Avocados (Revised)

General Information and Agronomic Aspects

Introduction

Avocado, is an evergreen tree that belongs to the Lauraceae (Laurel) family and genus Persea. Avocado is native to the tropical and subtropical regions of Central and South America but is now widely cultivated in many parts of the world, including most tropical countries of Africa. It is very nutritious with a high protein and oil content and is becoming increasingly important as an export crop. 
The avocado oil is cholesterol free, of the monounsaturated type believed to protect against heart disease and certain kinds of cancers. The fruit is nearly sodium free, is rich in potassium dietary fibre and vitamin B6, C, D and E. It is eaten as fresh fruit, in salads, soups, ice cream, also used to make avocado oil, perfume and avocado paste. The family is characterized by the presence of aromatic essential oils and flowers with six tepals and six stamens, and fruits that are often fleshy and contain a single or several seeds. 
It is best suited to warm climates with well-drained soils. In Africa, avocado is grown in various countries for both domestic consumption and export.
Avocado is widely cultivated for its fruit, which is consumed fresh or used in a variety of culinary applications, such as ice cream, salads, sandwiches, and smoothies. The fruit is a good source of healthy fats, dietary fiber, vitamin B6, C, D and E, and minerals, and has been associated with various health benefits, such as improved heart health, weight management, and digestion. In addition to its use as food, avocado has also been traditionally used in various medicinal applications. The pulp and seed of the fruit contain compounds that have been shown to have anti-inflammatory, antimicrobial, and antioxidant properties, and may have potential therapeutic applications for conditions such as arthritis, diabetes, and cardiovascular disease. The leaves are used for medicinal purposes. Avocado oil, extracted from the fruit is cholesterol free, of the monounsaturated type believed to protect against heart disease and certain kinds of cancers. The oil is also used in cosmetics, as it is rich in vitamins, minerals, and fatty acids that are beneficial for the skin and hair health (Yahia, E. M., & Woolf, A. B, 2011, Orwa et al, 2009, GBIF, 2021). 

Species account

Avocado is a medium to large evergreen tree that typically grows to a height of between 9-20. While the tree is categorized as an evergreen, some varieties may shed their leaves briefly prior to flowering. The tree canopy can vary from low and dense to upright and asymmetrical. Leaves: shape vary from elliptic to oval or lanceolate, with a typical length of 7-41 cm. These leaves are usually covered with fine hairs and display a reddish hue when young, but as they mature, they become smooth, leathery, and take on a dark green color. Flowers: yellowish-green and measure approximately 1-1.3 cm in diameter. Fruit: a large, pear-shaped berry that has a thick, rough, dark green or blackish skin and a soft, creamy, pale green or yellowish pulp. The skin of the fruit varies in thickness and texture, and the fruit itself can be green, black, purple, or reddish in color when mature, depending on the variety. Avocado fruits can range in shape from spherical to pear shaped and can weigh up to 2.3 kg (Orwa et al, 2009, GBIF, 2021).

Avocado varieties.
Avocados exhibit significant variation in size, shape, color, texture, flavor, and nutritional content. There are hundreds of different avocado varieties, which can be categorized into three main groups: West Indian, Guatemalan, and Mexican.
West Indian avocados are the largest and have a smooth, shiny skin and a slightly sweet taste. They are typically consumed raw and are primarily cultivated in tropical areas like the Caribbean. Guatemalan avocados are medium-sized with a thick, bumpy skin and a buttery, nutty flavour. They are the hardiest group and can withstand colder temperatures and higher altitudes, making them popular for commercial farming. Mexican avocados are the smallest and have a dark, wrinkled skin and a creamy texture, making them a preferred ingredient in traditional dishes like guacamole.
In addition to these three groups, there are also many hybrid varieties of avocados that have been developed through cross-breeding. Hybridization is a common cause of variation in avocados and other fruits, as it allows for the mixing of desirable traits from different parent plants. For instance, the popular Hass avocado is a hybrid of Guatemalan and Mexican avocados, with a thick, pebbly skin, a rich, creamy texture, and a nutty, buttery flavour. Other popular hybrid varieties like the Reed and Lamb Hass also exist. Farmers typically choose avocado varieties based on various factors, such as fruit size, yield potential, taste, texture, disease resistance, and market demand. These criteria play an essential role in determining the profitability of avocado farming (Wikifarmer, n.d).

