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Introduction
Avocado, scientifically known as Persea americana Mill, is a highly nutritious fruit that has gained significant importance in recent years due to its increasing demand and international trade. Belonging to the Lauraceae family, avocado is an evergreen tree native to Mexico and Central America, but its cultivation has spread to various tropical and subtropical regions worldwide
Origin and Distribution of Avocado
Avocado originated in Mexico and grew wild throughout South America. Spanish explorers introduced this fruit to Spain in 1601, and it later reached Jamaica around 1650, Florida in 1833, California in 1856, and Zanzibar in 1892. However, extensive cultivation in these areas began around the 1900s (Lahav & Whiley, 2002). In India, avocado was introduced to the south and west coasts approximately 100 years ago from Sri Lanka. Today, it is commercially grown in the United States, Israel, South Africa, and Australia, with the U.S., Israel, and South Africa being the main exporters (Whiley et al., 2007).
Nutritional Composition and Uses
The edible portion of avocado is its smooth, buttery pulp, which has an aroma resembling the interior of a tender coconut shell. The fruit is rich in fat content, with some varieties containing twice the energy value of bananas. Avocado is low in sugar and easily digestible, making it a recommended fruit for diabetic patients (Ding et al., 2007). Ripe avocado fruits are an excellent source of vitamins A, B, and C, as well as minerals like potassium, phosphorus, magnesium, and iron (Tango et al., 2004).
Ripe avocado fruits can be consumed fresh with added sugar or used in the preparation of ice creams, sandwiches, and milkshakes. The fruit is also a rich source of oil, ranging from 5% to 20% depending on the variety. Avocado oil finds applications in the cosmetic industry, although the fruit itself is not suitable for canning or cooking (Whiley et al., 2007).
Botany and Varieties
Avocado (Persea americana Mill) belongs to the Lauraceae family, which comprises about 7 genera and 1,900 species. The tree can reach a height of 20 meters and has shallow roots without visible root hairs. The leaves are spirally arranged, with variable size and shape. The inflorescence, or panicle, is produced terminally and consists of many fragrant, bisexual flowers subtended by brown, hairy bracts. The fruits are fleshy berries, large in size, pyriform or globose, single-seeded, and turn yellowish or purple upon ripening. The mesocarp (edible portion) is yellow or yellowish-green and has a butter-like consistency (Schaffer et al., 2013).
Other economically important species within the Lauraceae family include cinnamon (Cinnamomum zeylanicum) and camphor (C. camphora).
Ecological Races and Varieties
Avocado has three distinct ecological races: West Indian, Guatemalan, and Mexican. Each race possesses unique characteristics, as summarized below (Whiley et al., 2007):
- West Indian Race: Native to the lowlands of Central America, this race is best suited for hot, low tropical regions. The trees have lighter-colored leaves compared to other races. The fruits are medium-sized, with smooth, leathery, glossy skin and ripen 6-9 months after flowering. The seeds are large with rough cotyledons and loosely fitted in the cavity. The pulp has the lowest oil content (3-10%).
- Guatemalan Race: Native to the highlands of Central America, this race is more cold-tolerant than the Mexican type. The large fruits (600-900 g) are borne on long stalks with thick, brittle, hard, and warty skin. The fruits ripen in 9-12 months, and the seeds are small and tightly held in the cavity. The oil content ranges from 8% to 15%.
- Mexican Race: Native to the highlands of Mexico, this is the only race with anise-scented leaves. The fruits are small (225 g or less), with thin, smooth skin, and ripen 6-8 months after flowering. The seeds are large and loosely fitted, with smooth cotyledons. The pulp has the highest oil content (up to 30%). This race is the most resistant to cold but less commercially important
Climate and Soil Requirements
Climate
- Optimal temperature range: 12.8°C to 28.3°C
- Mild winters and temperatures below 25°C are essential for flower induction
- Cannot tolerate freezing temperatures or extreme heat
- Rainfall: 75 to 180 cm annually
Soil
- Sandy loams of alluvial origin or soils with 20-40% clay content are ideal
- Well-drained soils are necessary; waterlogging should be avoided
- Tolerant to alkaline or calcareous soils, but not to saline soils
- Optimal pH range: 5 to 7
Propagation
Avocado can be propagated through both sexual and asexual methods.
Seed Propagation
Seed propagation is easy, but the seedlings will not be true to type. Avocado seeds have low viability, remaining viable for only 2-3 weeks after extraction from ripe fruits. Seed viability can be increased by storing them in dry or sand at 5°C (Samson, 1980). Germination can be accelerated by removing the seed coat or soaking the seeds for 8 hours before sowing. More seedlings (clones) can be obtained from a single seed by lengthwise splitting into 4-6 parts, leaving a piece of the embryo in each part. Seeds typically require 50-95 days for germination.
