezing. If it is moist (holds together), do not irrigate;
if it crumbles in the hand, it may be watered. Watch soil moisture
carefully at the end of the irrigating season. Never enter winter with
wet soil. Avocados tolerate some salts, though they will show leaf tip
burn and stunting of leaves. Deep irrigation will leach salt
accumulation.

Fertilization: Commence feeding of young trees after one year of growth,
using a balanced fertilizer, four times yearly. Older trees benefit from
feeding with nitrogenous fertilizer applied in late winter and early
summer. Yellowed leaves (chlorosis) indicate iron deficiency. This can
usually be corrected by a chelated foliar spray of trace elements
containing iron. Mature trees often also show a zinc deficiency.

Frost Protection: It is important to choose a cultivar that is hardy in
your area. Mexican types are the best choice for colder regions. Plant
above a slope for air drainage, or near the house for added protection.
In youth, protect with rugs, towels and such spread overhead on a frame.
For further protection heat with light bulbs and wrap the trunk with
sponge foam. These measures also permit tender cultivars to become
established in borderline locations; established trees are much hardier
than young ones. The upper branches can also be top worked with hardy
Mexican types, which will protect a more tender cultivar on lower
branches, as well as serving as a pollinator. Harvest fruit before the
frost season begins. Cold-damaged fruit turns black. Avocados are often
in bloom at the time of frost and the flowers are killed, but the tree
tends to rebloom. This is especially true of Mexican types.

Pruning: Columnar cultivars require pinching at early age to form a
rounded tree. Others need no training. Current orchard practice avoids
staking. The best results are obtained by fencing the tree with plastic
mesh for the first two to three years. Container and dwarf trees will
need constant staking. The skirts of avocado trees are sometimes trimmed
to discourage rodents, otherwise the trees are usually never pruned.
Branches exposed to sun by defoliation are extraordinarily susceptible to
sunburn and will surely die. Such branches should always be whitewashed.
It is better to avoid any pruning. Most cultivars are ill-adapted to
espalier. They are too vigorous. Avocado fruit is self-thinning.

Propagation: Desired clonal rootstocks can be be propagated by a method
known as the etiolation technique. The largest seed are planted in gallon
cans and the seedlings are then grafted to a root rot tolerant clonal
scion. When the stem of the graft reaches about 1/4 inch in diameter, the
top is cut off leaving a whorl of buds just above the graft. A 4 inch
band of black tar paper is formed into an extension of the can and filled
with vermiculite and placed in a dark box with high temperature and
humidity. When growth is some 3 - 4 inches above the vermiculite, the
plant is removed into the light where the upper portion quickly assumes a
green color. The tar paper collar is removed, the shoot is severed from
the seed and then placed in flats where the cuttings are rooted in the
conventional manner. Any seed may also be used for rootstock, but Mexican
types make the strongest growth and are the most often used. Plant
cleaned seeds as soon as they are ripe. The seedling plants are ready to
bud the following year. Budding is done in January, when suitable buds
are available. Larger stocks are worked by bark grafts in the spring.
Scions are collected Dec - Jan after the buds are well-formed. Paint and
cover the graft with a moistened plastic bag and place a vented paper bag
over the whole.

Pests and diseases: Rats and squirrels will strip the fruit. Protect with
tin trunk wraps. Leaf-rolling caterpillars (Tortrix and Amorbia) may
destroy branch terminals. Avocado Brown Mite can be controlled by
powdered sulfur. Six-spotted Mite is very harmful; even a small
population can cause massive leaf shedding. A miticide may be required if
natural predators are absent. Snails can be a problem in California.

Two fungi and one virus cause more damage than any pests. Dothiorella
(Botryosphaeria ribis) canker infects the trunk, causing dead patches
that spreads to maturing fruit, causing darkened, rancid smelling spots
in the flesh. Flesh injury begins after harvest and is impossible to
detect on outside. Mexican types are immune to trunk cankers but the
fruit is not. The disease is rampant near the coast and has no economical
control. Root Rot (Phytophthora cinnamomi) is a soil-borne fungus that
infects many plants, including avocados. It is a major disease problem in
California. Select disease-free, certified plants and avoid planting
where avocados once grew or where soil drainage is poor. The disease is
easily transported by equipment, tools and shoes from infected soils.
Once a tree is infected (signs include yellowing and dropping leaves),
there is little that can be done other than cut back on water. Sun Blotch
is a viral disease that causes yellowed streaking of young stems,
mottling and crinkling of new leaves and occasional deformation of the
fruit. It also causes rectangular cracking and checking of the trunk, as
if sunburned. It has no insect vector but is spread by use of infected
scions, contaminated tools and roots grafted with adjacent trees. It is
important to use virus-free propagating wood.