Common Kenyan avocado varieties

Fuerte

Hybrid of Guatemalan and Mexican races with thin skinned green-pebbled fruit of very good flavour. Fuerte is one of the major varieties in export markets

The tree is broad, very productive but susceptible to both Anthracnose and Scab.

This variety has many lines with different shapes; the pear shaped fruit is smooth, medium-thin skin that peels easily, with dense, pale green flesh with a good flavour preferred in the export market. Matures 6-8 months after flowering

Pear-shaped smooth, medium-thin skin that peels easily, with dense, pale green flesh with a good flavor.

Skin remains green when fruits are mature

Creamy, pale green flesh

Matures 6-8 months after flowering

Hass

• Vigorous grower and bears medium-sized, rounded, rough-skinned, black fruits. Propagates well. Matures 8-9 months after flowering.
• The tree grows tall and requires cutting back
• Distinctive for its tumbled leathery fairly thick skin that turns near black when fully ripe.
• he flesh closest to the skin is a lush pale lime green color and as it nears the central pit it develops a rich creamy yellow tone and softer, oil rich texture
• Oval-shaped fruit with small to medium seed
•  Flavor creamy and smooth with a nutty and sweet finish 
•  Outstanding shelf-life and shipping characteristics
• One of the longest harvesting seasons.

‘Puebla’

• Spreading, dark green tree bearing deep purple to maroon round fruit. This variety is normally used as a rootstock. Matures 5-7 months after blossoming.
• Small compact fruits resembling “Hass”
• Fruit is deep maroon and smooth
• Fruit has a velvety flesh with nutty aroma and
• a rich butter tastes
• This variety is normally used as a rootstock

‘Pinkerton’

• Long, pear-shaped fruit with small seed
• Large fruit that weighs 225 – 510 g per fruit
• Medium thick green skin with slight pebbling
• Green skin deepens in color as it ripens
• Excellent peeling
• Creamy, pale green flesh

Nabal

Bears fruit in alternate years. Its green fruits have a good flavour. Matures 8-9 months after flowering.

Others: 'Reed', 'Simmonds', 'Booth 7&8', 'Pinkerton', 'Bacon', 'Lula' and 'Taylor'. These are grown but not exported ( Mugambi 2002)

Varieties in Uganda include: 
Hass avocado is the most commonly grown and can be found in different parts of the country. 

Varieties in Tanzania:
The Hass and Fuerte are the primary commercial avocado varieties cultivated for export, although they are also available in limited quantities in the domestic market. Conversely, local avocado cultivars, while popular in domestic markets, have a shorter shelf life when ripe and are therefore not commonly exported. Other varieties grown in Tanzania include Booth 7, Puebla, Pinkerton, Uyole line, X-ikulu, Simmonds, Ettinger, Zutano, Weisal and Nabal

Ecological information 

Depending on variety, avocado grows well from 0-2100 m above sea level (asl). The varieties 'Tonnage', 'Simmonds', 'Booth 7&8' are suited to lowland areas between 90 and 800 m asl. 'Hass' and 'Nabal' are suited to altitudes 800-2100 m asl. 'Fuerte' and 'Puebla' are suited to altitudes 1500-2100 m asl. 
Avocado grows successfully on many types of soil provided they are deep, with good water holding capacity and free draining. Water logged or saline soils are unsuitable because avocado plants are sensitive to excessive soil moisture and high salinity. The optimum pH is 5.5-6.5. Temperatures between 16 and 24degC are good for growing avocados. Maximum temperature for avocado is 33degC. Above this temperature avocado fruits and trees can be damaged. High temperatures and direct sunshine can cause sunburn damage to exposed fruit. Avocado will not grow where frost can occur. An annual rainfall 1200 mm is optimal for good production. Water requirements are not less than 1000 mm/year. If supplementary water is required not more than 50 mm at a time may be applied. Climatic conditions with alternating dry and rainy seasons are best for avocados.

Agronomic aspects

Propagation and planting

Select healthy seeds and plant them in boxes or seedbeds. Immediately after germination transplant the seedlings into 4 litre pots, tins or polythene bags. Water them regularly until they are pencil thick. Grafting should be carried out when the seedling reaches pencil thickness. Wedge grafting method is most successful. Grafting should be done at the point where rootstock is soft. The scion should be dormant at the time of grafting and should match the size of the stock. Wrap the grafting point thoroughly to exclude water from the union and to prevent it from drying out.

Land preparation
The fields should be cleaned by removing tree stumps, bushes and perennial weeds. Then the land should be mowed or cultivated.

