Avocado, The

Botany, Production and Uses

Bruce Schaffer, B Nigel Wolstenholme, Anthony W Whiley, Bruce Schaffer, B Nigel Wolstenholme, Anthony W Whiley

• Publisher: CAB International
• Year: 2013
• ISBN:
  9781789243727
• Edition: 2
• Topic: Ciencias biológicas
• Subtopic: Botánica

Although the avocado has been cultivated and consumed for more than 9000 years in itsnative Latin America, world production and trade have only developed over the past fewdecades, and now exceed 3.5 million tonnes.The avocado is now widely traded between warmer and colder countries as a result of theglobal community becoming increasingly aware of its versatility and nutritional value. Forthe past decade, the global avocado industry has been going through a period of rapidtechnological change and innovation. The dietary value of 'the most nutritious of all fruit' isalso receiving more attention.

1 Introduction

B. Schaffer,¹ B.N. Wolstenholme² and A.W. Whiley^3
1Tropical Research and Education Center, University of Florida, USA; ²University of KwaZulu-Natal, South Africa; ³Sunshine Horticultural Services, Queensland, Australia

The avocado (Persea americana Mill.) is a member of the family Lauraceae of the order Laurales, a large pantropical family of about 50 genera and 2500 to 3000 species of mostly trees and some shrubs (Rowher, 1993). This family includes species of economic and cultural importance, including the bay laurel (Laurus nobilis L.). According to Chanderbali et al. (2008), the ancient Greeks offered laurel leaves to reward accomplishment, giving rise to the modern term ‘laureate’, still used today to signify a reward recipient, i.e. Nobel Prize laureate. The genus Cinnamomum includes the cinnamon and camphor trees of commerce, and Aniba bases for perfume oils. Important timber trees are found in the genera Chlorocardium, Eusideroxylon, Persea and Beilschmiedia. Chanderbali et al. (2008) consider avocado as the most important commodity from the Lauraceae. World production of avocado fruit now exceeds 3.5 million tonnes, of which about 20% is traded among countries.

Evolutionary Aspects – ‘Evo-devo’ and ‘Evo-eco’

Lauraceae has long been regarded as a ‘primitive’ or relatively unspecialized angiosperm family, usually placed not too far from the very primitive Magnoliaceae (Heywood, 1978). Primitive floral features include many small, trimerous flowers with undifferentiated sepals and petals called tepals, and an inferior ovary. Using modern taxonomic techniques, Chanderbali et al. (2008; see also Chapter 3, this volume) note that P. americana (Laurales) occupies a phylogenetically pivotal position (clade) in the Magnoliids, near the base of the widely accepted Angiosperm phylogeny described by Soltis and Soltis (1990). This clade contains most of the ‘primitive’ or ‘basal’ angiosperms of older classifications. The avocado, in fact, has become an established genetic model plant to elucidate ancestral floral development of angiosperms, to aid in understanding the evolution of angiosperm flowers from non-flowering seed plants (gymnosperms) (Chanderbali et al., 2008, 2009).

The world avocado community is fortunate that leading taxonomists, evolutionary biologists, botanists and horticulturists regard the avocado as a model species for cutting-edge genomic research. Modern genomic research techniques such as transcriptome sequencing, genome mapping and partial genomic sequencing will be a major step in sequencing the entire avocado genome (Chanderbali et al., 2008; see the discussion of ‘The Avocado Genome Sequencing Project’ in Pliego-Alfaro et al., Chapter 10, this volume). Potential benefits to growers will include: improved cultivars and rootstocks, higher and more regular yields of quality fruit with better postharvest (as well as on-tree storage) characteristics, and improved tolerance of major diseases and pests (especially Phytophthora root rot).