Harvest: The time of harvest depends upon the variety. Commercial
standards requires fruit to reach 8% oil content before harvesting.
Mexican types ripen in 6 - 8 months from bloom while Guatamalan types
usually take 12 - 18 months. Fruits may continue enlarging on the tree
even after maturity. Purple cultivars should be permitted to color fully
before harvest. Guatamalan types can be stored firm, at 40 - 50° F. for
up to six weeks. Mexican types discolor quickly and require immediate
consumption.

Miscellaneous: Leaf and seed extracts have been used for a variety of
medical application, including treatment of diarrhea and dysentery and as
an antibiotic.

CULTIVARS


Anaheim
     Origin Otto Keup, Anaheim, 1910. Guatamalan. Tree columnar,
productive. Fruit very large, to 24 oz., elongated glossy green, seed
small, oil 15%. Tenderest of cvs. for coast only. To 32° F. Season July.

Bacon
     Origin James Bacon, Buena Park,1954. Hybrid. Tree broad, productive.
Fruit small to medium, to 12 oz., round-ovoid, smooth green. Flesh only
fair, almost colorless,seed cavity molds rapidly. Hardy for Bay Area,
Central Valley. To 25° F.Season December.

Creamhart
     Origin Orton Englehart, Escondido,1969. Hybrid. Seedling of Reed.
Tree open, upright, branching. Fruit medium, to 14 oz., skin green flesh
extraordinarily pale,buttery, nearly fiberless. Not alternate bearing. To
30x F.Season April - July.

Duke
     Origin Bangor (Oroville), 1912.Tree vigorous, open, resists wind.
Fruit small, 12 oz., elongated pyriform, waxy green, skin paper-thin.
Flesh excellent, oil 21%.Seeds commonly used for rootstocks, resist root
rot. Extraordinarily hardy, recovers quickly from freeze, to 22° F.
Season October

Fuerte
     Origin Atlixco, Mexico, intro. Carl Schmidt, 1911. Hybrid. Tree
open, spreading, tall. Fruit large to very large, 16 oz., elongated
pyriform, skin dark green with numerous small raised pale spots, waxy
bloom, skin thin. Flesh good, oil 18%, seed medium. Formerly standard cv.
of California industry. Tends to bear in alternate years, unproductive
near coast or in north. To 26° F. Season December.

Ganter
     Origin Albert Rideout, Whittier, 1905. Mexican. Tree tall,
spreading, open. Fruit small, to 8 oz., long pyriform, skin paper-thin,
pale waxy green. Flesh good, oil 18%. Oldest avocado cv. in California.
Quite hardy, for Central Valley floor and far north. To 23° F. Season
October.

Gwen
     Origin Riverside, Robert Whitsell, 1982, patented. Seedling of Hass.
Tree dwarf, to 14 ft., low vigor. Fruit small, to 8 oz., a Hass look
alike, elongated green, flesh good. Most productive of dwarf avocados,
best dwarf for outdoor use, also for containers, greenhouse. Not hardy,
to 30° F. Season February - October.

Hass
     Origin Rudolph Hass, La Habra Heights, 1926. Seedling of Lyon.
Guatamalan. Tree rather open, not tall. Fruit medium, to 12 oz.,
pyriform, skin thick, pebbled, coppery purple. Flesh good, oil 19%, seed
fairly small. Currently the standard of the industry. To 26° F. Season
July.

Jim
     Origin John Reinecke, San Diego, 1939. Hybrid. Tree upright. Fruit
small to medium, to 10 oz., olive green, with long neck, oil 12%. To 26°
F. Season June.

Lula
     Origin George Cellon, Miami, 1919. West Indian. Tree dense, broad,
prolific. Fruit round, slightly pyriform, to 20 oz., slightly rough
glossy green, oil 12%. Only West Indian type recommended for California,
rather hardy, to 28° F. Season April.

Lyon
     Origin R. Lyon, Hollywood, 1908. Central American. Tree columnar,
slow growing, difficult to propagate, often scion incompatible. Fruit
commonly over 24 oz., dark glossy green, rough, pyriform, oil 21%. High
quality. Tender, to 30° F. Season April.

Mexicola
     Origin Coolidge, Pasadena, 1910. Mexican. Tree tall and spreading,
vigorous. Fruit small, 5 oz., round pyriform, skin paper-thin, purplish
black, waxy bloom. Flesh highest quality, seed very large. Hardiest cv.
known, seedlings useful as rootstocks in far north. Recovers rapidly from
freeze. Defoliated at 20° F, trunk killed at 17° F. Season September.

Mexicola Grande
     Seedling selection of Mexicola. Mexican. Tree tall and spreading
similar to Mexicola. Fruit 15% - 25% larger than Mexicola and somewhat
rounder in shape with better seed/flesh ratio. Skin paper-thin,
purple-black. High quality flesh with high oil content. Hardy to about
18x F.

