Avocado

Land preparation

For avocado production, a thorough land preparation is important. The farmer has to clear the land, stump, plough and remove all weeds and grass. In addition, he/she has to put in place erosion control devices.

1.1. Propagation

Avocado trees are propagated by grafting or budding scions of desirable cultivars onto seedling or rootstocks. Avocados can be grown from seeds, but fruit quality and yield potential will be quite variable. Grafted or budded avocado trees usually produce fruits when three to five years old, while seedlings often require five to seven years.

1.2  Nursery care

Seeds should be harvested from selected tree (not picked from the ground) and after removal of the pulp, care should be taken, in planting them as soon as possible in the nursery in order to avoid drying out. It is common the infection of Phytophthora cinnamoni (avocado root rot) and to eliminate this fungus, seeds should be sunk in hot water at 50⁰C for 30 minutes before planting. Germination of the seed is hastened by removing the brown seed coats and cutting a thin slice from the apical and basal end of each seed before planting.

The seed coat can be removed by wetting the seeds and allowing them to dry in the sun. The seed should be placed in the soil with the large basal end down, just deep enough to cover the tips. Seedling should be ready for grafting after  5 -6 months. The propagation by seed-for rootstocks production is a necessary step before grafting with improved varieties (taken from well identified mother plants). The planting distances depend on altitude, soil type/fertility, cultivar and climatic influences. Wider than normal spacing is necessary if the soil fertility is poor, maintenance inputs are limited and plants depend mainly on rainfall. Overcrowding leads to increased infestation with pests and diseases and will consequently reduce yield and income. Depending on the above-mentioned variables, recommended tree spacings are indicated in the cultivar descriptions. In Rwanda, 4-5 m x 4-5 m spacing is recommended; planting density of 400-625plants/ha.

1.3 Transplanting

Prior to planting, planting holes measuring 80m x 80m x 80 m are recommended. Top soil should be placed in one pile and subsoil in another. The hole should then be refilled with a mixture of 50 % top soil and 50 % of well rotten manure, compost, or other decomposed organic matter (50 kg/hole).

Container grown plants should be removed from their pots and any circling roots should be cut off in several places or loosen them. Soil is removed from the prepared holes and any stakes should be inserted firmly into the planting hole (to give support to the tree for the first two years); the plants are then set and covered to the same depth that they were growing in the container.

1. Pollination

Avocado flowers need to be pollinated before the fruit will set. Flowers are either male or female, so pollination requires transfer of pollen from a male flower to the stigma of a female flower, even when a variety is self fertile. All varieties yield better when pollen comes from a different variety, and this even applies to Hass, which is sometimes considered to be highly self fertile. Ettinger is a particularly good pollinator for Hass. Pollination is usually carried out by insects, and if they are experimentally excluded from flowers, no fruit sets. Honey bees have been found to be the most important pollinators, even when wasps and flies are abundant, so the presence of honey bee hives in or near the orchard is recommended.

2.Tillage, weed control and cover crops

During the early years of an orchard it is desirable to plant a cover crop to protect and maintain the soil until the trees can cover it with shade. Such a crop must not, however compete with the trees and must be restricted to the strip between the tree rows. The area around the base of the tree must be free of grass and other weeds and. If possible, this area should be covered with an organic mulch.

3. Training and pruning

Pruning during the first years is desirable to encourage lateral growth and multiple framework branching. In order to reinforce this shaping of the tree, sucker growth must be continuously stopped by removing all shoots originating from below the grafting joint. These shoots grow from the rootstock and are not part of the grafted cultivar.

Training a tree means creating a strong central shoot from which extend spaced side branches at the desired height and the creation of a bare main trunk about 50 – 70cm in height. Keep in mind that excessive pruning of a young tree in its juvenile stage will definitely delay the productive phase.

In later years, pruning is mainly undertaken to influence the size, quality and quantity of the fruit. It includes the removal of undesired shoots inside the canopy, the pruning of broken or diseased branches and the shortening of those touching the ground. The best time to prune is during the dormant season.

4. Fertilization

Young trees must be fertilized very judiciously since their roots could easily be burnt. Avocados can take up and utilize adequate amounts of essential elements without much fertilization, provided a suitable fertile plot has been chosen for planting. However, manure applications of 50 kg per tree per year are common.