The Lauraceae provide most of the extant species richness of the Laurales, which were especially widespread and abundant in the mid-Cretaceous. They have high basic chromosome numbers (2n = 24 for avocado). It is believed that all extant Lauraceae resulted from an ancient polyploidy event(s) at least 100 million years ago, with a second later occurrence of polyploidy in Lauraceae. Avocado floral evolution and floral development research (‘evodevo’) is summarized by Chanderbali et al. in Chapter 3 of this volume. They conclude that avocado is an attractive model for further research on floral evolution. Avocado fruit, which are botanical ‘berries’, also have some unique features, including the potential for prolonged on-tree storage (pre-climacteric); continued cell division while still firmly attached to the tree; and a pronounced respiratory climacteric concomitant with ripening. The fruit therefore also lends itself to gene profiling research on upstream regulation of fruit ripening in general.

A seminal paper on the evolutionary ecology (‘evo-eco’) of large Meso-American forest fruits, with emphasis on dispersal ecology (Janzen and Martin, 1982) was the inspiration for Wolstenholme’s and Whiley’s (1999) speculative horticultural discussion of avocado evolutionary ecology. This theme was elaborated in Barlow’s (2000) book The Ghosts of Evolution. Humans only arrived in the Americas about 13,000 years ago, yet this highly nutritious, oil-storing fruit of both tropical lowland and tropical highland Meso-American forests, has a long evolutionary history. Janzen and Martin (1982) drew attention to the probability of a now extinct Pleistocene megafauna being attracted to certain forest fruits, often swallowed whole, and thereby effectively dispersed, with little damage to the seeds. These included elephant-like gomphotheres, toxodons, giant ground sloths and glypodonts, with high energy and nutritional needs, co-evolving with favoured fruits for millions of years. In Chapter 2 of this volume, Bost et al. provide a detailed account of the history of avocado.

Barlow (2000) concluded that the avocado, arguably the world’s most nutritious commercially grown fruit (Purseglove, 1968), fits the megafaunal dispersal syndrome. For approximately 11,000 years (since the megaherbivores became extinct) it has been a ‘ghost of evolution’, almost entirely dependent on human dispersal, and anachronistic and ‘overbuilt’ in today’s world. This would apply more to the highland tropical (‘subtropical’) type of avocado with its high oil content. This unique fruit has since been appreciated and utilized by indigenous people for at least 9,000 years, in and near its native habitat in Meso-America (Smith, 1966; Gama-Campillo and Gomez-Pompa, 1992; Chanderbali et al., 2008).

The avocado was also valued by the Mayan and Aztec civilizations, as evidenced by their iconography (picture writing) (Storey et al., 1986; Gama-Campillo and Gomez-Pompa, 1992). It is believed that these cultures selected for larger fruit size and improved eating quality, and gradually spread the fruit to new areas outside the presumed native range. A degree of semi-domestication and intermingling of genes occurred over thousands of years. Today, the highly variable avocado is horticulturally classified into three races, namely the tropical lowland West Indian race (a misnomer), the tropical highland (or ‘cool subtropical’) Mexican race and the tropical highland (or ‘warm subtropical’) Guatemalan race. Selection and vegetative propagation of superior cultivars only occurred in the last 110 years, starting in Florida, USA with ‘Pollock’ and ‘Trapp’ (Fairchild, 1945). Most modern vegetatively propagated ‘subtropical’ cultivars are at least partial hybrids between Mexican and Guatemalan races, selected from chance, superior seedlings. ‘Fuerte’, the first ‘standard of excellence’ for ‘subtropical’ cultivars, popularized in California, was first budded or grafted to a seedling rootstock in 1911. Hundreds of other cultivars were subsequently selected and released. The current standard of excellence for the subtropics, including cool Mediterranean climates, is ‘Hass’, which has been widely grown since the 1950s. ‘Tropical’ cultivars have been selected and vegetatively propagated mainly in Florida also for just over 100 years (see Crane et al., Chapter 8, and Ernst et al., Chapter 9, this volume). With respect to the demands of commercial orcharding, however, the avocado still has a long way to go to full domestication. In this respect, it is a relatively new fruit.

