1999. Revista
Chapingo Serie Horticultura 5: 49-54.
ECOLOGICAL
ADAPTATION AND THE EVOLUTION OF MODERN AVOCADO CULTIVARS
R. J. Knight,
Jr., C. W. Campbell.¹
¹ Tropical Research & Education
Center, University of Florida, 18905 S.W 280 Street, Homestead, FL
33031-1344; FAX 305-246 7003; e-mail rjk@icon.hmsd.ufl.edu.
Florida Agricultural Experiment Station
Journal Series N-01746
The geographic area in which the avocado originated,
stretching from Mexico through Guatemala and probably to Pacific coastal Costa
Rica, presents a diverse set of environments.
The three races that gave rise to modern avocado cultivars--Antillean
(West Indian), Guatemalan and Mexican--accordingly are adapted to quite
distinct environmental conditions. The
apparent Mexican-Guatemalan hybrids that originated around Atlixco in Puebla
State, Mexico, exemplified by ‘Fuerte’, showed remarkable adaptation when moved
to the Mediterranean-type subtropical microclimates common in southern
California, permitting a commercial industry to develop and later be
transferred to other parts of the world with similar climates, such as South
Africa, Israel and Chile. Selection
from the material originally brought from Mexico and Central America permitted
California to develop commercial cultivars exemplified by ‘Hass’, ‘Fuerte’ and
‘Reed’, among others, that lead the world in the area planted. Guatemalan cultivars are adapted to
elevated situations in Central America, yielding well under cool conditions and
forming an important dietary staple.
The Antillean race, which probably originated along the Pacific coast in
Central America, yields well in lowland tropical situations and warm
subtropical areas such as southern Florida.
Planting Guatemalan and Antillean avocados side-by-side in Florida early
in this century produced a new group of hybrid cultivars with quality superior
to that of most Antillean cultivars, adaptation to sea level locations, and an
extended range of season of maturity that permitted Florida to market fruit
into the autumn and winter, well past the season of pure Antillean
cultivars. The best of these hybrids,
‘Booth 7’, ‘Booth 8’, ‘Lula’, ‘Choquette’ and a few others have been exported
to warm locations about the tropical world for commercial planting. Thus, selection of avocado germplasm
imported from Mexico and Central America into California and Florida produced a
group of cultivars that are of significant commercial importance in the world.
Key
words: Persea americana, Plant introduction,
Varietal improvement, Germplasm
El área geográfica en donde el aguacate originó, que
incluye desde México, Guatemala y probablemente hasta la costa Pacífica de
Centroamérica, abarca una gran diversidad de ambientes ecológicos. Por ello, las tres razas que dieron origen a
los cultivares modernos de aguacate, la antillana, la guatemalteca y la
mexicana, muestran adaptación a condiciones ambientales muy distintas. Los que parecen híbridos entre las razas
mexicana y guatemalteca originados cerca de Atlixco en el estado de Puebla
(‘Fuerte’, por ejemplo) han mostrado una adaptación sobresaliente a los
microclimas subtropicales de tipo Mediterráneo en California. Esto permitió el desarrollo de una industria
comercial que después fue trasladada a otras partes del mundo de clima
semejante, como Sudáfrica, Israel y Chile.
La selección del material de México y Guatemala permitió el desarrollo
de cultivares comerciales, como por ejemplo ‘Hass’, ‘Fuerte’ y ‘Reed’, que ya
constituyen una mayoría de los aguacates sembrados en el mundo. Los aguacates guatemaltecos tienen
adaptación a condiciones elevadas, producen bien bajo condiciones frescas y son
un componente importante del alimento de la región. La raza antillana produce bien en los trópicos bajos y en
regiones cálidas y subtropicales como el sur de Florida. Cuando aguacates de raza guatemalteca y
antillana fueron sembrados juntos en Florida a principios de este siglo, se
produjo un grupo de cultivares nuevos cuya calidad resultó superior a la de la
mayoría de los cultivares de raza antillana.