Table. Nutrients recommendation per one avocado tree (g/year)

Age	N	P	K
1	42	22.6	75
2	84	45.2	150
3	126	67.8	250
4to5	168	101.7	300
6to7	224	135.6	400
8to9	280	169.5	500
10to12	336	203.4	600
Max	420	203.4	750

Zinc: Most soils are either naturally low in zinc or the zinc is not available. The following quantities should therefore be applied annually: Zinc oxide at 200 g/100 l water

Boron: Many avocado orchards are also low in boron and the trees should be sprayed every year with 100 g borax or 75 g Solubor/100 l water.

Table. Fertilizer recommendation per one avocado tree (g/plant) per year

Year	NPK (17-17-17)	Urea (46-0-0)	KCl (0-0-60)	Zinc	Boron
1	133	42	87	Zinc oxide at 200g/100L of water, to be applied annually	100g borax/100L of water, to be applied annually
2	266	84	175
3	399	127	304
4 to 5	598	144	331
6 to 7	798	192	441
8 to 9	997	240	551
10 to 12	1,196	288	661
Max	1.196	471	911

Time of application: 2 equal applications in beginning of season A and B.

5. Irrigation

Irrigation will be beneficial to plant growth and crop yields during prolonged dry periods. Avocados are sensitive to moisture stress. Critical stages of moisture demand occur during flowering, fruit-set and fruit development. Irrigation at 3 weeks interval resulted in early maturity, increased fruit size and oil content. In dry period, water is given at 10 days interval. Sprinkler irrigation is better than flood system as avocado plants have shallow root system and cannot stand water logging. Drip irrigation gives better results than other irrigation methods.

6. Intercultural and intercrops

Avocado orchards should be kept free from weeds. Shallow cultivation is preferred, otherwise it will damage roots. Mulching with grasses and leaves can be done as a practice that will impede weed growth and conserve soil moisture. Weedicide can be applied annually to check weed growth. Shallow rooted and leguminous crops are grown as intercrop in avocado plantations during first few years.

7. Thinning fruit

Larger fruits are preferred by the market, but when a tree bears a very high number of developing fruits they tend to be smaller. Removing (thinning out) some of the developing fruits will tend to produce the desirable heavier avocados. This process also discourages biennial bearing, which is a tendency for some trees to produce bumper crops one year and poor ones the next.

1. Avocado root rot (Phytophthora cinnamomi)

This is the most serious disease in nearly all avocado producing areas of the world. Every effort has been made to rectify the situation by uprooting or treatment of affected trees. Hot water and fungicide treatment of seeds for propagation purposes is highly recommended and grafting on Phytophthora-tolerant and/or resistant rootstocks have been included. In areas subject to flooding and in poorly drained soils, trees of any size and age are likely to be infected by this fungus.

Trees affected by the disease lose vitality, become sparsely foliated and do not produce a good crop. Leaves are pale-green and wilted, branches usually become sun burnt and die back in advanced stages. Feeder roots get darkened and decayed and trees will eventually die prematurely. The fungus can be spread or introduced to new areas by infested planting material, soil and irrigation water.

Management:

• Use diseased-free seeds.
• Treat seed for planting in a hot-water bath at 48 to 50⁰C for 20 minutes. After the hot-water treatment, rinse the seed immediately with clean, cold running water and spread out to dry thoroughly on a clean surface not in contact with the soil.
• Use clean nursery soil for container-grown plants. The soil should be well drained and carefully irrigated to prevent excessive moisture.
• Use tolerant/resistant rootstocks (e.g. Duke No. 6 and 7; G 6).
• Remove diseased trees from the field.
• Avoid movement of soil or water from diseased to non-infested areas.
• Use cultivation equipment first in healthy portion of the orchard before use in diseased areas.
• Washed and dry cultivation equipment after use in diseased areas.

For chemical control of this disease, Ridomil (active ingredient Metalaxyl) a soil-applied granular formulation that works by killing the pathogen, is recommended.

 2. Anthracnose (Colletotrichum gloeosporioides )

Anthracnose infection is important only on fruits, where it is the most serious disease affecting the avocado. Dry spots, dark brown in colour, form on the skin, leading to abnormal development. In severe attacks the young fruits soon drop. On fruits at a later point of their development, the fungal spores germinate and then enter a dormant stage. The infection remains hidden for a long period until the fruit ripens. It is mostly a post-harvest disease as the fruits are more susceptible when mature.