Avocado as a World Fruit

Samson (1986) grouped tree fruit crops into four groups based on size of annual production:

1. Those with production exceeding 10 million tonnes (t), e.g. grape, citrus, banana, apple, plantain and mango.

2. Production exceeding 1 million and up to 10 million t, e.g. pear, avocado, papaya, peach, plum, pineapple, date and fig.

3. Production between 0.1 and 1 million t, e.g. cashew nut

4. Fruits for which there are no reliable statistics, e.g. guava, Brazil nut, lychee, macadamia and soursop.

The production of grape, pome and stone fruits has doubled since 1950, citrus and banana has tripled and pineapple has increased sixfold. Avocado, kiwi fruit and lychee have experienced ‘rapid development’. In 2011, annual production, in million tonnes, was for citrus about 125, for banana about 93, for apple about 70, for mango more than 35, for pear and peach about 20 and for avocado more than 3.5 (Naamani, 2011). Thus on a world production basis, avocado is still a second tier fruit and likely to remain so, as it is generally not eaten as a prime ‘starchy staple’ food. Nevertheless, as this fruit becomes better known in Europe, most of the USA, and increasingly parts of rapidly industrializing Asia, there is scope for considerable growth, albeit from a relatively low base. Furthermore, the dietary value of avocado, with its largely monounsaturated (‘good’) fatty acid (oil) content and exceptional mineral, vitamin and other beneficial anti-oxidant phytochemical content, is increasingly becoming recognized (see Bost et al., Chapter 2, this volume for more about avocado nutrition).

The role of the USA in the recent expansion of world avocado production has been profound. Since the mid-1990s the USA has overtaken Europe in total avocado consumption, then about 170,000 t – overwhelmingly locally produced (California, Florida and Hawaii). Since the beginning of the new millennium, US consumption rose from about 230,000 t to nearly 600,000 t in 2009–2010. US imports rose from about 66,000 t in 1999–2000 to over 340,000 t in 2009–2010. This was made possible by step-wise lifting of phytosanitary restrictions of imports to the USA (initially only to certain north-eastern states) from Mexico, Chile and Peru in particular. This has led to the ‘South Americanization’ of the southern hemisphere export market, and the rapid rise of Chile as the world’s second largest avocado producer (after Mexico) (Imbert, 2010; Naamani, 2011).

The traditional northern hemisphere market for ‘subtropical’ avocados, Europe (including the UK), showed a steady growth trend to reach 274,000 t in 2009–2012. This market is now supplied year-round by Peru, Spain, Chile, South Africa, Israel, Kenya and Mexico, with France, the UK and Germany the main importers. Naamani (2011) believed that this market (plus Eastern Europe, Russia, etc.) with a target population of about 500 million, has been held back by a fluctuating and generally stagnant supply for the past two decades. For many countries and for most of Asia, in fact, avocado consumption is a novelty in a relatively early stage of commercialization compared with other fruit crops. In spite of the large increase in avocado production over the past 10–15 years, exports (overwhelmingly of ‘subtropical’ avocados with high oil content fruit) of about 680,000 t in 2008 still comprise only about 20% of the world crop (about 90% ‘Hass’).

The main avocado exporters in 2008–2009 were Mexico (380,000 t), Chile (100,000 t), Peru (45,000 t), South Africa and Spain (each about 40,000 t), Israel (32,000 t) and Kenya (11,000 t) (Naamani, 2011). World trade grew by more than 250,000 t in 4 years. Imbert (2010) noted that world avocado trade reached about 800,000 t in 2009–2010. The USA imported 56% from Mexico and 5% from South America. European Union imports were 29% of the total world imports, mainly from Africa (12%), Mexico (6%) and South America.

Adaptation to Modern Orcharding

Ecological aspects

The Meso-American origin of avocado includes habitats from sea level to a few hundred metres above sea level for tropical lowland type (so-called West Indian or Antillean race) avocados, and up to 3000 m for highland Mexican and Guatemalan (‘subtropical’) race avocados. The mountainous nature of the native habitat results in a wide range of climatic and soil types, mostly in high rainfall areas with a pronounced summer peak and a dry winter and spring. The great genetic diversity of avocado is therefore not surprising, and there has undoubtedly been evolutionary migration up and down mountain ranges in response to past climate changes.