Estos cultivares tienen buena adaptación a localidades cerca del nivel
del mar, y un tiempo extendido de producción, lo que permitió a Florida colocar
fruta en el mercado durante el otoño e invierno, más tarde que anterior
mente. Los mejores de éstos, ‘Booth 7’,
‘Booth 8’, ‘Lula’, ‘Choquette’ y otros más se han trasladado a regiones cálidas
tropicales para su cultivo
comercial. Así, la selección del germoplasma
traído de México y Centroamérica ha producido un grupo de cultivares que son de
gran importancia en el comercio mundial.
Geographic Origin of Avocado
The avocado, Persea americana Miller, apparently
originated in a broad geographic area stretching from the eastern and central
highlands of Mexico through Guatemala to the Pacific coast of Central America
(Smith 1966 and 1969, Popenoe 1927).
The Antillean (also called West Indian) race of avocado is believed to
have come into being on the western coast of Costa Rica (Ben Ya’acov, pers.
comm.).
Differentiation of the 3 Horticultural Races
Horticulturists
recognized three well-differentiated races, Mexican, Guatemalan and Antillean
(=West Indian) based upon morphology
and their climatic adaptations some 80 years ago, but the differences had first
been noted and recorded in the 17th century by Fray Bernabe Cobo in his Historia del Nuevo Mundo, written in
1653 (Popenoe 1934).
Origin of the Atlixco Population and Importation of Fruit to
California
Avocados have
been utilized and selected in what is now Puebla State of Mexico for some 9,000
years (Smith 1966 & 1969). The
first material involved was of the Mexican race native to the area, but in the
Valley of Atlixco in Puebla State, seedling avocado trees of the Mexican and
Guatemalan race have long been found, as well as apparent inter-racial hybrids
such as the original tree of ‘Fuerte’.
During the years from 1890 until 1911, when the Revolution interrupted
commerce, avocado fruit from the Atlixco area was sent to California to be
marketed in Los Angeles. Seeds from
this group were planted in considerable quantities about southern California,
so genes from Atlixco were well represented in the early cultivars selected for
asexual propagation when this became common. (Popenoe, 1926).
Selection of Cultivars from imported Atlixco Material in
California and Fuerte’s subsequent Importance to California and the world
In 1911, Fred
Popenoe, a nurseryman of Altadena, California underwrote a trip to Mexico by
Carl Schmidt, whose objective was to collect budwood of superior avocado
seedlings growing in the vicinity of Atlixco, in Puebla State. Avocados in Atlixco at that time were grown
from seed, but propagation by budding had recently become a standard practice
in California, making it possible to select and propagate named cultivars from exceptional
seedlings. One of the seedlings Carl
Schmidt collected in Atlixco, Number
15, named ‘Fuerte’ because of its strong growth in the nursery, has
subsequently had tremendous impact on the world avocado industry (Popenoe,
1926). By 1947 ‘Fuerte’ was the leading
commercial cultivar in present-day Israel (Oppenheimer, 1947), and by 1957, 60
percent of the avocado plantings in South Africa were of ‘Fuerte’ (Malan,
1957). ‘Fuerte’ has now been displaced
in most areas that have a Mediterranean-type climate by ‘Hass’, and other
adapted cultivars that are more dependably productive than is ‘Fuerte’, but for
many years it led the world in commercial production.
Ecological adaptations of Atlixco Avocados
Atlixco is
situated about 1,875 M above sea level and has an average rainfall of about 80
cm during the season from May to October.
The average mean annual temperature is about 12.5°C with a maximum of
32° C and a minimum of 2° C. (Shamel, 1936; Mosiño A. and García, 1974) Average mean temperatures available for two
California sites are higher than that for Atlixco: 18°C for Los Angeles and 17.2°C for San Diego, but the extreme
minimum temperature recorded at both the California cities, -2°C, is
considerably lower than the mean minimum for Atlixco (Court, 1974). The fact that a commercial industry was
built in southern California with germplasm brought for the most part from
Puebla State indicates that this material has the capacity to tolerate
occasional freezing weather, if this is of short duration. Additional evidence of its cold tolerance is
provided by success of the cultivars from California when they were introduced
into Palestine, which is now Israel (Oppenheimer, 1947).