Management:

• Remove dead branches and twigs since they harbour the fungus.
• Remove fallen rotten fruits from the field.
• Apply copper-based fungicides pre-flowering, at fruit formation, and after harvest. Also spray applications with chemicals such as: Benomyl, Mancozeb, Metiram, Propineb, Thiabendazole or Triforine are very successful.

3. Cercospora fruit spot (Pseudocercospora purpurea)

This is another important fruit disease impairing the quality of the avocado. The severity of infection varies from season to season and can cause losses of up to 60%. Small, light-yellow spots later changing to brownish appear on fruits and leaves which eventually become hard and crack, thus creating entry points for other decay organisms. Treatments are the same as those for anthracnose.

4.Scab

Scab is caused by Sphaceloma perseae is usually common in humid areas. The fungus readily infects young, succulent tissues of leaves, twigs and fruits. Lesions appear as small dark spots slightly raised, oval to elongated. These coalesce, giving a corky appearance to the surface of the fruits – impairing the appearance but not the internal quality of the fruit. Recommended treatments are again the same as for anthracnose.

5. Scale insects

Two main groups of scale insects include the “soft scales (Coccus sp.)” and the “armoured scales”. Scales are small, stationary brown greenish insects occasionally found sucking sap from avocado leaves.

Soft scales such as Coccus spp. excrete large amount of honeydew, which lead to the development of sooty mould on leaves, branches and fruit. Honeydew attracts ants, which while feeding on the honeydew protect scales from natural enemies. Armoured scales such as the coconut scale (Aspidiotus destructor) may encrust  young twigs, leaves and fruit. They do not produce honeydew.

Scales are usually not a problem in avocado orchards; however, damage can be serious on young tress and small twigs may be killed.  The presence of scales on the skin of fruit does not cause internal damage, it may lead to rejection of fruit, especially if grown for export.

Management:

• Conserve natural enemies. Scales are usually kept under control by parasitic wasps, ladybird beetles and lacewings, provided no broad spectrum pesticides are used and no ants are present.
• Chemical control recommendations are the application of systemic or contact insecticides like Imidaclopride.

6. Fruit flies (Bactrocera invadens, Ceratitis spp.)

Avocados, especially thin-skinned varieties might be attacked by various species of fruit flies.  Some fruit flies lay eggs under the skin of the fruit that is just beginning to ripen, but others attack young and old fruit.

When the fruit reaches about the size of a golf ball a sting lesion appears as a slight puncture mark surrounded by a white exudate.  As the fruit develop the lesion becomes dry and turn into distinct star-shaped crack in the skin surface.

Management:

The few natural enemies of the fruitfly cannot control the problem effectively. There are 2 methods of control, namely:

• Eradication of unwanted host plants: eg. wild-growing guavas, etc. These plants should be eradicated in an area of up to 200 m around the orchards.
• Baiting: Using poisoned bait to lure the flies should be considered if damage exceeds 5 %.

Harvesting

Fruits should reach the market in good condition, without disease or bruising. To help to ensure good quality, fruit should be cut, not pulled from the tree at harvest, otherwise the stalk comes away and diseases will enter and rot the fruit. Fruit ripeness is a key issue in harvesting and marketing. The fruits ripen much more quickly after they have been picked, but the process should not be complete until ready for sale. Ripening can be slowed after picking by keeping the fruit cool.

Care must be taken throughout the ripening process as fruit can be damaged and might be rejected. Even light physical damage allows pathogens to enter the fruit.

In avocado, yield varies from 50-500 kgs  per tree.

Post-harvest management

After harvesting avocadoes, the farmer must carefully handle avocado fruits while transporting them and storing them, otherwise they can suffer from post harvest disorder. These disorders are mainly external: chilling injury fungal damages and mechanical friction damage as well as internal: grey pulp, stained pulp, vesicular browning, vesicular stain and peduncle rot.

Thereafter, avocado fruits are ripened so as to be eaten either alone or combined with other fruits or processed into various products such as: avocado salad recipes, baked products ( cakes),fine soup mixes, appetizers, avocado oil, skin lotions, soaps, shampoos,……