Vegetative Propagation
- Cuttings: The rooting capacity varies depending on the race and cultivar. The Mexican race exhibits the best rooting percentage, followed by the Guatemalan race, while the West Indian race is difficult to root. The use of mist or bottom heat (25°C), etiolation, and treatment with rooting hormones like IBA can promote rooting percentage and hasten root development.
- Grafting and Budding: These are feasible methods of vegetative propagation in avocado. Veneer grafting on 2-4-month-old rootstocks with 5-7 cm long terminal shoot scions is successful in Florida, while older rootstocks are used in California. Approach grafting has shown 75% success in Tamil Nadu, India. Chip budding and shield budding are also successful methods of budding. Seedling trees with inferior quality attributes can be top-worked through cleft grafting and converted into high-yielding trees.
- Micropropagation: In vitro multiplication through axillary buds and micro-cuttings of adult grafted plants has been successful in avocado. Micro-propagated plantlets initially exhibit a slow growth rate, which can be enhanced by inoculation with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus fasciculatum. Gamma radiation up to a certain level has also been reported to increase the development of somatic embryos in the embryogenic cultures of the Fuerte variety (Witjaksono and Litz, 2004).
Rootstocks
Rootstocks have a significant influence on growth, vigor, yield, fruit quality, mineral composition of leaves, and resistance/tolerance to frost, diseases, and salinity in avocado. Mexican rootstocks, commercially used in California, Israel, and South Africa, are resistant to cold and impart dwarfness. West Indian rootstocks are widely used in warmer climates and saline soils. The Duke series of rootstocks are resistant to root rot and frost, while Duke-7 and Green rootstocks impart vigor to the scion. The Pollock rootstock overcomes salinity stress. Clonal rootstocks produce trees with uniform growth and early bearing habits compared to trees budded on seedling rootstocks. Phytophthora-resistant rootstocks are in high demand by avocado growers, but the ones developed so far have been found incompatible for grafting and hybridization with P. americana
Cultivation Practices
Planting:
Before planting, the main field should be plowed, harrowed, and leveled. The planting distance varies depending on the variety, climatic conditions, and soil characteristics. Generally, a spacing of 6-12 m is recommended for avocado planting. Recently, high-density planting systems have been adopted, comprising 400 trees per hectare (5m x 5m spacing). Later, selective tree removal is carried out, retaining 220 plants per hectare for the Fuerte variety and 270 plants per hectare for Hettinger and Hass varieties to obtain higher yields.
Pits of about 60 cm3 should be prepared 10-15 days before planting and filled with a mixture of 25 g dried farmyard manure, leaf mold, and topsoil. The grafted or budded plant should be planted along with the ball of earth around the roots, ensuring that the graft union remains above the soil level. In areas prone to water stagnation, grafts can be planted on mounds. Planting can be done at any time during the year, except during hot summer periods. After planting and until the plants are established, regular watering is essential. Buds/grafts should be supported with stakes.
Manures and Fertilizers
The nutrient requirements in avocado vary according to the variety, spacing, and soil type. Apart from the major elements like nitrogen, phosphorus, and potassium (NPK), avocado also requires calcium, magnesium, boron, and iron for normal growth and yield. Periodic soil and leaf analysis are recommended to determine the exact fertilizer requirements.
Leaves of healthy avocado plants typically contain 1.8% N, 0.15% P, 1.5% K, 2.2% Ca, 125 ppm Fe, 50 ppm Mn, 50 ppm Zn, 10 ppm Cu, and 45 ppm B. In general, young avocado trees require NPK in an equal ratio (1:1:1), while older trees should receive these elements in a 1:2:1 proportion (Samson, 1980). Nitrogen and calcium levels in the leaves markedly influence yield, fruit size, and quality in avocado. Micronutrients like Mg, B, and Fe are also important for this crop.
Training and Pruning
No regular training or pruning is given to avocado plants. For the first 8-10 years, no pruning is carried out as the tree naturally develops a well-balanced head consisting of a smooth central stem with radiating whorls of horizontal branches at regular intervals. In upright varieties like Pollock, the central shoot is headed back after a particular height to encourage the growth of laterals. In spreading types like Fuerte, interlocking branches are cut and shortened. Properly timed pruning can replace chemical thinning. Pruning terminal branches will reduce yield as flowers emerge on shoots produced from the previous season’s wood.
Intercultural Operations and Intercrops: Avocado orchards should be kept weed-free. Shallow cultivation is preferred to avoid damaging the roots. Mulching with grasses, leaves, rice hulls, rice hulls mixed with paper, and yard waste can be practiced to suppress weed growth and conserve soil moisture. Herbicides like simazine and monouron can also be applied annually to control weed growth.
During the first few years of establishment, shallow-rooted and leguminous crops can be grown as intercrops in avocado plantations. Although the initial establishment costs are high, permanent cover crops like Polygonium capitatum, Lippa canescens, and Arachis gladrata are reported to be beneficial for checking weed growth and reducing labor charges.