Origin of Florida’s Antillean Population
The first
English-speaking settlers to arrive in the Miami area of south Florida in 1850
found Antillean avocado seedlings naturalized in the hardwood vegetation (hammocks) of the area. The origin of these trees is not known but
it is reasonable to believe they were introduced from Cuba during the Spanish
occupation of Florida, which ended in 1819 with the purchase of Florida from
Spain by the United States. (Antillean
avocados growing at St. Augustine to the north of Miami, killed by the severe
winter weather of 1835, were presumably of the same origin.) (Wolfe et
al., 1949)
Development of Asexually Propagated Cultivars in Florida and
their Relationship to Development of the Avocado Industry in Florida
Early in the
20th century George Cellon, a nurseryman of Miami, devised the first known
method for propagating avocados asexually by budding (Anonymous, 1946). This method was later replaced in Florida by
grafting, but it is still employed in California and other areas of commercial
production. Superior Antillean
seedlings of local origin were selected by Cellon as the first named cultivars
and two of these, ‘Pollock’ and ‘Trapp’ are still classified as commercial varieties in south Florida (Fla.
Avocado Administrative Committee,
1999).
Ecological Adaptations of Cuba’s and Florida’s Antillean
Avocados
The climate of
the part of Florida where Antillean Avocados have long been grown is classified
as Humid Semi-hot Semi-tropical, 4.45 by Papadakis’ (1966) classification. Climates in Cuba, where the Antillean avocado
is widely distributed, are classified by Papadakis as Humid Semi-hot Tropical
(Habana) or Moist Monsoon with 4-5 dry months (Cienfuegos). The annual mean temperature in Cuba is 25.5°
C, with a mean for January of 22.5º C and a mean for August of 27.8ºC, and
freezing weather never occurring (Inst. Econ., 1974). The main difference between south Florida’s climate and the
conditions common in Cuba is that freezing weather occasionally occurs in
Florida (a minimum of 0°C at Miami in February of some years) but this rarely damages the trees because
freezes are usually of short duration.
The annual mean temperature at Miami is 23.9°C, with a mean in January
of 19.4°C and a mean in August of 27.9°C (Court, 1974). In these conditions trees of the Antillean
race grow well, and thus it was possible to establish an industry based on
cultivars of this race.
Introduction of Guatemalan Avocados to Florida and their
Evaluation
The first
Guatemalan avocados ‘Colla’, ‘Collins’ and ‘Winslow’, were introduced into
Florida in 1906 (Fairchild, 1939).
Their ecological adaptations were quite different from those of the
Antillean cultivars that had been selected from Cuban importations. Climates prevailing in the Guatemalan
highlands have been classified as “humid tierra templada,” suitable for arabica
coffee, maize and rice, subdivided into moist monsoon with 4 (or less) dry
months, or ever humid (Papadakis, 1966).
Genotypes of avocado that thrive under these conditions would be
expected to undergo some stress when moved to warm, lowland climates such as
those normal in Cuba and southern Florida.
The base of the Guatemalan race is made up of a group termed the
“Guatemalan Criollos” by Schieber and Zentmyer (1980) which grows in Guatemala
and parts of Mexico and El Salvador at elevations from 1500 to 2400 meters (m)
and in some cases as high as 2600 m.
Plants at the last-named elevation are subject to frost in winter, and
all Guatemalan trees grow best at cooler temperatures than those preferred by
trees of Antillean race. Guatemalan
avocados brought to Florida survived but in most cases did not thrive,
apparently because of Florida’s year-round warm temperatures. Only one Guatemalan cultivar, ‘Taylor’, has
persisted to the present as a minor commercial variety in Florida.