P. americana is believed to have evolved mostly within geographically tropical latitudes (23.5°N to 23.5°S), and hence avocado is often listed as a ‘tropical’ fruit. This however is misleading as the tropicality is purely geographic. Storey et al. (1986) gave the native range as from about 24°N to 8°N. Horticulturally and climatically, only the West Indian race is tropical in adaptation, having evolved at low altitude in western coastal areas of Central America – and therefore sometimes called the ‘lowland race’. The West Indian origin has been discounted, but the name is firmly embedded in the avocado literature. This category of avocado is therefore ‘tropical lowland’ in adaptation and is widely grown in the hot, humid tropics and semi-tropics. Its range is limited by frost, to which it has only token tolerance. The fruit tends to be large and has a relatively low oil content (2–5% in ‘pure’ West Indian race cultivars) and higher sugar content than ‘subtropical’ race avocados. The flavour of the ripe fruit is blander and distinctly different from that of ‘subtropical’ avocados, but is highly appreciated in lowland tropical countries, where it helps to upgrade the diet of local people. Research in semi-tropical Florida, USA, has done much to select improved West Indian race cultivars. Some of these and those hybridized with the Guatemalan race have higher fruit oil content (6–12%) (see Crane et al., Chapter 8, this volume). Most lowland tropical avocado production, however, is from seedling (non-grafted) trees, usually managed at a low level of technology in smallholder multiple cropping systems. Very little of this fruit enters international trade.

In contrast, germplasm of the commercially more important (from a world trade perspective) Guatemalan race, and to a lesser extent the Mexican race, evolved in upland montane cloud forests. The Guatemalan race is truly tropical highland, geographically and climatically. The more northerly Mexican race is both tropical highland and borderline ‘cool subtropical’ in adaptation. For both races, some frost tolerance was a prerequisite for survival, more so for the Mexican race. Good air drainage on mountain slopes and an evergreen forest environment explain why wild avocado trees can survive at altitudes of 2500 to 3000 m above sea level in Meso-America. The surprising tolerance of ‘subtropical’ avocados to seasonal drought (Whiley and Schaffer, 1994), whether in the winter months in summer rainfall areas, or in summer in Mediterranean climates, probably evolved in response to the dry winter and spring in the native montane area (see Wolstenholme, Chapter 5, this volume for further discussion on avocado ecology).

The fruits of Guatemalan and Mexican race cultivars and their hybrids, collectively characterized in this book as ‘subtropical’ type avocados, have higher (9–30%) oil content, lower sugar content, and a richer ‘nutty’ flavour than the ‘tropical’ type avocados. This has been described as an acquired taste, and certainly the avocado was slow to be accepted in the western world. In fact, there may be some truth to the assertion that only after some clever ‘reverse psychology’ advertising which indirectly alluded to the widely accepted aphrodisiac properties of avocado by native American people, did avocado sales take off in Europe. Today, this type of avocado is logically grown in tropical highlands, such as the state of Michoacán, Mexico and highlands of the Dominican Republic, Guatemala and other Central American countries, with potential in elevated a...

Table of contents

• Cover Page
• Title Page
• Copyright Page
• Contents
• Contributors
• Preface
• 1 Introduction
• 2 History, Distribution and Uses
• 3 Taxonomy and Botany
• 4 Genetics and Breeding
• 5 Ecology: Climate and Soils
• 6 Reproductive Biology
• 7 Ecophysiology
• 8 Cultivars and Rootstocks
• 9 Propagation
• 10 Biotechnology
• 11 Irrigation and Mineral Nutrition
• 12 Crop Management
• 13 Foliar, Fruit and Soilborne Diseases
• 14 Insect and Mite Pests
• 15 Harvesting, Packing, Postharvest Technology, Transport and Processing
• Index
• The colour plates