Irrigation
Avocado plants do not require regular irrigation. After establishment, irrigation is necessary only during severe summer and prolonged dry periods. Irrigation at three-week intervals has been shown to result in early maturity, increased fruit size, and higher oil content. In summer, water is typically applied at 10-day intervals. Sprinkler irrigation is preferred over flood irrigation, as avocado plants have a shallow root system and cannot tolerate waterlogged conditions.
Flowering and Fruit Set
Seedling trees require 5-6 years for flowering, while vegetatively propagated plants need only 3-4 years. Many varieties exhibit a biennial bearing tendency. Girdling increases yield, and temperature significantly influences flowering.
Avocado inflorescences are compound panicles of racemes with a unique flowering behavior called protogynous diurnally synchronous dichogamy (Davenport, 1986). The bisexual flowers first mature as functional females (protogynous) and later as functional males. Each flower opens twice: first as a female and then as a male. Based on the timing of these openings (forenoon or afternoon), avocado varieties are classified into two types, A and B.
In type A, the flower opens as a female in the forenoon, closes near midday, and reopens as a male in the afternoon of the same day. In type B, the first opening is in the afternoon as a female, and the second opening is in the forenoon of the next day as a male.
For successful fruit set, cross-pollination is essential, and planting both type A and type B cultivars is recommended. Environmental factors like frost, cloud, rain, and temperature can affect this dichogamous nature of avocado flowers. Honey bees are the primary pollinators.
Despite producing many flowers, fruit set is low due to factors like self-sterility, pollination problems, degeneration of young fruits, and defective seed development. Fruit development in avocado follows a sigmoid pattern, with rapid cell division in the initial phase and continued cell division in the mesocarp until maturity. Environmental factors (rainfall, humidity, temperature) and endogenous factors (hormones, nutrients) influence fruit growth (Davenport, 1986; Degani et al., 2003).
Harvesting
Fruit size is considered an index for maturity in avocado. Seeded fruits are typically 8-10 times larger than seedless ones. Changes in fruit size, color, and oil content serve as harvest indices. In California, the change in seed coat color from yellowish-white to dark brown is considered a maturity index for harvest. In South India, fruits are typically harvested during August-September. Mature fruits can be retained on the tree for several months as they do not soften as long as they are attached to the mother plant. Immature fruits will not soften even after proper maturation. In general, avocado fruits require about 6-12 months for maturity from fruit set.
Yield
Avocado yield varies from 50-500 fruits per tree. Purseglove (1974) reported high yields of 6,000 to 10,000 fruits from well-managed avocado orchards in California.
Post-harvest Management
Avocado is a climacteric fruit, and its ripening can be accelerated by ethylene exposure. Fruits exposed to 10 ppm ethylene for 36-48 hours at 18°C start ripening within 3-4 days. During ripening, the middle lamella disappears, and pectin is removed from the cell wall, causing the fruits to soften.
Avocado fruits can be stored for about one month at low temperatures (5-7°C). Storage life can be extended by waxing the fruits, packing them in polyethylene bags followed by low-temperature storage, and controlled atmospheric storage (2% O2, 10% CO2). Post-harvest life of the fruits can be extended to two months by exposing them to hot air (38°C) at 50% relative humidity (RH) for 6 hours and then storing them at 5°C (Paz et al., 2003). The firmness of avocado fruits stored at low temperatures can be assessed by acoustic measurements (Flores, 2003).
Crop Protection
Important insect pests of avocado include scale insects, mites, and mealybugs.
Physiological Disorders:
a) Mesocarp discoloration: The mesocarp shows grey to brown discoloration, mainly in the distal half of the fruit. Factors affecting the polyphenol oxidase enzyme level are responsible for this phenomenon.
b) Grey pulp: This disorder is more prevalent in fruits grown in warmer areas, where the moisture content drops below 75%, making the fruits unfit for storage and marketing.
c) Pulp spot: Pulp spot is severe early in the season and is characterized by black discoloration of the vascular bundles at the cut ends of the fruit stalk.
d) Tip burn: This is a chloride toxicity issue prevalent in spring. Affected leaves show necrosis of the tips and margins and eventually fall off. In severe cases, yield is reduced. Deep ploughing, incorporating organic manures, and providing irrigation can control this disorder.
Diseases
a) Fruit rot: Caused by Colletotrichum gloeosporioides, this is an important disease affecting avocado trees. Infected fruits drop, and remaining ones fail to develop proper size or become malformed. Controlled atmosphere storage at around 7.2°C can prevent the disease.
b) Root rot: This is the most serious disease in avocado plantations, caused by Phytophthora cinnamomi. Severe attacks can kill the trees. Application of the fungicide Ridomil (metalaxyl) at 1 g a.i./10 kg soil before planting and soil drenching at 1 g a.i./10 liters around established plants can control the disease. Proper drainage facilities are also necessary.
Other diseases affecting avocado include stem-end rot (Dothiorella aromatica), cercospora spot (Cercospora purpurea), anthracnose (Glomerella cingulata var. minor), and fruit softening (Fusarium solani and F. sambucinum).