Planting out
Generally a spacing for pure stands of avocadoes of 9m x9m will be suitable. Dig holes of 60cmx60cm by 60cm deep keeping the top soil and the sub soil separate. Mix the top soil with 1 bucket (20 litre) of well rotted manure or compost and 1-2 handfuls of Mijingu rock phosphate. Remove the plant from its pot keeping the root and soil structure intact (easiest if it has been properly wetted ahead of time) and plant out using only the topsoil mixture to fill the hole.

The subsoil may be used to make a basin around the tree. Water the seedlings immediately after planting if it is not in the rainy season. Shade the young plants with banana leaves or similar material after planting and when they are putting up a new flush. If planted in a windy area, a windbreak will be necessary to protect the plants from leaning to one side and to help prevent leaf shedding fruit drop and bruising.

Manuring
One avocado tree can yield 250-300 kg of fruit per harvest season. This causes a high demand on soil nutrients. To determine the right amount of manure to apply the soil should be tested annually, otherwise the following table can be used as a guide for organic farmers for manuring individual trees:

Age of tree (years)	Nitrogen and potassium Litres animal urine mixed 1:4 with water in split applications	Mijingu rock phosphate	Additional potassium Ashes (handfuls)	Manure or compost decomposed
1-3	8	500 g	-	15 Kg
4-5	16	900 g	-	15 Kg
6-7	30	1.4 Kg	1	30 Kg
8-9	46	1.4 Kg	2	30 Kg
10-14	60 (three 20 litre buckets/year, mixed with 12 buckets of water, divided into several applications)	2 Kg	4	30 Kg
15+	80 (four 20 litre buckets/year mixed with 16 buckets of water, divided into several applications	2.5 Kg	8	30 Kg

Adapted from AIC advice for conventional farmers replacing conventional fertilizers with natural products to get the same amount of nutrients.
If chicken manure is available, it can be used instead of animal urine as follows, but spread on top of the soil far enough from the tree stem to avoid burning the stem:
 

Age of tree (years)	Chicken manure Kg/tree/year	Additional Mijingu Rock phosphate	Additional potassium Ashes - (handfuls)	Compost decomposed
1-3	1.5	150 g	-	15 Kg
4-5	3	300 g	-	15 Kg
6-7	6	450 g	1	30 Kg
8-9	9	450 g	2	30 Kg
10-14	12	800 g	4	30 Kg
15+	15	1 Kg	8	30 Kg

Trace elements: In spite of heavy applications of manures and compost, deficiencies may occur in avocado orchards. Usually these are seen as various degrees of discoloration. Accurate analysis can be made from plant analysis laboratories. The following list can be used as a guideline:

Mineral	Symptom	Control
Zinc	Mottled leaves with light yellow areas between the veins and abnormal development of growing shoots	Apply 250 g zinc sulphate for each year of age to a maximum of 4.5 kg/tree. The application should be done in a 60 cm circle around the tree
Manganese	Progressive yellowing of the margins and interveinal parts of the leaves while the veins remain green	Spray the young leaves with foliar spray of manganese sulphate
Iron	Loss of green colour in leaves	Apply 360 g iron chelate per tree in acid soils. In normal soils 250 g of iron sulphate in 10 l of water/per tree will correct iron deficiency
Chlorine	High concentration of chlorine in the soil may cause leaf burn, also damage the root system	Apply lime and compost to neutralise the chlorine.

Pruning
Initial pruning may be done to give the tree a good shape. Otherwise pruning is limited to the removal of dead wood and parasitic plants. 
•    Remove all sucker and dead branches from main trunk branches.
•    Prune canopy to keep the tree to a height of 5-8 m and for ease of picking. The tree is very susceptible to sunburn, therefore pruning should be minimized.
•    Prune the larger tree roots by cultivating to a depth of 50 cm around the edge of the tree canopy.
The area around the tree should be kept clean by weeding and removal of all fallen fruits.
 

Nutritive Value per 100 g of edible Portion

Adapted from AIC advice for conventional farmers replacing conventional fertilisers with natural products to get the same amount of nutrients.

If chicken manure is available, it can be used instead of animal urine as follows, but spread on top of the soil far enough from the tree stem to avoid burning the stem:

Trace elements: In spite of heavy applications of manures and compost, deficiencies may occur in avocado orchards. Usually these are seen as various degrees of discolouration. Accurate analysis can be made from plant analysis laboratories. The following list can be used as a guideline:

Pruning

Initial pruning may be done to give the tree a good shape. Otherwise pruning is limited to the removal of dead wood and parasitic plants. 