Murrieta Green
     Origin Colima, Mexico, intro. by Juan Murrieta, 1910. Hybrid. Tree
slow growing, easily trained. Fruit large, to 18 oz., oblate, green,
resembling Fuerte. Flesh exceptional, oil 18%. Only cv. readily adaptable
to espalier. For coast and intermediate. To 27° F. Season September.

Nabal
     Origin Antigua, Guatemala, intro. by F.W. Popenoe, 1917. Tree dense,
columnar. Fruit handsome, large pyriform, to 17 oz., green, skin
resembles Fuerte. Flesh exceptionally high quality, oil 16%. Young trees
require pinching to force low branching. Tends to bear alternate years.
To 27° F. Season July.

Pinkerton
     Origin John D. Pinkerton, Saticoy, 1972, patented. Guatamalan. Tree
dense, productive. Fruit variable in size, 7 to 12 oz., skin thick,
pebbled, green. To 30° F. Season November.

Queen
     Origin Antigua, Guatemala, intro. by E.E. Knight, 1914. Guatamalan.
Tree broad. Fruit exceptionally large, to 24 oz., elongated, purple,
flesh excellent, oil 13%. Fairly hardy for large cv., worth trying in Bay
Area. To 26x; F. Season August.

Puebla
     Origin Atlixco, Mexico, intro. by Carl Schmidt, 1911. Mexican. Tree
broad, high branching. Fruit beautiful, medium to large, to 18 oz.,
ovoid, skin thin, lacquered maroon purple. Flesh excellent, oil 20%.
Least hardy Mexican type, to 29° F. Season December.

Reed
     Origin James S. Reed, Carlsbad, 1948. Hybrid. Tree columnar. Fruit
large, to 15 oz., round, skin thick, pebbled, black. Flesh good. To 30°
F. Season August.

Rincon
     Origin Carlsbad, Sam Thompson, 1944. Hybrid. Tree small. Fruit small
to medium, 10 oz., green, resembling Fuerte. Flesh good. For coast, Santa
Barbara and Ventura. To 27° F. Season January.

Ryan
     Origin Albert Rideout, Whittier, 1927. Hybrid. Tree low, spreading.
Fruit medium, to 14 oz., elongated, otherwise resembles Hass, skin thick,
pebbled, purple. Flesh good, oil 25%. For Inland Empire, Bay Area. To 26°
F Season August.

Spinks
     Origin E. Bradbury, Bradbury, 1911. Hybrid. Tree spreading. Fruit
medium, to 15 oz., round with small neck, tangelo shaped. Lacquered,
coppery purple, outstanding flavor, oil 16%. To 27° F. Season April.

Topa Topa
     Origin E.S. Thatcher, Ojai,1912. Mexican. Tree columnar, vigorous.
Fruit handsome, elongated pyriform, small to medium, 8 oz., smooth dark
purple with white waxy bloom. Skin paper-thin. Flesh rather poor, oil
15%, seed elongated. Seedlings commonly used for rootstocks. Hardy, for
far north. To 23° F.

Whitsell
      Origin Robert Whitsell, Riverside,1982, patented. Hybrid. Hass
seedling. Tree dwarf, to 12 feet, low vigor. Fruit small, 6 oz.,
elongated Hass look alike. Flesh good. Bears in alternate years. For
containers and greenhouse only, not hardy. To 30° F. February to October.

Wurtz (syn. Littlecado)
      Origin Roy Wurtz, Encinitas, 1935. Hybrid. Tree prostrate,
difficult to train, low vigor. Fruit dark green, medium, to 10 oz. For
containers and greenhouse. To 26° F. Season July.

Zutano
       Origin R.L. Ruitt, Fallbrook,1926. Hybrid. Tree columnar. Fruit
small to medium, to 10 oz. elongated smooth green, resembles Fuerte but
inferior, has fibers. Hardy for Bay Area, Central Valley. To 25° F.
Season November.

FURTHER READING

     See generally: California Avocado Society Yearbook, 1915 to present.
     Davenport, T.L. Avocado Flowering, Hort. Reviews 8: 257-289.
     Koch, F.D. Avocado Grower's Handbook, Bonsall Publications, 1983.
     Morton, Julia F. Fruits of Warm Climates. Creative Resources
			Systems, Inc. 1987. pp. 91-102.
     Ortho Books. All About Citrus and Subtropical Fruits. Chevron
			Chemical Co. 1985. pp. 16-19.

See Index of CRFG Publications, 1969 - 1989 and annual indexes of Fruit
Gardener for additional articles on the avocado.

© Copyright 1996, California Rare Fruit Growers, Inc.
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