Origin of Guatemalan-Antillean Hybrid Cultivars
The first
Guatemalan-Antillean hybrids to be recorded originated at the U. S. Department
of Agriculture (USDA) Plant Introduction Garden at Miami and were seedlings of
‘Colla’, ‘Collins’ and ‘Winslow’ each pollinated by an Antillean parent, and
were named ‘Collason’, ‘Collinson’ and ‘Winslowson’ (Fairchild, 1939). They constituted a new type of avocado on
the face of the earth, and were of immediate interest because they combined
enhanced fruit quality from the Guatemalan parent with adaptation to warm
lowland conditions derived from the Antillean parent. Furthermore, their fruit matured at a later season than did
Antillean cultivars, opening the autumn and winter markets to Florida fruit. ‘Collinson’ bears an attractive fruit that
unfortunately has some marketing defects, namely shy bearing under some
conditions, sensitivity to low storage temperatures and a tendency for the seed
to germinate while still in the fruit nevertheless it continues to be a minor
commercial cultivar in Florida. The
Guatemalan-Antillean hybrid cultivars that have achieved greatest commercial
success, however, resulted from the collaboration of a resident of Homestead
named Will Booth with W. J. and Isabelle Krome, proprietors of a nursery in
southern Dade County, Florida. The
Kromes had an experimental planting of mixed cultivars and selections of both
races. In 1920, Will Booth planted seeds
of Guatemalan cultivars from the Krome planting and grew them to fruiting age,
at which time their hybrid origin became evident (W. H. Krome, pers.
comm.). The tree designated ‘Booth 2’
first fruited in 1927, as did others in the numbered series of Booth
cultivars. Booth seedlings 1, 3, 5, 7
and 8 were propagated commercially in 1935 (Wolfe et al., 1949). They
appeared on the scene when Florida was experiencing severe competition from
Cuba in marketing its Antillean cultivars, which matured in summer and early
autumn (Brooks, 1929). The
Guatemalan-Antillean hybrids growing in Florida matured fruit in autumn and
winter, at a time when Cuba did not have fruit available to market, and thus
Florida’s avocado industry survived through growing these new cultivars. When trade between Cuba and the U.S.A. was
interrupted in the early 1960s, production of Antillean cultivars again became
important in Florida, as production of hybrid cultivars continues to be.
Effect of the Success of ‘Fuerte’ in California on Avocado
Culture Elsewhere; Establishment of ‘Fuerte’ Orchards in Israel, South Africa,
Chile, Australia
Avocados were
introduced to what is now Israel in 1908, but the named cultivars ‘Fuerte’ and
‘Dickinson’ were not brought in for trial before 1924. In 1934, ‘Benik’, ‘Nabal’ and ‘Queen’ were
imported from California. By 1947,
‘Fuerte’ was the leading commercial cultivar, but because of irregular bearing
a more dependable cultivar was sought.
Other cultivars under trial at that time were ‘Duke’, ‘Northrup’,
‘Mexicola’, ‘Ganter’, ‘Puebla’, ‘Caliente’ and ‘Anaheim’. ‘Hass’ was introduced into Israel in the
late 1940s and was added to the list of recommended varieties by the
mid-sixties (Ohad, 1965; Ticho and Gefen, 1965). ‘Fuerte’ continued to be the most commonly planted cultivar into
the late 1970s (Gustafson, 1967), and remains important. In recent years, cultivars which developed
within the country as well as some of California’s newer cultivars have been
planted in Israel. The subtropical Mediterranean
climate in that part of the world is characterized by winter weather (November
to mid-March) which is generally cool but may be as warm as 30°C on cloudless
days. (Occasional temperatures of 40°C
or above may occur during extremely hot “khamsin days” in springtime.) Freezing temperatures may rarely occur on
the coastal plain, with temperatures in some pockets lacking air drainage
having descended as low as -4.5°C (Oppenheimer, 1947). The success of Israel’s avocado industry,
based on germplasm from California, most of which came originally from Mexico,
is exemplified by production figures over a 35-year period, rising from 800
metric tons (mt) in 1961 to 75,900 mt in 1996 (FAOSTAT database, 1999). Avocados are widely grown in South Africa,
principally in the eastern and northern Transvaal and also in frost-free
coastal belts of the Cape Province and Natal, as well as Swaziland (Malan,
1957). The climate for Nelspruit in the
Transvaal is classified as Low Tierra Fria with a dry monsoon, suitable for
wheat, maize, rice and cotton (Papadakis, 1966). Nelspruit is also a center of citrus production. In the 35 years from 1961 to 1996, South
African avocado production increased by more than 10-fold, from 4,700 mt to
49,100 mt (FAOSTAT Database, 1999).