• Remove all sucker and dead branches from main trunk branches.
• Prune canopy to keep the tree to a height of 5-8 m and for ease of picking. The tree is very susceptible to sunburn, therefore pruning should be minimised.
• Prune the larger tree roots by cultivating to a depth of 50 cm around the edge of the tree canopy.

The following specifications constitute raw material purchasing requirements.

 

Ⓒ S. Kahumbu, Kenya

Nutritional value

Avocado is a nutrient-dense fruit that is well-known for its health benefits. The fruit is rich in healthy fats which mainly consist of monounsaturated and polyunsaturated fats which can help reduce cholesterol levels, lower inflammation and improve heart health. Avocado is a good source of minerals particularly potassium, essential for regulating blood pressure and fluid balance in the body. It also contains magnesium which plays a vital role in bone health, muscle function, and energy production. Furthermore, avocado provides a moderate amount of calcium which is crucial for bone and teeth health.
The fruit is a powerhouse of bioactive compounds that offer several health advantages. Avocado is a rich source of dietary fiber, which aids digestion, supports gut health, and promotes satiety. Additionally, it is loaded with vitamins, including vitamin K, vitamin B6, vitamin C, vitamin E, and folate, and also contains small amounts of thiamin, riboflavin, niacin, and vitamin B12. The vitamin K in avocado supports bone health by increasing calcium absorption and reducing the urinary excretion of calcium as this may prevent osteoporosis. The folate content in avocados supports fetal health during pregnancy by reducing the risk of miscarriage and neural tube abnormalities. The abundance of antioxidants in avocados, such as beta-carotene can help protect against oxidative damage, inflammation, and chronic diseases (Ingram et al., 201, Hurtado et al., 2018, Hyson, P. (2023). 

Table 1: Proximate nutritional content of 100 g of avocado (Persea americana) ripe raw fruit.

Source (Nutrient data): FAO/Government of Kenya. 2018. Kenya Food Composition Tables. Nairobi, 254 pp. http://www.fao.org/3/I9120EN/i9120en.pdf
$ Draining the water several times leaches away water soluble nutrients significantly.
a Lewis, J. 2019. Codex nutrient reference values. Rome. FAO and WHO
b NHS (refers to saturated fat)
c https://www.hsph.harvard.edu/nutritionsource/water/
d British Heart Foundation
e FDA
f NIH
g Mayo Clinic
hWest African food composition table, https://www.fao.org/3/ca7779b/CA7779B.PDF

Recipes

Avocado is a versatile and nutritious fruit that can be used in a variety of recipes, from salads, Sandwiches and smoothies.
Avocado Smoothie
Ingredients:
•    1 ripe avocado, pitted and sliced
•    1 banana
•    1 cup spinach or kale
•    1 cup almond milk (or your choice of milk)
•    1 tablespoon honey or maple syrup (optional)
•    Ice cubes (optional)
Preparation instructions
•    Blend all the ingredients until smooth. 
•    Add more milk if needed for desired consistency.
Ref: Barber, 2023
Guacamole
Guacamole is a Mexican dip/spread made from mashed avocados, lime juice, salt, and cilantro, often with tomatoes and onions.
•    2 ripe avocados
•    1 small onion, minced
•    2 cloves garlic, minced
•    1 ripe tomato, diced
•    1 lime, juiced
•    Salt and pepper to taste
•    Fresh cilantro, chopped (optional)
Preparation instructions
•    Cut the avocados in half, remove the pit, and scoop the flesh into a bowl.
•    Mash the avocados with a fork.
•    Stir in the minced onion, garlic, diced tomato, lime juice, salt, and pepper.
•    Add chopped cilantro for extra flavour if desired.
Ref: Maryellen., 2023
Avocado Toast
Ingredients:
•    2 slices of whole-grain bread
•    1 ripe avocado
•    Salt and pepper to taste
•    Optional toppings: poached egg, sliced tomatoes, red pepper flakes, or feta cheese
Preparation instructions 
•    Toast the bread to desired level of crispiness.
•    While the bread is toasting, slice the avocado and remove the pit.
•    Mash the avocado onto the toasted bread.
•    Season with salt and pepper.
•    Add your favourite toppingsComments: A quick and healthy breakfast or snack option.