Chile has a
variety of climates, among them subtropical semiarid Mediterranean and marine
Mediterranean (Papadakis, 1966), which are well suited to avocado culture. Seedlings of Mexican race were known in the
country from colonial times, but California commercial cultivars were first
imported by the Ministry of Agriculture in 1928. Because of climatic problems where they were planted, in
Santiago, they attracted little attention.
In 1932, however, a shipment of 14 cultivars of Mexican, Guatemalan and
hybrid origin was received and planted at La Cruz, Chile, followed by additional
cultivars in a couple of other shipments.
These formed the nucleus of a developing avocado industry. ‘Hass’ was imported in 1944. In 1958, all production was marketed within
Chile, and prices were occasionally depressed when fruit supplies exceeded
demand (Magdahl, 1958). By 1965,
however, demand was good and the industry was growing (Schmidt, 1965). This trend continued and accelerated as
Chile entered the export market; its avocado production rose from 8,000 t in
1961 to 55,000 t by 1996 (FAOSTAT Database, 1999).
Avocados are
grown in a variety of locations in Australia along the eastern coast of the
country (Schulz 1961), with climatic classifications ranging from humid
subtropical through semi-hot semitropical, semi-Mediterranean subtropical, cool
winter hot tropical and humid semi-hot tropical (Papadakis, 1966). Some
of the earliest cultivar importations came from California to Queensland in
1933, and very few introductions from Florida are grown there (Zentmyer,
1965). The fruit was not well known
within Australia for some years after the first introductions, but the arrival
of American servicemen in 1942 established a demand for it, and demand within
the country grew slowly in the years after World War II (Sharpe, 1950; Storey,
1960). By 1961 four main cultivars were
grown: ‘Hass’, ‘Fuerte’ and ‘Rincon’,
all imported from California, and a cultivar that originated in Australia,
‘Sharwil’ (Schulz, 1961). The increased
popularity of the avocado in recent years is demonstrated by the fact that only
400 mt of fruit were produced in 1961, but production by 1996 had risen to
16,400 mt (FAOSTAT Database, 1999).
Effect of Success of Antillean and Guatemalan-Antillean
Hybrid Cultivars on Production Elsewhere, as Reflected in Exports of Germplasm
In the 65
years from 1933 to 1998, 179 distributions (one or more cultivars or seeds) of
avocado germplasm were sent from USDA’s clonal repository at Miami to much of
the world including equatorial Africa, the Caribbean and tropical America, and
various parts of Asia and Oceania (USDA 1933-98). Some distributions were in response to individual requests, and
others were made in cooperation with various aid-oriented organizations. These
distributions have without question been instrumental in the three-fold
increase in world avocado production that came about in the 35 years from 1961
to 1996. There is no record of
germplasm distributions from Miami to Cameroon, but that country’s record is
impressive; its avocado production increased from 12,000 t in 1961 to 45,000 t
in 1996. The increase was less in Ghana, which produced 4,000 t in 1961 and
6,300 t in 1996. Production figures for
Senegal are not available before 1991, when production was 16,000 t, and
increased to 20,000 t by 1996.
Obviously figures varied from one country to another, but the change in
avocado production for Africa as a whole rose from 44,000 t in 1961 to 204,000
t in 1996. One Asian country that
received germplasm from the USDA in Florida was the Philippines, where
production rose from 13,300 t in 1961 to 26,000 t in 1996. The upward change for Asia as a whole during
this period was from 44,200 t in 1961 to 273,900 t in 1996. (FAOSTAT Database, 1999) Looking at the total world, a dramatic
increase in avocado production is apparent in the immediate past, from 696,900
t in 1961 to 2,231,500 t in
1996. This 3.2-fold increase was
reached in large part by the transfer of germplasm from Mexico through
California to parts of the world where Mediterranean-type climates predominate,
and from the Caribbean and Guatemala through Florida to those areas which have
warm lowland-type climates.
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